WO2015000720A1

WO2015000720A1 - System for determining a quantitative number being indicative of the concentration of target elements in a fluid

Info

Publication number: WO2015000720A1
Application number: PCT/EP2014/063001
Authority: WO
Inventors: Joannes Baptist Adrianus Dionisius Van Zon; Theodorus Petrus Henricus Gerardus Jansen; Ron Martinus Laurentius Van Lieshout
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-07-01
Filing date: 2014-06-20
Publication date: 2015-01-08

Abstract

System for determining a quantitative number indicative of the concentration of target elements in a fluid, wherein the target elements (36) are attachable to particles (35) in the fluid (37) and wherein the particles (35) are bindable in a binding region (32) on a binding surface (31), the system (1) comprising: - a scattered light detection system (49) including a light source (51) for illuminating the binding region (32) and a light detector (40) for detecting light scattered by the particles (35) in the binding region (32) on or close to the binding surface (31), wherein the scattered light detection system (49) is adapted to detect the whole intensity of the light scattered by the particles (35) in the binding region (32) towards the light detector (40) within a field of view of the scattered light detection system (49), wherein the whole intensity corresponds to the integrated intensity of the light scattered by the particles (35) in the binding region (32), and - a quantitative number determination device (60) for determining the quantitative number, wherein i) the scattered light detection system (49) is further adapted to detect spots of scattered light in the binding region (32) and the quantitative number determination device (60) is adapted to, in a first mode, determine the quantitative number based on the number of detected spots in the binding region (32), if the detected whole intensity of the scattered light is smaller than a predefined first reference intensity, and ii) the quantitative number determination device (60) is adapted to, in a second mode, determine the quantitative number based on the detected whole intensity of the scattered light, if the detected whole intensity of the scattered light is larger than a predefined second reference intensity. The invention also relates to a method for determining a quantitative number indicative of the concentration of target elements in a fluid.

Description

System for determining a quantitative number being indicative of the concentration of target elements in a fluid

FIELD OF THE INVENTION

The invention relates to a system, method and computer program for determining a quantitative number being indicative of a concentration of target elements in a fluid.

BACKGROUND OF THE INVENTION

US 8,415,171 B2 discloses a system for determining a measure of the concentration of analyte molecules in a fluid. A plurality of capture objects, each including a binding surface having affinity for at least one type of analyte molecules, is exposed to a solution containing or suspected of containing the at least one type of analyte molecules, wherein at least some of the capture objects become associated with at least one analyte molecule. At least a portion of the capture objects subjected to the exposing step is spatially segregated into a plurality of locations, wherein at least some of the plurality of locations are addressed and a measure indicative of the percentage of the locations containing a capture object associated with at least one analyte molecule is determined, wherein the addressed locations are locations which contain at least one capture object. Either a measure of the concentration of analyte molecules in the fluid sample is determined based at least in part on the number of locations containing a capture object associated with at least one analyte molecule, or a measure of the concentration of analyte molecules in the fluid sample is determined based at least in part on a measured intensity level of a signal indicative of the presence of a plurality of analyte molecules, wherein the decision which of these

determination methods is used is made based upon the determined percentage of locations containing a capture object associated with at least one analyte molecule.

This process involves several steps of determining objects on a surface and is therefore complex to implement.

US 2003/153023 Al is directed to an optically-based method and system for analyte detection using solid phase immobilization, specific analyte labels adapted for signal generation and corresponding processes for the utilization thereof. The enumeration detection method narrows the area for signal observation, thus, improving detectable signal to background ratio. The system is comprised of a platform/support for immobilizing a sample stage having a labeled sample (analyte complex) bound thereto, a radiation source, an optical apparatus for generating and directing radiation at said sample and a control that obtains data and then conducts analyses using digital image data. Upon engagement of the system, the sample generates a signal capable of differentiation from background signal, both of which are collected and imaged with a signal detector that generated a sample image to a data processing apparatus. This apparatus receives signal measurements and, in turn, enumerates individual binding events. Generated signal may be increased via selected mass enhancement. The enumeration assay methodology detecting individual binding events may be used, for example, in analyses to detect analyte or confirm results in both research, commercial and point of care applications.

US 2003/096302 Al discloses methods for enhancing the dynamic range for specific detection of one or more analytes in assays using scattered-light detectable particle labels. The methods involve utilizing variations in detection technique and/or signal processing to extend the dynamic range to either or both of lower and higher concentrations.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system, method and computer program for determining a quantitative number being indicative of the

concentration of target elements in a fluid, which are technically less complex to implement.

In a first aspect of the present invention a system for determining a quantitative number being indicative of the concentration of target elements in a fluid is presented, wherein the target elements are attachable to particles in the fluid and wherein the particles are bindable in a binding region on a binding surface, wherein the system

comprises:

a scattered light detection system including a light source for illuminating the binding region and a light detector for detecting light scattered by the particles in the binding region on or close to the binding surface, wherein the scattered light detection system is adapted to detect the whole intensity of the light scattered by the particles in the binding region towards the light detector within a field of view of the scattered light detection system, wherein the whole intensity might corresponds to the integrated intensity of the light scattered by the particles in the binding region, and

a quantitative number determination device for determining the quantitative number, wherein

i) the scattered light detection system is further adapted to detect spots of scattered light in the binding region and the quantitative number determination device is adapted to, in a first mode, determine the quantitative number based on the number of detected spots in the binding region, if the detected whole intensity of the scattered light is smaller than a predefined first reference intensity, and

ii) the quantitative number determination device is adapted to, in a second mode, determine the quantitative number based on the detected whole intensity of the scattered light, if the detected whole intensity of the scattered light is larger than a predefined second reference intensity

Thus, the decision whether the quantitative number is determined based on detected spots in the binding region or based on the whole intensity of the scattered light is just based on the whole intensity, which can be measured in a technically relatively simple way. In particular, it is not required to apply data processing steps on measured data for determining a percentage of locations containing a capture object associated with a target element. This leads to a technically more simple system for determining the quantitative number being indicative of the concentration of target elements in the fluid.

The particles are preferentially labels like magnetic beads specifically bound to the target elements, which may be target molecules. The light detector is preferentially adapted to generate a two-dimensional image of the binding surface. For instance, the light detector may comprise a CCD camera. If the light detector is adapted to generate a two- dimensional image of the binding surface, the whole intensity of the light scattered by the particles in the binding region on or close to the binding surface is preferentially the intensity of the binding region in the two-dimensional image. The binding surface may be an inner surface of a cartridge comprising the fluid with the particles and the target elements.

The quantitative number can be the concentration of the target elements in the fluid or another quantitative number which depends on or is related to the concentration of the target elements in the fluid. The light can be scattered by the particles bound in the binding region to the binding surface, but also by particles, which are not bound to the binding surface in the binding region and which are on or close to the binding surface in the binding region within the field of view of the scattered light detection system.

