GB1590899A

GB1590899A - Blood particle counter having threeelectrode counting head

Info

Publication number: GB1590899A
Application number: GB42602/77A
Authority: GB
Original assignee: Becton Dickinson and Co
Current assignee: Becton Dickinson and Co
Priority date: 1977-09-15
Filing date: 1977-10-13
Publication date: 1981-06-10
Also published as: FR2403604A1; DE2817719B2; FR2403604B1; DE2817719A1

Description

(54) BLOOD PARTICLE COUNTER HAVING THREE-ELECTRODE COUNTING HEAD (71) We, BECTON, DICKINSON AND COMPANY, a Corporation organised and existing under the laws of the State of New Jersey, United States of America located at Stanley and Cornelia Streets, East Rutherford, New Jersey, 07073, United States of America, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention concerns a DC transducer for use in the counting of blood particles and particularly, but not exclusively, blood cells suspended within a liquid medium which minimizes the generation of bubbles and associated noise.

Blood cell counting transducers which have previously been utilized typically employ a pair of electrodes, each of which is positioned on the opposite side of an aperture through which a diluted blood sample flows. An electronic signal is connected across the electrodes above and below it. As a cell, suspended in diluent, is drawn through the aperture, it alters the current flow between the electrodes to produce a pulse. This change in current is sensed by a detecting circuit connected across the electrodes.

Examples of cell counting devices having the above capabilities are disclosed in U.S.

Patent Nos. 3,783,376; 3,812,425; 3,861,800 and 3,973,194.

Blood cell counters having more than two electrodes have been used to accomplish functions such as compensating for changes in electrolyte conductivity and aperture diameter. U.S. Patent Nos. 3,944,917 and 3,982,182 disclose systems utilizing more than two electrodes in a counting device to remedy these problems.

The use of a DC transducer within a counting apparatus is desirable in a number of situations, particularly where the counting of small cells is to be accomplished.

Because of its greater sensitivity to small cells, such as red blood cells and platelets, it has an advantage over the use of an AC transducer. However, there is a bubble generation problem which must be overcome before the DC transducer can be most successfully employed.

Bubble generation is caused by electrolysis at either the detecting or biasing electrodes within the system. If the bubbles are small, they could be erroneously counted by the system along with the blood cells or platelets. If large they tend to clog the aperture.

Another problem which may affect the detection system is changes in the resistance of the diluent. The detector is unable to tell if the change in current flow is due to a blood cell in the aperture or due to changes in the diluent, and errors may occur in this manner.

It is accordingly an object of the present invention to provide a new or improved DC transducer assembly for counting blood particles suspended within a liquid medium.

According to the present invention, there is provided a DC transducer assembly for counting blood particles suspended within a liquid medium, comprising; a conduit for transporting suspended blood particles in a downstream direction, said conduit having a small diameter; a supporting structure in fluid communication with said conduit having an aperture therein through which, in use, suspended blood particles from said conduit may flow; first and second electrodes in fluid communication with and located on downstream and upstream sides of said apertures respectively;; a DC bias source connected across said first and second electrodes, said source providing an electronic signal across said first and second electrodes and there being a high resistance through said conduit between said second electrode and said aperture when a suspension of blood particles flow therethrough due to the small diameter of said conduit; a third electrode located within said conduit in fluid communication with and positioned between said second electrode and said aperture, said third electrode being located in proximity with respect to said aperture so as to eliminate any effects of the resistance path within the conduit between the second and third electrodes;; a detection circuit connected between said first and third electrodes, said detection circuit being capable, in use, of sensing current changes corresponding to the passage of blood particles through said aperture while operating substantially independently of any changes in resistivity of the suspension of blood particles within a portion of the conduit between said second and third electrodes, the electrodes and said aperture being arranged such that a suspension of blood particles may pass in a downstream direction by and in fluid communication with the second electrode, said third electrode, said aperture, and said first electrode, respectively; and wherein the magnitude of the bias current, the length of the third electrode, and the diameter of the conduit are arranged such that the voltage drop across the length of the third electrode is less than about 1.2 volts when a suspension of blood particles passes through said conduit.

The present invention will now be described in more detail with reference to the accompanying drawings in which: Figure 1 is a sectional view of a counting head and transducer assembly of an embodiment of the present invention; Figure 2 is a top view of the counting head and transducer assembly of Figure 1; Figure 3 is a sectional view of a jeweled aperture slide which is inserted into the counting head; Figure 4 is a view of the counting head and transducer assembly of Figure 1 incorporated within a blood cell counting instrument; and Figure 5 is a simplified view of a counting head having three electrodes.

