EP0729569A1

EP0729569A1 - Apparatus for determining the erythrocyte sedimentation rate

Info

Publication number: EP0729569A1
Application number: EP95901660A
Authority: EP
Inventors: Bertil Jacobson; Tian-xian Dept. of Med. Physics and Clin. ZHAO
Original assignee: Swelab Instrument AB
Current assignee: Swelab Instrument AB
Priority date: 1993-11-15
Filing date: 1994-11-15
Publication date: 1996-09-04
Also published as: CN1135258A; SE9401646D0; JPH09505146A; WO1995014224A1; AU1080795A; SE9401646L; SE507563C2

Abstract

Apparatus for determining the erythrocyte sedimentation rate, ESR, of a sample of blood comprises a measuring cell (3) receptive of the sample of blood, electrical means (4-6) connected with the measuring cell for determining the capacitance (C) of the sample of blood received in the measuring cell (3), means (10-12; 15) for determining the haematocrit of the sample of blood, and electrical means (15) for determining the ESR using an empirical relationship expressing the ESR as a function of the capacitance (C) and the haematocrit (H) of the sample of blood.

Description

Apparatus for determining the erythrocvte sedimentation rate

This invention concerns determination of the erythrocyte sedimentation rate, ESR, using an empirical relationship bet¬ ween the ESR value and certain electrically measured quanti- ties of the blood sample the ESR value of which is sought. Determination of the ESR value is a common diagnostic step. Usually, the ESR value is determined by first aspirat¬ ing an anticoagulated sample of the patient's blood into a narrow transparent tube so that the tube is filled to a pre- determined height, and then leaving the sample undisturbed for a certain time, typically one hour, whereupon the height of the more or less colourless supernatant column of blood plasma formed as a result of the sedimentation of the red blood cells is measured. The ESR value .^' equal to the height of this column in millimetres if the segmentation time is one hour.

The rather long time required to arrive at the ESR value using this technique is a serious drawback, and the object of this invention is to render possible a rapid determination of the ESR value using only a small volume of blood.

To this end, there is provided in accordance with the invention an apparatus for determining the ESR value of a sample of blood which has the characterising features set forth in the independent claim. The dependent claims recite features of preferred embodiments of the apparatus.

As is previously known, the electrical impedance of blood is primarily determined by plasma resistance, erythro¬ cyte interior fluid resistance and erythrocyte membrane capa¬ citance. We have found that the ESR of blood is related to elec¬ trical impedance parameters of the blood, especially the erythrocyte membrane capacitance, and that an alternative and fast method for determination of the ESR value can be based on impedance measurement. The invention will be described in greater detail here¬ inafter, reference being had to the accompanying diagrammatic drawings, in which: Fig. 1 is a diagrammatic view of a first embodiment which incorporates a particle counter used for determining the haematocrit of the blood;

Fig. 2 is a diagrammatic view of a second embodiment in which the haematocrit is determined from the impedance of the blood;

Fig. 3 is a diagrammatic view showing a modification of the embodiment shown in Fig. 2.

Referring to Fig. 1, the sample of blood the ESR value of which is to be determined and to which an anticoagulant has been added, is received in a suitable vessel, such as a test tube 1. A tube 2 which extends into the sample in the test tube 1 is connected to a measuring cell 3 provided with four electrodes, two outer electrodes 4 and two inner elec- trodes 5 positioned between the outer electrodes. In use of the apparatus, blood is aspirated from the test tube 1 into the measuring cell and a current is passed through the blood in the measuring cell 3 by way of the outer electrodes 4. The voltage then produced between the two inner electrodes 5 is measured, and circuitry generally designated 6 determines the electrical impedance of the blood in accordance with well- known techniques.

For example, the current may be fed to the outer elec¬ trodes 4 at three different frequencies (the highest of which should be many times higher than the lowest) ; this permits simultaneous determination of the capacitance C of the blood and the resistance R of the blood plasma and at the same eli¬ minates errors due to the interior resistance of the erythro- cytes. A further possibility is to use known methods involv- ing only two frequencies and a measurement of phase shift in order to determine C and R.

The blood is transported through the tube 2 and the mea¬ suring cell 3 by means of a pump 7. Another pump 8 feeds a liquid diluent from a reservoir 9 into the tube downstream of the measuring cell 3 to dilute the blood before the blood is passed through a very narrow and short passage 10 which forms part of an electronic blood cell counter of a well-known type. This blood cell counter comprises a pair of electrodes 11 which are positioned on opposite sides of the passage 10 and connected with circuitry 12 for counting the erythrocytes and determining the haematocrit of the blood.