The first and second reference intensities are preferentially predefined by, for instance, calibration measurements, wherein the reference intensities can be predefined such that the respective selected procedure for determining the quantitative number allows determining the quantitative number with a desired accuracy. The second reference intensity is preferentially equal to or smaller than the first reference intensity.

The field of view of the scattered light detection system is defined by the spatial limit of detection of the scattered light detection system, i.e. the field of view of the scattered light detection system is defined by, for instance, the spatial region within which the light detector can detect the scattered light.

In an embodiment the quantitative number determination device is adapted to, in the first mode, provide a correlation between the number of detected spots in the binding region and the quantitative number and to determine the quantitative number based on the correlation and the detected number of spots in the binding region, if the whole intensity of the scattered light is smaller than the first reference intensity. If the whole intensity is relatively small, a relatively small amount of particles is bound in the binding region. If this amount is small enough, each detected spot in the binding region can correspond to a single particle, which allows for an accurate determination of the quantitative number of the target elements in the fluid based on the number of detected spots in the binding region. In this embodiment the first reference intensity is therefore preferentially predefined such that the corresponding relatively small whole intensity corresponds to a situation, in which each detected spot substantially corresponds to a respective single particle only. The correlation can be in the form of, for instance, a mathematical formula, a lookup table, wherein discrete values in the lookup table may be interpolated, if required, et cetera. It can be determined by calibration measurements, wherein the number of detected spots is determined, while the quantitative number is known.

The correlation can be provided as a direct correlation, i.e. as a correlation directly correlating the number of spots with the quantitative number, or as an indirect correlation, wherein in the latter case firstly an intermediate value may be determined which may be used together with assignments between intermediate values and quantitative numbers for determining the concentration. For instance, firstly an intensity may be determined based on the number of spots and this intensity can be used together with an intensity-quantitative number curve for determining the quantitative number. The intensity may be determined, for instance, by multiplying the number of spots with an intensity factor being indicative of the expected intensity of a spot, wherein the intensity factor may be predetermined by calibration measurements.

It is further preferred that the quantitative number determination device is adapted to, in the second mode, provide a correlation between the whole intensity of the scattered light and the quantitative number and to determine the quantitative number based on the correlation and the whole intensity of the scattered light, if the whole intensity of the scattered light is larger than the second reference intensity. If the amount of particles bound in the binding region is relatively large such that a single detected spot may not correspond to a single particle or such that single spots are not even detectable anymore, the quantitative number can be determined more reliable based on the whole intensity. The second reference intensity is therefore preferentially predefined, especially by corresponding calibration measurements, such that for whole intensities being larger than the second reference intensity the quantitative number can be determined most accurately based on the whole intensity only. By using the whole intensity based quantitative number determination procedure for relatively large whole intensities and the spot based quantitative number determination procedure for relatively small whole intensities, the quantitative number can be determined over a relatively large concentration range.

If in an embodiment in the case of the whole intensity being smaller than the first reference intensity firstly an intensity is determined based on the number of spots and if this intensity is used together with an intensity-quantitative number curve for determining the quantitative number, this intensity-quantitative number curve for whole intensities being smaller than the first reference intensity can be combined with an intensity-quantitative number curve for whole intensities being larger than the second reference intensity for providing a combined intensity-quantitative number curve for determining the quantitative number based on this combined curve and a provided intensity, wherein this provided intensity may be the calculated intensity, which is based on the detected number of spots, if the whole intensity is smaller than the first reference intensity, and it may be the whole intensity, if the whole intensity is larger than the second threshold.

In an embodiment the second reference intensity may be larger than the first reference intensity, wherein in this case the quantitative number determination device may be adapted to, in the first mode, detect the intensity of each spot of scattered light in the binding region, to determine the number of particles per respective spot of scattered light from the respective detected spot intensity, in order to determine the total number of particles in the binding region, and to determine the quantitative number based on the determined total number of particles in the binding region, if the whole intensity of scattered light is larger than the first reference intensity and smaller than the second reference intensity. Also in this case the quantitative number determination device may be adapted to provide a correlation between the determined number of particles in the binding region and the quantitative number and to determine the quantitative number based on the correlation and the determined number of particles in the binding region, if the whole intensity of scattered light is larger than the first reference intensity and smaller than the second reference intensity. In a situation in which at least some of the detected spots do not correspond to a single particle bound in the binding region, but in which still single spots are detectable, this quantitative number determination procedure may lead to a better accuracy than the quantitative number determination procedure based solely on the number of spots and the quantitative number determination procedure based on the whole intensity only. This situation can be defined by first and second reference intensities, which are different and which may be determined by calibration measurements.

In an embodiment the second reference intensity may be larger than the first reference intensity, wherein the scattered light detection system is further adapted to detect spots of scattered light in the binding region and the quantitative number determination device is adapted to determine a first quantitative number being indicative of the

concentration of the target elements in the fluid based on the number of detected spots in the binding region, to determine a second quantitative number being indicative of the

concentration of the target elements in the fluid based on the whole intensity of the scattered light and to weightedly average the first and second quantitative numbers, if the whole intensity of the scattered light is larger than the first intensity reference and smaller than the second intensity reference. Also this average based quantitative number determination procedure can lead to an improved accuracy of determining the quantitative number in a region between the first and second intensity thresholds.

Preferentially, the scattered light detection system is adapted to measure a background intensity by measuring the light received from the binding surface outside the binding region, wherein the quantitative number determination device is adapted to subtract the background intensity from the whole intensity for providing a background corrected whole intensity. Preferentially, this corrected whole intensity is used for the further steps like the decision on which concentration determination procedure should be used and like the determination of the concentration based on the whole intensity. In this case also the reference intensities have preferentially been predefined with reference to the background corrected whole intensity by using, for instance, calibration measurements in which the background corrected whole intensity is used, and, if correlations are used for determining the quantitative number based on the background corrected whole intensity, also these correlations are preferentially predefined with respect to the background corrected whole intensity by using, for example, calibration measurements in which the background corrected whole intensity is used. This can further improve the quality of the overall concentration determination process.

In a preferred embodiment the scattered light detection system is adapted to detect the spots of scattered light in the binding region by generating a first image being an image of the binding region showing the spots, wherein the first image also shows the binding surface outside the binding region, wherein the quantitative number determination device is adapted to determine the quantitative number based on the detected spots in the binding region by a) generating a threshold image by thresholding the first image, b) determining a background intensity from the part of the first image showing the binding surface outside the binding region, c) subtracting the background intensity from the first image for generating a second image, d) multiplying the second image with the threshold image for generating a third image, e) determining the whole intensity in the binding region in the third image, and f) determining the quantitative number based on the determined whole intensity in the binding region in the third image. This threshold image based technique is preferentially performed, if the first reference intensity is smaller than the second threshold intensity and if the whole intensity of the detected scattered light in the binding region is larger than the first threshold and smaller than the second threshold. This can lead to a more accurate determination of the concentration of the target elements in the fluid close to the first reference intensity.