Referring now to Figure 1 there is shown a counter head and transducer assembly in detail with the exception of an aperture slide, which is inserted when the counter head and transducer assembly is in use, and a reservoir containing a sample.

The counter head and transducer assembly comprises a non-metallic conduit 10 through which the diluted blood sample will flow. Three electrodes 12, 14 and 16 are located along this conduit. The lower electrode 14 is positioned outside conduit-l0. A filter 18 is included at the lower end of the conduit.

An aperture slide 74 including a.jeweled aperture 80 as shown in Figure 3 is designed to fit between an upper housing 20,of the transducer, and a housing 22 which includes an assembly for purging the system of debris which may clog the aperture. "0" rings 24 and 26 insure the leakproof insertion of the slide, and provison is made so that the jeweled aperture 80 will be positioned within the conduit 10. The aperture is described in more detail in U.S. Patent No. 3,783,376.

Upper housing 20 and upper electrode 12 are interfaced by an O-ring 28 at the point where a plate 30 is secured to this electrode.

A wire 32 leads from plate 30, and is connected to circuitry which will later be described.

The purge assembly, which is more clearly shown in Figure 4, includes housing 22, a conduit 34, an annulus 33, and a solenoid valve 36. An "0" ring 38 is positioned between the purging housing 22 and a housing 40. Electrodes 14 and 16 are within housing 40, which is separated from a base 42 by a gasket 44. The base 42 contains a spring 46 which is held by a retainer 48 against a protrusion 50. A latch 52 is also secured to the base by bolts 54. The latch 52 has a protrusion 56 upon which the base of a sample reservoir may rest.

A number of bolts, as shown at 58j arid screw 60, are used to maintain the structure of the transducer assembly, which is comprised of the several housing and other components described above.

A reservoir position detecting switch 62 is also secured to the transducer assembly. An insulator 64 separates the, switch from a mounting bracket 66. The switch 62 includes the mounting bracket 66, an actuating lever 68, and a button 69.

Referring to Figure 2, a top view of the transducer is shown. Terminals 70 and 72 connect electrodes 16 and 14 to the apprbp- riate circuitry, and a fitting 75 is shown for attachment of the purge line 34. Annulus 33 is in fluid communication with both the sample conduit 10 and the purge line 34.

The aperture slide 74, as shown in Figure 3, is inserted between housing assembly 22 and upper housing 20 before the counting procedure can begin. O-rings 24 and 26 provide a leak-proof seal when the slide is in position. The slide is formed of a polyester film such as Mylar (Registered Trade Mark), having a thickness of approximately 0.015 inch and a length of approximately 2 inches. However, any suitable material may be used. A hole 76 is formed near one end of the aperture slide and positioned to be aligned with conduit 10. The forward edge is beveled to facilitate its insertion. A jewel 78, preferably a ruby, is press fit into hole 76. The jewel has a thickness of about 0.25 mm and an outside diameter of approximately 1.20 mm. The aperture 80 is formed in the jewel, and in this embodiment has a diameter of 70 microns.

Figure 4 illustrates the basic invention as applied to a simplified counting system. The lower end of the conduit 10 is immersed in a diluted blood sample contained within a reservoir 82. A sump 84 is in fluid communication with the conduit, and a pump 86 controls the vacuum which is exerted on the system through tube 88 and sump 84. As can most clearly be seen in this figure, the counting circuitry 90 is connected between electrodes 12 and 16, and the DC bias source 92 between electrodes 12 and 14. The counting circuitry is connected to a microcomputer 94 which performs calculations using the data received to generate all parameters at the end of the test. Test results are displayed by either cathode ray tube display 96 or printer 98, although any conventional device such as a meter can also be employed.

Figure 5 is included for purposes of illustration, and is simplified to provide a clearer understanding of the structure and dimensions of the transducer. This embodiment comprises generally an upper portion 100 which is secured to the upper housing 102 by means of bolt 104. An "0" ring 106 is also provided between these members.

Provision is made at 103, between the upper housing 102 and middle and lower housings 108, 110, for insertion of the aperture slide.

"0" ring 112 is provided between housings 108 and 110. A lower portion 114 is secured to portions 102, 108, and 110 by means of a screw 116.