A temperature sensing member 13, such as a thermistor, is associated with the measuring cell 3 and connected to cir¬ cuitry 14 for determining th-*- temperature of the blood in the measuring cell.

Determination of the ESR value of the blood is effected by means of circuitry 15 using an empirical relationship of the kind described below. A display 16 indicates the deter¬ mined ESR value. The circuitry 16 processes the inputs recei¬ ved from the impedance determination circuitry 6, the haema¬ tocrit determination circuitry 12 and the temperature deter¬ mination circuitry 14. Before the blood sample is aspirated into the measuring cell 3 it should be slightly diluted by the addition of a predetermined, relatively small quantity of a liquid diluent. The quantity is not very critical but the volume of added diluent should not exceed the volume of the undiluted blood sample. A preferred ratio of the volume of diluent to the volume of undiluted blood is 1/4 to 1/2 (the volume of anti¬ coagulant is included in the volume of diluent) .

The addition of the diluent to the blood may be effected from the diluent reservoir 9 by means of the pump 8. Alterna- tively, a separate diluent pump may be provided as described below.

The addition of a small quantity of diluent is advanta¬ geous in that it inhibits certain saturation effects which have been found to occur with undiluted blood and affect the precision of the determination for high ESR values. It is important that only a small amount of diluent is added, be¬ cause the addition of a diluent also has been found to impose requirements for an increased accuracy of the ESR determina¬ tion circuitry 15. The dilution can be dispensed with if ESR values in the high range need not be determined very accura¬ tely.

Preferably, a diluent of low electrical conductivity is used, such as an aqueous solution of glycine (monoaminoacetic acid) . The diluent should be isotonic so that any change of volume of the erythrocytes is avoided, or the determination of the haematocrit should be carried out on undiluted blood. Naturally, the portion of the blood sample used for de- termining the haematocrit need not pass through the measuring cell 3; this portion of the blood may be fed to the cell counter through a separate line.

In the embodiment shown in Fig. 2 a separate double pump 23 is provided to draw a precisely measured volume of blood from the test tube 1 through a narrow tube 22 and a precisely measured volume of diluent from a diluent reservoir 24 and to feed the blood and the diluent to a vessel 25 in which a thorough mixing is effected by means a suitable mixing device, not shown. As in the previous embodiment, pump 7 draws the diluted blood through tube 2 into the measuring cell 3 which is provided with a pair of outer electrodes 4 and a pair of inner electrodes 5. These electrodes are con¬ nected to impedance determination circuitry 6 which feeds its output to ESR determination circuitry 15. The latter also re- ceives the output of temperature measuring circuitry 14 and drives display 16.

Fig. 3 illustrates a modification of the means for pro¬ viding a diluted sample to be drawn into measuring cell 3. Apart from this modification, the embodiment of Fig. 3 is identical with that shown in Fig. 2.

In this case, the blood sample is taken from the patient into a special cylindrical test tube 36 having a closure 37 through which a hollow needle may be passed. Before the blood sample is taken, the test tube 36 is filled with a predeter- mined volume of diluent and an anticoagulant. After the blood sample has been taken, the test tube 36 is positioned in a holder 38 associated with a level indicator comprising a light source 39 and an array of photosensitive elements 40 on a scale 41. A computing circuit 42 connected to the level indicator 39-41 determines the total volume of the diluted sample in the test tube 36 and the degree of dilution. The output of circuit 42 is fed to ESR determination circuitry 15 (not shown in Fig. 3, see instead Fig. 2). It ,.s also possible to obtain the desired dilution by using a sealed test tube w ich is pre-filled with a predeter¬ mined volume of diluent (including an anticoagulant) and the interior of which is under a partial vacuum. When the blood sample is taken, blood is allowed to flow into the test tube until pressure equilibrium has been reached, i.e. until a predetermined volume of blood has been introduced.

The diluted blood in the test tube 36 is fed to the mea¬ suring cell through a hollow needle 43 and tube 2. In the embodiment shown in Fig. 2 and Fig. 3, the haematocrit of the blood is determined from impedance measu¬ rements by means of the electrodes 4 and 5, and as in the embodiment of Fig. 1 the ESR value is determined using an empirical relationship. A number of examples of relatio "hips for use in determining the ESR value will be given herein¬ after.