Preferentially, the particles are magnetic particles adapted to be attached to target elements, wherein the binding surface is adapted to bind the particles in the binding region, if the particles are attached to the target elements, wherein the system further comprises a magnet assembly for forcing the particles to the binding surface, where the particles with attached target elements are bound in the binding region, and for removing unbound particles from the binding surface. Thus, the particles can have two functions, labeling the target elements and actuating the quantitative number determination process by being forced by the magnetic field. It is therefore not necessary to use two kinds of particles, a first kind for labeling the target elements and a second kind for actuating purposes, thereby rendering the system for determining the quantitative number technically even more simple.

In an embodiment the scattered light detection system is adapted to detect the whole intensity of the scattered light several times under different illumination conditions, wherein the quantitative number determination device is adapted to provide for the different illumination conditions different first and second reference intensities and to select one of the different illumination conditions based on the detected corresponding whole intensities, wherein i) the scattered light detection system is further adapted to detect spots of scattered light in the binding region based on the scattered light which has been detected under the selected illumination condition, and the quantitative number determination device is adapted to, in the first mode, determine the quantitative number based on the detected spots in the binding region, if the detected whole intensity of the scattered light, which has been detected under the selected illumination condition, is smaller than the first reference intensity which has been provided for the selected illumination condition, and ii) the quantitative number determination device is adapted to, in the second mode, determine the quantitative number based on the detected whole intensity of the scattered light, which has been detected under the selected illumination condition, if the detected whole intensity of the scattered light, which has been detected under the selected illumination condition, is larger than the second reference intensity which has been provided for the selected illumination condition. The illumination conditions are, for instance, the detection times, which may also be regarded as being shutter times, during which the scattered light is detected by, for instance, acquiring an image of the binding region based on the scattered light by using a camera like a CCD camera. The respective illumination condition can be selected such that the light detector of the scattered light detection system, for instance, the CCD camera, is not saturated and the detected intensity is larger than a predefined threshold during the detection of the scattered light, if the selected illumination condition is used. The selection can be performed by thresholding the detected intensities. This can further increase the range of concentrations for which a quantitative number can be determined.

In a further aspect of the present invention a method for determining a quantitative number being indicative of the concentration of target elements in a fluid is presented, wherein the target elements are attached to particles in the fluid and wherein the particles with attached target elements are bound in a binding region on a binding surface, the method comprising:

illuminating the binding region by a light source of a scattered light detection system and detecting light scattered by the particles in the binding region on or close to the binding surface by a light detector of the scattered light detection system, wherein the whole intensity of the light scattered by the particles in the binding region towards the light detector within a field of view of the scattered light detection system is detected, wherein the whole intensity might correspond to the integrated intensity of the light scattered by the particles (35) in the binding region (32), and determining the quantitative number by a quantitative number determination device,

wherein

i) spots of scattered light in the binding region are detected and, in a first mode, the quantitative number is determined based on the number of detected spots in the binding region, if the whole intensity of the scattered light is smaller than a predefined first reference intensity, and

ii) in a second mode the quantitative number is determined based on the whole intensity of the scattered light, if the whole intensity of the scattered light is larger than a predefined second reference intensity.

In a further aspect of the present invention a computer program for determining a quantitative number being indicative of the concentration of target elements in a fluid is presented, the computer program comprising program code means for causing a system as defined in claim 1 to carry out the steps of the method as defined in claim 11 , when the computer program is run on a computer controlling the system.

It shall be understood that the system of claim 1, the method of claim 11, and the computer program of claim 12 have similar and/or identical preferred embodiments, in particular, as defined in the dependent claims.

It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims or above embodiments with the respective independent claim.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

Fig. 1 schematically and exemplarily shows an embodiment of a system for determining a quantitative number being indicative of the concentration of target elements in a fluid,

Figs. 2 and 3 schematically and exemplarily show some elements of the system shown in Fig. 1 in more detail,

Fig. 4 schematically and exemplarily illustrates a binding of particles with attached target elements on a binding surface, Fig. 5 shows a flowchart exemplarily illustrating a method for determining a quantitative number being indicative of the concentration of target elements in a fluid, and

Figs. 6 and 7 schematically and exemplarily show dose-response curves. DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows schematically and exemplarily an embodiment of a system for determining a quantitative number being indicative of the concentration of target elements in a fluid. In this embodiment the quantitative number is the concentration, i.e. the system is a system for determining the concentration of the target elements in the fluid. Fig. 2 shows a perspective view and Fig. 3 shows a side view of a part of the system 1 for determining the concentration of target elements in the fluid. The system 1 comprises a microscope 42 and above the microscope 42 two magnetic subunits 22a, 22b at arms 21a, 21b of a magnet assembly 10 with end sections 24a, 24b. Above the end sections 24a, 24b a cartridge 30 with a binding surface 31 is positioned. The two subunits 22a, 22b are positioned perpendicular to the longitudinal direction of the microscope 42.

The end sections 24a, 24b of the corresponding magnetic subunits 22a, 22b project in an essentially perpendicular direction to the magnetic subunits 22a, 22b, this is the direction essentially parallel to the longitudinal axis of the microscope 42, in the perspective shown in Fig. 2 upwards. The cartridge 30 is positioned above the magnetic subunits 22a, 22b and corresponding end sections 24a, 24b. The magnet assembly 10 generates a magnetic field that exerts magnetic forces in the area of the binding surface 31 of the cartridge 30 to accomplish a magnetic actuation. The magnetic field exerts forces especially at magnetic particles within the cartridge 30 and enables the magnetic actuation process. A lens 43 below the microscope 42, the microscope 42, and a camera 44 below the lens 43 are parts of a light detector 40. An incoming ray 54a of a light source 51 , for instance, an LED or laser coming from the left side in Fig. 1 , impinges at the binding surface 31. Particles present at the binding surface 31, especially magnetic particles, scatter parts of the incoming ray 54a, wherein the scattered light 46 is detected by the light detector 40, in order to detect the particles on the binding surface 31. Scattered light 46 in Fig. 1 is depicted as a light cone 41 starting from the binding surface 31 and spreading in the direction of the lens 43 of the light detector 40. An outgoing ray 54b leaves the cartridge 30 originating from the incoming ray 54a after impinging at the binding surface 31 and being scattered. The system 1 is preferentially integrated into a housing (not shown) and may be applied in a portable handheld device (not shown). The view of Fig. 3 makes clear the combination of the magnet assembly 10 for providing a magnetic field at the cartridge 30 and the optical detection of the particles by the light detector 40. Again, the incoming ray 54a is reflected at the binding surface 31 of the cartridge 30, wherein a small part of the incoming ray 54a is reflected at the particles and detected by the light detector 40. The reflected light is again depicted as a cone 41 in a schematic way. Especially, the end sections 24a, 24b of the magnetic subunits 22a, 22b are directed vertical, whereas the magnetic subunits 22a, 22b are aligned horizontal. The end sections 24a, 24b are directed upwards essentially in a right angle to the magnetic subunits 22a, 22b, and essentially parallel to the longitudinal axis of the microscope 42, this is the longitudinal direction of the plane of Fig. 3. The structure described allows for the magnet assembly 10 and the light detector 40 to be positioned at the same side with regard to the cartridge 30.