The electrodes and their connections are shown in detail. The lower electrode 14 is located outside the sample conduit 10 in such a manner that bubbles generated by electrolysis are shown approaching the surface of the diluted sample within the reservoir 82. These bubbled-will not enter the sample conduit where they could then flow through the aperture 80 and impair the system.

The DC bias source 92 is connected to electrodes 14 and 12. Bubbles formed at electrode 12 will rise harmlessly above the sensing zone between electrodes 16 and 12.

Electrodes 16 and 12 are connected to the detecting/counting circuits 90, and aperture 80 is located between these electrodes. It is therefore of utmost importance that a minimum of bubbles is generated at electrode 16, as these bubbles would flow directly into aperture 80 thereby clogging it or causing errors within the detection system. It is also important that electrodes 16 and 12 are located near the aperture to minimize the effects of any variance in the resistance caused by the presence of diluent in the conduit. Because the detector is not always able to distinguish pulses caused by varying resistance of the diluent from those caused by the passage of a pulse through the aperture, keeping the distance between the detection electrodes small is necessary.

The sensitivity of the device is also improved by keeping the distance between the sensing electrodes small. The resistance of the liquid in the sample conduit is typically about 25 Kohms in this embodiment, whereas the resistance of the 70 11 aperture is about 20 Kohms. By placing electrodes 12 and 16 in close proximity to the aperture, the effects of the 25 Kohm segment ' is eliminated. Instead of detecting a resistance change caused by a blood cell with respect to a total resistance of 45 Kohms in the system, this change is measured with respect to only 20 Kohms. Although the resistance within the conduit could be greatly diminished by use of a larger diameter sample conduit the small tube is desirable to reduce carryover. In a successful application of this invention, conduit 10 has a diameter of approximately 0.07".

Bubble generation caused by electrolysis at electrode 16 is minimized or eliminated by two features. First, the detection circuitry is connected such that significant current is not drawn from the electrode. A high impedance return path to the circuitry is therefore provided. Secondly, the length of the electrode 16 in the diluent path and the magnitude of the DC bias current are arranged to keep the voltage drop across the electrode lengthlto less.than about 1.2 volts.

The resistance R of a uniform conductor is defined by the expression R = t/A, where f in its length, A is its cross-sectional area, and p is its resistivity. In this preferred embodiment, the length of the electrode 16 is 0.095 inches, the diameter of the sample conduit is 0.07 inches, and the resistivity of the diluent is about 20 ohm-inches. Applying the above formula, the resistance is found to be 494 ohms. The voltage drop V is defined by the formula V = I R,.and for a 1 ma DC current, the voltage drop is 0.494 volts. It should be understood that other conduit diameters, electrode lengths, and biasing currents may be utilized in providing a voltage drop of less than about 1.2 volts.

To begin the testing procedure, reservoir 82 is positioned against retainer 48 so as to compress spring 46. The spring is compress ed until the reservoir bottom rests against the protrusion 56 provided on latch 52.

Switch 62 provides information to the computer as to whether the reservoir is inserted.

Actuating lever 68 presses button 69 to actuate the switch once the reservoir is in place.

The reservoir 82 is positioned such that the sample may be aspirated through filter 18, conduit 10, aperture 80, conduit 10, and into sump 84. The pump operates to provide the negative pressure necessary for the aspiration.

As the sample is aspirated through the conduits and electrodes, the blood cells are counted in a conventional manner based upon the change in current between the detection electrodes each time a blood cell enters the aperture. Pulses are produced and counting circuitry 90 counts the number of blood cells. This information is relayed to computer 94 which causes printer 98 to record the results and/or cathode ray tube display 96 to visually display the information. The counting process is described in greater detail in U.S. Patent 3,812,425 and 3,973,194, as are the basic circuit components.

It should be understood that although the invention has been described as a blood cell counter, the same device can be used for counting platelets and other particles in the blood and also to produce a hematocrit determination, this latter process being described in detail in U.S. Patent No. 3812425.