As will become apparent from the examples, most of which are based on determination of the electrical impedance of the blood at three different frequencies, different degrees of complexity in the determination can be required, depending on the required accuracy. The determination as described herein¬ after comprises three steps: (1) correction for varying sample temperature, (2) determination of the haematocrit, and (3) determination of the ESR value. Temperature correction of the measured impedances can be accomplished using the empirical formula

Z₀ = ZJ1 ₊ c(t-g]

in which Z₀ is the normalised value of a certain impedance value Z_t measured at temperature t in relation to a certain reference temperature t₀ for which the following empirical formulas have been established. Constant c is a constant for the temperature dependency of the impedances; this dependency varies slightly with the frequency of the current.

The haematocrit H can be measured by means of a cell counter as in the embodiment of Fig. 1. In the embodiment of Fig. 2 the haematocrit H, expressed as a volume percentage, is calculated using an empirical relationship which includes resistances measured at different frequencies and ratios of these resistances, such as the following relationship which includes the ratios rj and r₂:

H = c₁R₁ + c₂R₃ + 03^ + c₄r₂ + c₅

in which, for a temperature of 26°C, constant c^-0.0223, constant c₂=0.200, constant c₃=122,8, constant c₄=-172.2 and constant c₅=51.3, and R_lf R₂ and R₃ are the resistances of blood anticoagulated with EDTA, as determined at frequencies of 100 kHz, 800 kHz and 1200 kHz.

A rough ESR value can be calculated from the following empirical relationship

ln(ESR) = _?- + c, H⁰⁷

in which C is the capacitance of the diluted blood, H₂ is the haematocrit, expressed as a percentage by volume, and con¬ stant c₆=0.98, constant c₇=l and constant c₈=-7.14 at 37°C when citrate has been added to the blood to prevent coagulation. A more accurate value is obtained if the resistance R of plasma is taken into consideration using the following empir¬ ical relationship

ln(ESR) = -^55 + _Cl1

|_| Cl0

in which constant c₉=0,207, constant c₁₀=2 and constant c_n=-4,62. A further improvement of the accuracy is achieved if the difference, ΔC, between the capacitance values measured at two different points in time after the diluted blood has been aspirated into measuring cell 3 and become stationary, and if corrections for the haematocrit are also introduced. Then, the following empirical relationship can be used: iInn_/(PfcS_QmR) = ^C 2°^{2 +}— ^C 3^{ΔC 2} ₊+ c₁₄R ÷ c₁₅C ⁺ c₁₆ΔC + c₁₇H + c₁₈H + c₁₉

in which C is the capacitance as measured at the second point in time, constant Cι₂=-0.0646, constant Cι₃=0.0166, constant c₁₄=0.414, constant Cι₅=1.836, constant c₁₆=0,283, constant c₁₇=- 0.0066, constant c_lg=0.397 and constant c₁₉=-18.27 when the points in time for the determination of ΔC are 2 and 90 seconds after the blood has been aspirated, the blood has been anticoagulated with EDTA and diluted with one part of a 0,1 mol/1 saline per three parts of blood, and the measure¬ ments have been carried out at 26°C. If the apparatus of Fig. 2 includes or is associated with a cell counter supplying the number of at least one type of cells in the blood, such as the number of white cells and/or the number of erythrocytes, the ESR value can be calculated using a relationship of the following type

■nCESB) ■ *-°* ^ » -^■» ^{» ■} - * *+⁰ r cB* . o^B - _-

in which B is the number of erythrocytes per 10^"12 litre of blood, constant c₂₀=-0.078, constant c₂₁=0.084, constant c₂₂=-0.463, constant c₂₃=l.973, constant c₂₄=-0.0108, constant c₂₅=-o.267, constant c₂₆=1.974 and constant c₂₇=-11.03 when the blood has been anticoagulated with EDTA and diluted with one part of a 0,1 mol/1 saline per three parts of blood, and the measurements have been carried out at 26°C.

The influence of the resistance of the interior fluid of the erythrocytes has been found not to have a significant in¬ fluence on the ESR value. It therefore need not be taken into account.

The various constants of the empirical relationships can readily be determined from a number of blood samples of known ESR values.