The magnetic subunits 22a, 22b define a gap 25 between each other. Adjacent to the gap 25 and above the gap 25 in this view some of the light of the incoming light ray 54a is scattered to the direction of the light detector 40. The gap 25 is formed in a way to house the scattered light 46 depicted by the light cone 41, the light cone 41 fits into the gap 25, as is shown in Fig. 3. The described structure allows for the light detector 40 and the magnet assembly 10 for magnetic actuation to be positioned at the same side of the cartridge 30. The system 1 is adapted to actuate particles and detect particles simultaneously.

In Fig. 1 the light source 51 is positioned at the left side of the light detector

40 for generating the incoming light ray 54a impinging at the binding surface 31 of the cartridge 30. A part of the light of the incoming ray 54a is scattered in the direction downwards between the magnetic subunits 22a, 22b, wherein the scattered light 46 passes the lens 43 and reaches the camera 44 at the bottom of the light detector 40 for automatic detection of the image of the microscope 42 and recording. The light source 51 and the light detector 40 can therefore be regarded as forming a scattered light detection system 49.

A further magnet unit 48 is provided above the cartridge 30 for generating a further magnetic field to support the magnetic actuation process or for removing excessive particles by magnetic forces, a method known as magnetic washing. The top side of the cartridge 30 remains fully accessible, allowing the positioning of the further magnet unit 48 for magnetic washing, a heater for temperature control or other devices. For more details regarding the scattered light detection system, the magnet assembly and the cartridge reference is made to WO 2011/036634 Al, which is herewith incorporated by reference. The light detector 40 is connected to a quantitative number determination device 60 for processing the fed signals such that, in this embodiment, the concentration of the target elements in the fluid is determined.

As schematically and exemplarily illustrated in Fig. 4, the particles 35 are adapted for being attached to the target elements 36 within the fluid 37, wherein the particles 35 are bindable in a binding region 32 on the binding surface 31 by using binding elements 34 arranged in the binding region 32. The fluid is, for instance, blood, saliva, urine, or another fluid, particularly another bodily fluid.

The scattered light detection system 49 is adapted to detect spots of scattered light in the binding region 32 and the quantitative number determination device 60 is adapted to, in a first mode, determine the concentration of the target elements 36 in the fluid 37 based on the detected spots in the binding region 32, if the whole intensity of the scattered light is smaller than a predefined first reference intensity. Moreover, the quantitative number determination device 60 is adapted to, in a second mode, determine the concentration of the target elements 36 in the fluid 37 based on the whole intensity of the scattered light, if the whole intensity is larger than a predefined second reference intensity being larger or equal to the first reference intensity.

The quantitative number determination device 60 is adapted to provide a correlation between the number of detected spots in the binding region 32 and the concentration of target elements 36 in the fluid 37 and to, in the first mode, determine the concentration of the target elements 36 in the fluid 37 based on the correlation and the detected number of spots in the binding region 32, if the whole intensity is smaller than the first reference intensity. The correlation can be in the form of, for instance, a mathematical formula, a look-up table, wherein discrete values in the look-up table may be interpolated, if required, et cetera. It can be determined by calibration measurements, wherein the number of detected spots is determined, while the concentration of the target elements in the fluid is known.

The correlation can be provided as a direct correlation, i.e. as a correlation directly correlating the number of spots with the concentration of the target elements 36 in the fluid, or as an indirect correlation, wherein in the latter case firstly an intermediate value may be determined which may be used together with assignments between intermediate values and concentrations of target elements 36 for determining the concentration of the target elements 36. For instance, firstly an intensity may be determined based on the number of spots and this intensity can be used together with an intensity-concentration curve for determining the concentration of the target elements within the fluid. The intensity may be determined, for instance, by multiplying the number of spots with an intensity factor being indicative of the expected intensity of a spot, wherein the intensity factor may be

predetermined by calibration measurements.

The quantitative number determination device 60 is further adapted to provide a correlation between the whole intensity of the light scattered by the particles in the binding region 32 and the concentration of target elements 36 in the fluid 37 and to, in the second mode, determine the concentration of the target elements 36 in the fluid 37 based on the correlation and the whole intensity, if the whole intensity is larger than the second reference intensity.

If in an embodiment, in the case of the whole intensity being smaller than the first reference intensity, firstly an intensity is determined based on the number of spots and if this intensity is used together with an intensity-concentration curve for determining the concentration of the target element within the fluid, this intensity-concentration curve for whole intensities being smaller than the first reference intensity can be combined with an intensity-concentration curve for whole intensities being larger than the second reference intensity for providing a combined intensity-concentration curve for determining the concentration of the target elements within the fluid based on this combined curve and a provided intensity, wherein this provided intensity may be the calculated intensity, which is based on the detected number of spots, if the whole intensity is smaller than the first reference intensity, and it may be the whole intensity, if the whole intensity is larger than the second threshold.

The first and second reference intensities can be equal, or the second reference intensity can be larger than the first reference intensity. In the latter case the quantitative number determination device 60 may be adapted to, in the first mode, detect the intensity of each spot of scattered light in the binding region 32, to determine the number of particles 35 causing a respective spot of scattered light from the respective detected intensity, in order to determine the total number of particles 35 in the binding region 32, and to determine the concentration of the target elements 36 in the fluid 37 based on this determined total number of particles 35 in the binding region 32, if the whole intensity is larger than the first reference intensity and smaller than the second reference intensity. The quantitative number determination device 60 is then also adapted to provide a correlation between the number of particles 35 in the binding region 32 determined in this way and the concentration of target elements 36 in the fluid 37 and to determine the concentration of the target elements 36 in the fluid 37 based on the correlation and the determined number of particles 35 in the binding region 32, if the whole intensity is larger than the first reference intensity and smaller than the second reference intensity.