WHAT WE CLAIM IS: 1. A DC transducer assembly for counting blood particles suspended within a liquid medium, comprising: - a conduit for transporting suspended blood particles in a downstream direction, said conduit having a small diameter; a supporting structure in fluid communication with said conduit having an aperture therein through which, in use, suspended blood particles from said conduit may flow; first and second electrodes in fluid communication with and located on downstream and upstream sides of said aperture, respectively;; a DC bias source connected across said first and second electrodes, said source providing an electronic signal across said first and second electrodes and there being a high resistance through said conduit between said second electrode and said aperture when a suspension of blood particles flow therethrough due to the small diameter of said conduit; a third electrode located within said conduit in fluid communication with and positioned between said second electrode and said aperture, said third electrode being located in proximity with respect to said aperture so as to eliminate any effects of the resistance path within the conduit between the second and third electrodes;; a detection circuit connected between said first and third electrodes, said detection circuit being capable, in use, of sensing current changes corresponding to the passage of blood particles through said aperture while operating substantially independently of any changes in resistivity of the suspension of blood particles within a portion of the conduit between said second and third electrodes, the electrodes and said aperture being arranged such that a suspension of blood particles may pass in a downstream direction by and in fluid communication with said second electrode, said third electrode, said aperture, and said first electrode, respectively; and wherein the magnitude of the bias current, the length of the third electrode, and the diameter of the conduit are arranged such that the voltage drop across the length of the third electrode is less than about 1.2 volts when a suspension of blood particles passes through said conduit.

2. An assembly according to Claim 1 wherein a high impedance path is provided between the third electrode and the detection electrode to minimise bubble generation at the third electrode.

3. An assembly according to Claim 1 or Claim 2 wherein the conduit is constructed so that its upstream end with respect to the aperture is capable of extending into a reservoir containing the blood cells suspended in the liquid medium.

4. An assembly according to Claim 3 wherein the second electrode is positioned on the exterior of the upstream end of the conduit so as to be immersed within the blood particle suspension when the reservoir is appropriately positioned, this location of the second electrode ensuring that bubbles generated at said second electrode will rise within the reservoir rather than through the conduit.

5. An assembly according to any one of the preceding claims wherein between the second electrode and the aperture the conduit is of uniform diameter.

6. An assembly according to Claim 5 wherein between the second electrode and the aperture the conduit diameter is less than 0.25".

7. An assembly according to Claim 6 wherein between the second electrode and the aperture the conduit diameter is 0.07".

8. A DC transducer assembly for counting blood particles suspended within a liquid medium substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The reservoir 82 is positioned such that the sample may be aspirated through filter 18, conduit 10, aperture 80, conduit 10, and into sump 84. The pump operates to provide the negative pressure necessary for the aspiration. As the sample is aspirated through the conduits and electrodes, the blood cells are counted in a conventional manner based upon the change in current between the detection electrodes each time a blood cell enters the aperture. Pulses are produced and counting circuitry 90 counts the number of blood cells. This information is relayed to computer 94 which causes printer 98 to record the results and/or cathode ray tube display 96 to visually display the information. The counting process is described in greater detail in U.S. Patent 3,812,425 and 3,973,194, as are the basic circuit components. It should be understood that although the invention has been described as a blood cell counter, the same device can be used for counting platelets and other particles in the blood and also to produce a hematocrit determination, this latter process being described in detail in U.S. Patent No. 3812425. WHAT WE CLAIM IS:

1. A DC transducer assembly for counting blood particles suspended within a liquid medium, comprising: - a conduit for transporting suspended blood particles in a downstream direction, said conduit having a small diameter; a supporting structure in fluid communication with said conduit having an aperture therein through which, in use, suspended blood particles from said conduit may flow; first and second electrodes in fluid communication with and located on downstream and upstream sides of said aperture, respectively;; a DC bias source connected across said first and second electrodes, said source providing an electronic signal across said first and second electrodes and there being a high resistance through said conduit between said second electrode and said aperture when a suspension of blood particles flow therethrough due to the small diameter of said conduit; a third electrode located within said conduit in fluid communication with and positioned between said second electrode and said aperture, said third electrode being located in proximity with respect to said aperture so as to eliminate any effects of the resistance path within the conduit between the second and third electrodes;; a detection circuit connected between said first and third electrodes, said detection circuit being capable, in use, of sensing current changes corresponding to the passage of blood particles through said aperture while operating substantially independently of any changes in resistivity of the suspension of blood particles within a portion of the conduit between said second and third electrodes, the electrodes and said aperture being arranged such that a suspension of blood particles may pass in a downstream direction by and in fluid communication with said second electrode, said third electrode, said aperture, and said first electrode, respectively; and wherein the magnitude of the bias current, the length of the third electrode, and the diameter of the conduit are arranged such that the voltage drop across the length of the third electrode is less than about 1.2 volts when a suspension of blood particles passes through said conduit.