Determination of the ESR value using the method and the apparatus according to the invention requires only a relati¬ vely small volume of the sample of blood, 1 ml or less.

Claims

1. Apparatus for determining the erythrocyte sedimen¬ tation rate, ESR, of a sample of blood, characterised by a measuring cell (3) having a compartment receptive of the sample of blood, electrical means (4-6) connected with the measuring cell for determining the capacitance C of the sample of blood received in the measuring cell (3) , means (10-12; 15) for determining the haematocrit of the sample of blood, and electrical means (15) for determining the ESR using an empirical relationship expressing the ESR as a func¬ tion of the capacitance C and the haematocrit H of the sample of blood.

2. Apparatus according to claim 1, characterised in that said relationship is of the following type: c„C ln(ESR) = -i- - c₈

in which In(ESR) is the natural logarithm of the ESR value ex¬ pressed in mm/h, C is the capacitance of the blood in the measuring cell, H is the haematocrit value of the blood, ex¬ pressed as a percentage by volume, and c₆, c₇ and c₈ are con¬ stants.

3. Apparatus according to claim 1 or 2, characterised in that said electrical means (4-6) for determining the capa¬ citance C includes means for determining the electrical resis¬ tance R of the plasma of the sample of blood and said rela¬ tionship is of the following type:

ln(ESR) = -^5 + c_n _H ^Cιo

in which c₉, c₁₀ and c_n are constants.

4. Apparatus according to any one of claims 1 to 3, characterised in that said electrical means (15) for deter¬ mining the ESR is associated with means for determining the temperature t of the blood received in the measuring cell (3) and in that the said relationship includes R as a factor for correction for the deviation of t from the temperature t₀ for which the constants have been determined.

5. Apparatus according to any one of claims 1-4, characterised in that said means for determining the haemato¬ crit H comprises an electronic cell counter (10-12) connected with said ele .rical means (15) for determining the ESR.

6. Apparatus according to any one of claims 1-4, characterised in that said electrical means (4-6) connected with the measuring cell (3) for determining the capacitance C of the sample of blood received in the measuring cell inclu¬ des means for determining for three frequencies of an elec- trie current passed through the sample of blood received in the measuring cell both the capacitance C and the resistance R of the plasma of the sample of blood, and in that said means for determining the haematocrit H of the sample of blood comprises electrical means for determining H from the measured values of R.

7. Apparatus according to any one of claims 1-6, characterised in that said relationship includes the differ¬ ence ΔC in the capacitances of the blood measured at two dif¬ ferent times after the sample of blood has been received in the measuring cell (3) .

8. Apparatus according to claim 7, characterised in that said relationship is of the following type:

_ln(ESR) . ^C"^{C 2 ÷ C}"^{ΔC 2} _r °»* ^» lfi * CAC _f ^ ₊ ^ ^ _0]9 H rl

in which C is the capacitance as measured at the second point in time, constant Cj₂, c₁₃, c₁₄, c₁₅, c₁₆, c₁₇, c₁₈ and c₁₉ are constants.

9. Apparatus according to any one of claims 1-8, characterised in that a blood cell counter is associated with sa electrical means (15) for determining the ESR and in that said relationship includes the number of one or more types of cells, such as the number of white cells and/or the number of erythrocytes, per unit volume of the sample of blood.

10. Apparatus according to any one of claims 1-9, characterised by a device (23,24; 38-42) for diluting the sample of blood with a predetermined volume of diluent before the sample of blood is received in the measuring cell (3) , said predetermined volume preferably not exceeding the volume of the sample of blood before it is diluted.

11. A method for determining the erythrocyte sedi en- tation rate, ESR, of a sample of blood, comprising the steps of introducing the sample of blood in a measuring cell (3) , determining by electrical means the capacitance C of the sample of blood in the measuring cell, determining the haema¬ tocrit of the sample of blood, and determining by electrical means the ESR using an empirical relationship expressing the ESR as a function of the capacitance C and the haematocrit H of the sample of blood.

12. A method according to claim 11, characterised in that the difference ΔC in the capacitances of the blood is measured at two different times after the sample of blood has been received in the measuring cell (3) and that the said relationship includes said difference.

13. A method according to claim 11 or 12, characterised in that the sample of- blood is diluted with a predetermined volume of diluent before it is introduced in the measuring cell, said volume preferably not exceeding the volume of the sample of blood before it is diluted.