If the second reference intensity is larger than the first reference intensity, in a further embodiment the scattered light detection system 49 can be adapted to detect spots of scattered light in the binding region 32 and the quantitative number determination device 60 can be adapted to determine a first concentration of the target elements 36 in the fluid 37 based on the number of detected spots in the binding region 32, to determine a second concentration of the target elements 36 in the fluid 37 based on the whole intensity of the light scattered by the particles 35 in the binding region 32 and to weightedly average the first and second concentrations. For instance, the first and second concentration can be equally weighted.

The scattered light detection system 49 may also be adapted to detect the spots of scattered light in the binding region 32 by generating a first image being an image of the binding region 32 showing the spots, wherein the first image also shows an outside region 33 on the binding surface 31 outside the binding region 32, wherein the quantitative number determination device 60 is adapted to determine the concentration of the target elements 36 in the fluid 37 based on the detected spots in the binding region 32 by a) generating a threshold image by thresholding the first image, b) determining a background intensity from the part of the first image showing the outside region 33, c) subtracting the background intensity from the first image for generating a second image, d) multiplying the second image with the threshold image for generating a third image, e) determining the whole intensity in the binding region 32 in the third image, and f) determining the concentration of the target elements 36 in the fluid 37 based on the determined whole intensity in the binding region 32 in the third image. This can lead to a more accurate determination of the concentration of the target elements 36 in the fluid 37 close to the first reference intensity. This thresholding techhnique may particularly be used, if the second reference intensity is larger than the first threshold intensity and the whole intensity is in between the first and second reference intensities.

Preferentially, the scattered light detection system 49 is adapted to measure a background intensity by measuring the light received from the binding surface 31 outside the binding region 32, wherein the quantitative number determination device 60 is adapted to subtract the background intensity from the whole intensity for providing a background corrected whole intensity. This corrected whole intensity is preferentially used for the further steps like the decision on which concentration determination procedure should be used and like the determination of the concentration based on the whole intensity, i.e. all steps related to the whole intensity are preferentially performed by using the background corrected whole intensity. In this case also the reference intensities and the correlation for determining the quantitative number based on the background corrected whole intensity have preferentially been predefined with reference to the background corrected whole intensity by using, for instance, calibration measurements in which the background corrected whole intensity is used. However, in an embodiment the steps like the decision on which concentration determination procedure should be used and like the determination of the concentration based on the whole intensity, i.e. all steps related to the whole intensity, may also be performed by using a non-corrected whole intensity, i.e. a whole intensity that has not been background corrected.

In the following an embodiment of a method for determining the concentration of the target elements 36 in the fluid 37 will be exemplarily described with reference to a flowchart shown in Fig. 5.

After the target elements 36 have been attached to the particles 35 in the fluid 37 and bound in the binding region 32, in step 101 the binding region 32 is illuminated by the light source 51 of the scattered light detection system 49 and the light scattered by the particles 35 in the binding region 32 on or close to the binding surface 31 is detected by the light detector 40 of the scattered light detection system 49, wherein the scattered light detection system 49 detects the whole intensity of the scattered light scattered. In step 102 it is determined whether the whole intensity is below the first reference intensity or above the second reference intensity, wherein in this embodiment the first and second reference intensities are equal. If the whole intensity is smaller than the first reference intensity the method continues with step 103. If the whole intensity is larger than the second reference intensity the method continues with step 104.

In step 103 the scattered light detection system 49 detects spots of scattered light in the binding region 32 and the quantitative number determination device 60 determines, in the first mode, the concentration of the target elements 36 in the fluid 37 based on the detected spots in the binding region 32, and in step 104 the quantitative number determination device 60 determines, in the second mode, the concentration of the target elements 36 in the fluid 37 based on the whole intensity of the scattered light.

If in another embodiment the first and second reference intensities are not equal, in step 102 it can further be determined whether the whole intensity is larger than the first reference intensity and smaller than the second reference intensity, wherein in this case one of the concentration determination procedures described above for this situation can be performed.

In an embodiment the data processing steps are separated from the scattered light measuring steps such that a corresponding concentration determination method can be provided, which just comprises data processing steps and which can be provided as a separate computer program. This concentration determination method separately implemented as a computer program may comprise the steps of determining whether the whole intensity of the light scattered by the particles 35 in the binding region 32 is smaller than the predefined first reference intensity or larger than the second reference intensity, wherein in the first case the concentration of the target elements 36 in the fluid 37 may be determined based on the detected spots in the binding region 32 and wherein in the second case the concentration of the target elements 36 in the fluid 37 may be determined based on the whole intensity.

The above described systems for determining the concentration of the target elements in the fluid can be adapted for the detection of DNA (molecular diagnostics) and/or proteins (immuno-assays), both important markers for all kind of diseases in the human body. The particles are preferentially superparamagnetic labels, which may also be regarded as being beads, to detect the presence of target elements, which are preferentially target molecules, in a solution. These labels are coated with antibodies which specifically catch the target elements. After binding to the binding surface which is also coated with functional antibodies, the magnetic labels are detected by means of the optical detection technique.

The above described system allows for a detection of individual magnetic labels in certain concentration regimes. Due to the detection of individual labels, the instrumental limit of detection is relatively low. The counting of individual labels is achieved by imaging the surface of the cartridge onto the camera being preferentially a CCD camera by means of the objective lens, which preferentially has a high numerical aperture (NA) of, for instance, 0.4. A lens with NA=0.4 has an optical resolution around 1.8 μιη which is of the order of a preferred size of the magnetic labels to be detected. For instance, the particles, i.e. the magnetic beads in this example, may have a diameter of 500 nm. As long as the distance between the magnetic labels on the binding surface is larger than a few times their diameter, the labels can be detected as individual particles. However when beads are very close together, i.e., for example, almost touching, they cannot be discriminated as individual particles anymore and they are just still detected as one single particle. Above a certain surface density of beads, the number of counted beads starts to deviate from the real number of beads on the binding surface, because for very high surface densities individual beads cannot be detected anymore. However, the integrated intensity of the beads, i.e. the whole intensity of the light scattered in the binding region, can still be used as a measure for the number of beads in the binding region. This may pose a problem in the sense that at low bead densities a signal and used for determining the concentration of the target elements in the fluid, which is indicative of the number of detected spots inside the binding region, while at high bead densities a signal is provided and used for determining the concentration of the target elements in the fluid, which is indicative of an integrated intensity, i.e. the whole intensity of the binding region. The system described above is adapted to handle these two signals used for determining the concentration of the target elements in the fluid.

At high bead densities inside the binding region the measured intensity is generally a superposition of the collective intensities of all the beads and a background intensity. The background intensity is an intensity level which is not caused by bound beads. It is generally an intensity contribution caused by light scattering within a fluidic channel of the cartridge. Assuming that no binding of beads takes place outside of the binding region, the intensity measured outside of the binding region can be taken as the background intensity. By subtracting this background intensity from the intensity measured inside of the binding region, a bound bead-dependent intensity signal is obtained, which is preferentially used as whole intensity of the light scattered by the particles, i.e. the beads in this example, in the binding region.

At low bead densities inside the binding region each individual bead can be counted. This number is independent of the background intensity. By multiplying each bead with an average intensity per bead factor, the bead count can be converted to an intensity signal. Since the bead count does not include a background signal, the converted intensity signal also does not contain a background signal. It is therefore compatible with the intensity signal as described previously for high bead density spots.

For high target molecule concentrations, wherein the target molecule may be, for instance, cTnl, the bead density in the binding region will be high and the total binding region intensity will be measured. For low target molecule concentrations the bead density in the binding region will be low and individual beads can be counted. For each concentration regime the intensity may be determined or corrected, respectively, as described above, i.e. for the low concentration regime the intensity may be determined by multiplying the detected number of spots with the per bead intensity factor for determining an intermediate value and for the high concentration regime the intensity measured inside the binding region, from which the background intensity has been subtracted, may be used. By plotting the determined or corrected, respectively, intensity signal as a function of the target molecule concentration, a so-called dose-response curve is obtained. In this case the dose-response curve contains two separate sections, one for low and one for high concentrations. Ideally these two regions show an overlap. The per bead intensity factor, which may be an average bead intensity factor, as used in the low concentration regime may be adapted such that a continuous total dose-response curve is obtained, i.e. showing no discontinuities.

Fig. 6 shows schematically and exemplarily such a total dose-response curve comprising a first part 61 for the low concentration regime and a second part 62 for the high concentration regime, wherein in Fig. 6 / indicates the respective intensity in arbitrary units and C indicates the concentration in pM. The first part 61 represents assignments between intermediate values, i.e. the calculated intensity values, and concentrations of target elements, wherein these assignments can be determined by calibration measurements. These

assignments together with the relation between the number of detected spots in the binding region and the calculated intensity forms an indirect correlation between the number of detected spots in the binding region and the concentration of target elements in the fluid, which can be used for determining the concentration of target elements in the fluid based on the detected spots in the low concentration regime. The second part 62 represents a direct correlation between the whole intensity of the light scattered by the particles in the binding region and the concentration of target elements in the fluid, which can be used for

determining the concentration of target elements in the fluid based on the whole intensity in the high concentration regime.

The second part 62 of the dose-response curve for the high concentration regime, which may be defined by concentrations between 10 to 10000 pM, shows a saturation level. This saturation level is basically determined by the maximum intensity of a pixel of the camera 44, which is a CCD camera, but which may also be another camera like a CMOS camera. At the lower end of the second part 62 of the dose-response curve, i.e. in this example at about 10 pM, the whole intensity, which is the difference between the integrated intensity within the binding region and the background intensity, is relatively small, in particular, in the order of the background intensity. Since two intensities are subtracted from each other for providing the whole intensity, wherein the two intensities are of the same order of magnitude, the finally determined whole intensity is relatively inaccurate for relatively low integrated intensities within the binding region. The first part 61 of the dose-response curve shows two saturation levels. At the low concentration side of the second part 61, which may be defined by concentrations being smaller than 0.1 pM, the saturation may be caused by an assay effect, i.e. by non-specific binding. At the high concentration side of the first part 61 of the dose-response curve, which may be defined by concentrations being larger than 5 pM, the decrease in slope is caused by the optical resolution of the light detector. For lower optical resolutions the change in slope would start already at even lower concentrations. This effect is caused by two particles, which are very close together, and which therefore show up as only one beat on the camera 44, i.e. a single spot detected by the camera 44 may be caused by two or more particles. For higher concentrations of particles in the fluid there is an increased probability that this will happen. Therefore, for higher concentrations less and less particles will be counted by using the spot-based concentration determination procedure.

The transition region of the dose-response curve, in which the first part 61 of the dose-response curve comprises a decreased slope at its high concentration side, which is between about 5 and 20 pM in Fig. 6, can be improved by using another method for generating an intensity for the low concentration part of the dose-response curve. Two beads, which are in close proximity, cannot optically be resolved and will therefore be counted as a single particle, i.e. a single spot is detected, which is caused by two particles. However, the intensities of the two particles will still add such that the detected spot will have a double intensity. This spot intensity information can be used by the quantitative number

determination device by using the above described threshold image based concentration determination procedure. That means that a threshold image can be generated, wherein each pixel having an intensity value above a background level, which may be locally determined around each respective spot, may be represented by a logical one and each pixel having an intensity below the background level may be represented by a logical zero. Such a threshold image, in which the spot regions are represented by ones and the other regions are represented by zeros, can be calculated by the quantitative number determination device from a spot image of the binding surface 31. Moreover, the quantitative number determination device can be adapted to determine the average background intensity outside the binding region 32 from the spot image and subtract this average background intensity from the spot image for providing a background-corrected image. The quantitative number determination device may be further adapted to multiply this background-corrected image with the threshold image for providing a final image, wherein from this final image the integrated intensity of the binding region 32 can be determined as an intermediate value. This reconstructed intensity can be used together with assignments between intensities and concentrations, i.e. together with a corresponding part of a dose-response curve, for determining the concentration of the target elements within the fluid in the low concentration regime. This procedure basically extends the linear region of the first part of the dose- response curve as exemplarily illustrated in Fig. 7.

In Fig. 7 the dose-response curve comprises a first part 71 for the low-dose regime determined by using the threshold image based procedure and a second part 72 which corresponds to the second part 62 shown in Fig. 6. As can be seen in Fig. 7 the threshold image based procedure basically extends the linear region of the first part 71 of the dose- response curve such that a larger overlap region is obtained between the first and second parts 71, 72 of the dose-response curve.

In order to allow the quantitative number determination device to decide which concentration determination procedure should be used, the quantitative number determination device can comprise a table of reference intensities, which may also been regarded as being reference intensities. In principle the quantitative number determination device can be adapted to provide more than the above described concentration determination procedures and can correspondingly have a table with more than first and second reference intensities. The set of reference intensities divides the intensity scale into intensity intervals, wherein each intensity interval corresponds to a dedicated concentration determination procedure. Each concentration determination procedure defines a procedure for obtaining an established signal from a measurement performed by the scattered light detection system and to convert the established signal into a corresponding concentration of target elements in the fluid by using a calibrated correlation, i.e. a calibrated relationship, between the established signal and the concentration. The established signal may be a signal which is obtained directly from a measured signal provided by the scattered light detection system like an analog intensity signal or after applying an algorithm to the measured signal provided by the scattered light detection system like applying an image processing algorithm. The established signal can be different for different concentration determination procedures. In particular, the established signal can be a signal representing the whole intensity of the light scattered by the particles in the binding region, which can be obtained by providing a first intensity by measuring the light received from the binding surface outside the binding region, by providing a second intensity by measuring the light received from the binding region and by subtracting the first intensity from the second intensity. Moreover, the established signal can be calculated by multiplying an intensity factor, which corresponds to the intensity of a single spot detected in the binding region, with a determined number of the spots in the binding region. The established signal can also be determined in another way in accordance with the respective concentration determination procedure. For example, the above described threshold image based algorithm can be used for determining an intensity represented by the established signal.

The calibrated relationship between the established signal and the concentration is a description which links a concentration to the established signal. This description can be, for instance, an analytical expression like a mathematical formula or an algorithm to do a lookup in a discrete table, followed by an interpolation, which may be a linear, cubic, et cetera interpolation, between the discrete table elements. The calibrated relationship between the established signal and the concentration can be obtained from calibration measurements in which samples with a known concentration are measured and the respective established signal is generated. The calibrated relationships for different concentration determination procedures can be different.

The quantitative number determination device is preferentially adapted to compare the whole intensity signal with the reference intensities for determining an intensity interval, which corresponds to a concentration determination procedure, wherein then the concentration is determined in accordance with this concentration determination procedure. In this way each whole intensity measurement may result in a concentration either directly or via calculating other intermediate intensities, which can be used for determining the final concentration of the target elements within the fluid.

In an embodiment the light source of the scattered light detection system is a coherent light source like a laser. If coherent light is used, the average intensity per bead increases with increasing concentration due to mutual illumination. The product of bead count (under linear) and average bead intensity (over linear) becomes linear over a longer concentration range because both effects substantially compensate each other.

In an embodiment the scattered light detection system is adapted to detect the whole intensity of the scattered light several times under different illumination conditions, i.e. in this embodiment the scattered light detection system is adapted to acquire images of the binding region with different illumination times, which may also be regarded as being shutter times. For instance, images of the binding region can be acquired, wherein the illumination time for the different images may be 0.075 s, 0.15 s, 0.3 s, 0.6 s and 1.2 s. In this embodiment the quantitative number determination device is adapted to provide for the different illumination conditions different first and second reference intensities and to select one of the different illumination conditions based on the detected corresponding whole intensities. In particular, the quantitative number determination device is adapted to select an illumination time, i.e. a shutter time, such that the acquired corresponding image does not contain pixel saturation and also does not contain a too low pixel intensity to avoid the effects of camera noise. For performing this selection procedure intensity thresholding can be applied, wherein the detected whole intensity may be compared with predefined intensities, which may have been determined in a previous calibration step. The quantitative number determination device is further adapted to, in the first mode, determine the concentration based on the detected spots in the binding region, if the detected whole intensity of the scattered light, which has been detected under the selected illumination condition, is smaller than the first reference intensity which has been provided for the selected illumination condition, and to, in the second mode, determine the concentration based on the detected whole intensity of the scattered light, which has been detected under the selected illumination condition, if the detected whole intensity of the scattered light, which has been detected under the selected illumination condition, is larger than the second reference intensity which has been provided for the selected illumination condition. For these determinations of the concentration in the first and second modes corresponding correlations a) between the number of detected spots in the binding region and the concentration and b) between the whole intensity of the scattered light and the concentration may be provided by the quantitative number determination device depending on the selected illumination condition. The quantitative number determination device can therefore comprise several of these correlations for different illumination conditions, wherein the quantitative number determination device uses from these correlations one or several correlations, which correspond to the selected illumination condition.

Thus, since on beforehand it is unknown whether the respective sample contains a low or a high concentration, in an embodiment several images are taken with different illumination conditions, preferably with different shutter times. During the processing of the output signal of the scattered light detection system these images are investigated with respect to their intensity and suitable images are selected. Preferably the selected images do not contain pixel saturation and also do not contain a too low pixel intensity to avoid the effects of camera noise. By using multiple images with different illumination conditions the dynamic range of the intensity measurement may be extended.

Although in above described embodiments the target elements are target molecules, in other embodiments also larger moieties may be detected like cells, viruses, fractions of cells or viruses, tissue extracts, et cetera. The measurement of the scattered light can be performed as an end-point measurement as well as by recording corresponding signals kinetically or intermittently. Moreover, the labels, i.e. the particles, may be detected directly by the system, or the particles may be further processed prior to detection. An example of further processing is that materials are added or that the chemical, in particular, the biochemical, and/or physical properties of the particles are modified to facilitate detection. The system and method can be used with several biochemical assay types like a

binding/unbinding assay, a sandwich assay, a competition assay, a displacement assay, an enzymatic assay, et cetera. The system and method can be suited for sensor multiplexing, i.e. the parallel use of different scattered light detection systems and/or different binding surfaces, label multiplexing, i.e. the parallel use of different types of labels, and/or chamber multiplexing, i.e. the parallel use of different reaction chambers, wherein each reaction chamber comprises a corresponding binding surface. The described systems and methods can be used as rapid, robust and easy to use point-of-care biosensors for small sample volumes. The reaction chamber with the binding surface, in particular, the cartridge, can be a disposable item to be used with a compact reader comprising the scattered light detection system and the quantitative number determination device and preferentially also comprising the magnet assembly. The described systems and methods can be adapted to be used in automated high-throughput testing. Instead of a cartridge comprising the reaction chamber another element comprising the reaction chamber can be used like a well plate, a cuvette, et cetera, which preferentially fit into the compact reader.

The particles are preferentially magnetic nanoparticles having at least one dimension ranging between 3 nm and 5000 nm further preferred ranging between 10 nm and 3000 nm and even further preferred between 50 nm and 1000 nm.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Procedures like the determination of the whole intensity, the determination of the number of spots in the binding region, the determination of the concentration of the target elements in the fluid, the decision which concentration determination procedure should be used, et cetera performed by one or several units or devices can be performed by any other number of units or devices. For example, steps 102 to 104 can be performed by a single unit or by any other number of different units. These procedures and/or the control of the above described systems and/or devices in accordance with the above described respective methods can be implemented as program code means of a computer program and/or as dedicated hardware.

A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A system for determining a quantitative number being indicative of the concentration of target elements in a fluid, wherein the target elements (36) are attachable to particles (35) in the fluid (37) and wherein the particles (35) are bindable in a binding region (32) on a binding surface (31), the system (1) comprising:

a scattered light detection system (49) including a light source (51) for illuminating the binding region (32) and a light detector (40) for detecting light scattered by the particles (35) in the binding region (32) on or close to the binding surface (31), wherein the scattered light detection system (49) is adapted to detect the whole intensity of the light scattered by the particles (35) in the binding region (32) towards the light detector (40) within a field of view of the scattered light detection system (49), wherein the whole intensity corresponds to the integrated intensity of the light scattered by the particles (35) in the binding region (32), and

a quantitative number determination device (60) for determining the quantitative number,

wherein

i) the scattered light detection system (49) is further adapted to detect spots of scattered light in the binding region (32) and the quantitative number determination device (60) is adapted to, in a first mode, determine the quantitative number based on the number of detected spots in the binding region (32), if the detected whole intensity of the scattered light is smaller than a predefined first reference intensity, and

ii) the quantitative number determination device (60) is adapted to, in a second mode, determine the quantitative number based on the detected whole intensity of the scattered light, if the detected whole intensity of the scattered light is larger than a predefined second reference intensity.

2. The system as defined in claim 1, wherein the quantitative number determination device (60) is adapted to, in the first mode, provide a correlation between the number of detected spots in the binding region (32) and the quantitative number and to determine the quantitative number based on the correlation and the detected number of spots in the binding region (32), if the whole intensity of the scattered light is smaller than the first reference intensity.

3. The system as defined in claim 1, wherein the quantitative number determination device (60) is adapted to, in the second mode, provide a correlation between the whole intensity of the scattered light and the quantitative number and to determine the quantitative number based on the correlation and the whole intensity of the scattered light, if the whole intensity of the scattered light is larger than the second reference intensity.

4. The system as defined in claim 1, wherein the second reference intensity is larger than the first reference intensity, wherein the quantitative number determination device (60) is further adapted to, in the first mode, detect the intensity of each spot of scattered light in the binding region (32), to determine the number of particles (35) per respective spot of scattered light from the respective detected spot intensity, in order to determine the total number of particles (35) in the binding region (32), and to determine the quantitative number based on the determined total number of particles (35) in the binding region (32), if the whole intensity of scattered light is larger than the first reference intensity and smaller than the second reference intensity.

5. The system as defined in claim 4, wherein the quantitative number determination device (60) is adapted to provide a correlation between the determined number of particles (35) in the binding region (32) and the quantitative number and to determine the quantitative number based on the correlation and the determined number of particles (35) in the binding region (32), if the whole intensity of scattered light is larger than the first reference intensity and smaller than the second reference intensity.

6. The system as defined in claim 1, wherein the second reference intensity is larger than the first reference intensity, wherein

the scattered light detection system (49) is further adapted to detect spots of scattered light in the binding region (32) and the quantitative number determination device (60) is adapted to determine a first quantitative number being indicative of the concentration of the target elements (36) in the fluid (37) based on the number of detected spots in the binding region (32), to determine a second quantitative number being indicative of the concentration of the target elements (36) in the fluid (37) based on the whole intensity of the scattered light and to weightedly average the first and second quantitative numbers,

if the whole intensity of the scattered light is larger than the first intensity reference and smaller than the second intensity reference.

7. The system as defined in claim 1, wherein the scattered light detection system (49) is adapted to measure a background intensity by measuring the light received from the binding surface (31) outside the binding region, wherein the quantitative number determination device (60) is adapted to subtract the background intensity from the whole intensity of scattered light for providing a background corrected whole intensity of scattered light, to use the background corrected whole intensity for the comparison with the reference intensities and to, in the second mode, determine the quantitative number based on the background corrected whole intensity.

8. The system as defined in claim 1, wherein the scattered light detection system (49) is adapted to detect the spots of scattered light in the binding region (32) by generating a first image being an image of the binding region (32) showing the spots, wherein the first image also shows the binding surface (31) outside the binding region (32), wherein the quantitative number determination device (60) is adapted to determine the quantitative number based on the detected spots in the binding region (32) by:

generating a threshold image by thresholding the first image, determining a background intensity from the part of the first image showing the binding surface (31) outside the binding region (32),

- subtracting the background intensity from the first image for generating a second image,

multiplying the second image with the threshold image for generating a third image,

determining the whole intensity in the binding region (32) in the third image, and

determining the quantitative number based on the determined whole intensity in the binding region (32) in the third image.

9. The system as defined in claim 1, wherein the particles (35) are magnetic particles adapted to be attached to target elements (36), wherein the binding surface (31) is adapted to bind the particles (35) in the binding region (32), if the particles (35) are attached to the target elements (36), wherein the system further comprises a magnet assembly (10, 48) for forcing the particles (35) to the binding surface (31), where the particles (35) with attached target elements (36) are bound in the binding region (32), and for removing unbound particles (35) from the binding surface (31).

10. The system as defined in claim 1, wherein the scattered light detection system (49) is adapted to detect the whole intensity of the scattered light several times under different illumination conditions, wherein the quantitative number determination device (60) is adapted to provide for the different illumination conditions different first and second reference intensities and to select one of the different illumination conditions based on the detected corresponding whole intensities,

wherein

i) the scattered light detection system (49) is further adapted to detect spots of scattered light in the binding region (32) based on the scattered light which has been detected under the selected illumination condition, and the quantitative number determination device (60) is adapted to, in the first mode, determine the quantitative number based on the detected spots in the binding region (32), if the detected whole intensity of the scattered light, which has been detected under the selected illumination condition, is smaller than the first reference intensity which has been provided for the selected illumination condition, and

ii) the quantitative number determination device (60) is adapted to, in the second mode, determine the quantitative number based on the detected whole intensity of the scattered light, which has been detected under the selected illumination condition, if the detected whole intensity of the scattered light, which has been detected under the selected illumination condition, is larger than the second reference intensity which has been provided for the selected illumination condition.

11. A method for determining a quantitative number being indicative of the concentration of target elements in a fluid, wherein the target elements (36) are attached to particles (35) in the fluid (37) and wherein the particles (35) with attached target elements (36) are bound in a binding region (32) on a binding surface (31), the method comprising:

illuminating the binding region (32) by a light source (51) of a scattered light detection system (49) and detecting light scattered by the particles (35) in the binding region (32) on or close to the binding surface (31) by a light detector (40) of the scattered light detection system (49), wherein the whole intensity of the light scattered by the particles (35) in the binding region (32) towards the light detector (40) within a field of view of the scattered light detection system (49) is detected, wherein the whole intensity corresponds to the integrated intensity of the light scattered by the particles (35) in the binding region (32), and

determining the quantitative number by a quantitative number determination device (60),

wherein

i) spots of scattered light in the binding region (32) are detected and, in a first mode, the quantitative number is determined based on the number of detected spots in the binding region (32), if the whole intensity of the scattered light is smaller than a predefined first reference intensity, and

12. A computer program for determining a quantitative number being indicative of the concentration of target elements in a fluid, the computer program

comprising program code means for causing a system (1) as defined in claim 1 to carry out the steps of the method as defined in claim 11 , when the computer program is run on a computer controlling the system (1).