WO1999049318A1

WO1999049318A1 - Composition and method for hemoglobin and cell analysis

Info

Publication number: WO1999049318A1
Application number: PCT/US1999/005755
Authority: WO
Inventors: Yi Li; Carole Young
Original assignee: Coulter International Corp.
Priority date: 1998-03-24
Filing date: 1999-03-16
Publication date: 1999-09-30
Also published as: EP1066529A1; CN1294682A; EP1066529A4; CN1153972C; JP2002507749A; US5882934A

Abstract

A cyanide-free lytic reagent composition and method for measuring the total hemoglobin concentration in a blood sample, for counting the number of leukocytes and for deferential counting of leukocyte subpopulations are described. The cyanide-free lytic reagent composition comprises a quaternary ammonium salt or a pyridinium salt to lyse erythrocytes and release hemoglobin, and an organic ligand selected from the group consisting of triazole and its derivatives, tetrazole and its derivatives, alkaline metal salts of oxonic acid, melamine, aniline-2-sulfonic acid, quinaldic acid, 2-amino-1,3,4-thiadiazole, triazine and its derivatives, urazole, DL-pipecolinic acid, isonicotinamide, anthranilonitrile, 6-aza-2-thiothymine, 3-(2-thienyl)acrylic acid, benzoic acid and alkali metal and ammonium salts of benzoic acid, and pyrazine and its derivatives to form a stable chromogen with hemoglobin, and a salt to adjust conductivity of the reagent for impedance measurement. The reagent composition is mixed with a blood sample without pre-dilution and the UV absorption of the sample mixture is measured at the predetermined absorption wavelength. Counting the number of leukocytes and differential counting of leukocyte subpopulations are accomplished simultaneously on an automated cell counter utilizing DC impedance measurement.

Description

COMPOSITION AND METHOD FOR HEMOGLOBIN AND CELL ANALYSIS

CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of copending application

U.S. Serial Number 08/786,505 filed January 21 , 1997.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to lytic reagent compositions, diluents and methods for use in measuring total hemoglobin concentration in a blood sample, either manually or automatically, and for use in combination with a simultaneous leukocyte counting or differential counting of leukocyte subpopulations.

DISCUSSION OF THE PRIOR ART

The determination of total hemoglobin is indicative of the oxygen- carrying capacity of whole blood. An ability to measure hemoglobin (Hgb) in blood samples is an essential part of diagnostic analysis and is also important for monitoring responsiveness to therapies directed towards diseases which affect hemoglobin and to therapies which are directed towards other diseases but which may have adverse side effects on the hemoglobin level.

Leukocytes in the peripheral blood of normal subjects consist of five types, i.e., lymphocytes, monocytes, neutrophils, eosinophils and basophils. The latter three types of leukocytes are collectively referred to as granulocytes. Different types of leukocytes have different biological functionalities. Counting and differentiating different types of leukocytes in a blood sample provides valuable information for clinical diagnosis. The classification and counting of leukocytes has most commonly been conducted by the differential counting method which is also referred to the manual method. Automatic blood analyzers are also commonly used for counting leukocytes, which employs a hemolytic reagent to lyse erythrocytes and prepares a sample only containing leukocytes. The sample mixture then is analyzed by impedance method. A more sophisticated apparatus has been developed that counts different types of leukocytes (differential counting) including lymphoid (lymphocyte) and myeloid (monocyte and granulocyte) populations (U.S. Patent No. 4,286,963 to Ledis et al.). Leukocytes have also been further differentiated into three subpopulations, i.e., monocytes, lymphocytes and granulocytes (U.S. Patent No. 4,485,185 to Ledis et al.). Ideally, one would like to be able to accomplish multiple diagnostic analyses such as hemoglobin measurement and counting the number of leukocytes or differential counting of leukocyte subpopulations in a single automated step.

Among the many well-known methods for hemoglobin determination, the cyanide hemoglobin method has been recommended as a standard by the International Committee for Standardization in Hematology. Modification of this method by Matsubara and Okuzono has led to its wide usage in clinical laboratories. In this method, the iron ion of heme group in all forms of hemoglobin of the red cells are oxidized to methemoglobin by potassium ferricyanide. The methemoglobin is then complexed with cyanide anion, which has a very high affinity to iron ion of the heme group, to form a cyanmethemoglobin chromogen. This extremely stable chromogen has a maximum absorption at 540 nm, which is measured manually by UV spectrometry.

Despite of the stable chromogens formed by the standard cyanmethemoglobin method and its modified automatic methods, however, because of the potassium cyanide used, the reagent waste has 3 caused enormous environmental concern. In last ten years, a tremendous effort has been given to develop automated hemoglobin analysis methods without utilizing cyanide.

Oshiro et al, Clin. Biochem. 1583 (1982), teach the use of a reagent for hemoglobin analysis which comprises sodium laurylsulfate (SLS) and Triton X-100 (a nonionic surfactant) in a neutral pH (7.2). The SLS is used to lyse erythrocytes and is believed to further produce a SLS-hemoglobin complex which has a maximum absorption at 539 nm and a shoulder at 572 nm. The reaction completes within 5-10 minutes and the total hemoglobin measurement is quantitative. However, as later explained in U.S. Patent No. 5,242,832 (to Sakata), it is not possible with Oshiro's method to analyze leukocytes simultaneously with hemoglobin measurement.

U.S. Patent No. 5,242,832 (to Sakata) discloses a cyanide-free lysing reagent for counting leukocytes and measuring the hemoglobin concentration in blood samples. The lysing reagent comprises at least one first surfactant which is a quaternary ammonium salt, at least one second surfactant which includes cationic and amphoteric surfactants, and at least one hemoglobin stabilizer selected from the group including Tiron, 8-hydroxyquinoline, bipyridine, 1-10-phenanthroline, phenolic compounds, bisphenol, pyrazole and derivatives, second phenyl 5- pyrazolone and derivatives, phenyl 3-pyrazolone, and imidazole and its derivatives. Sakata teaches that fractionation of the leukocytes into two or three groups including an aggregate of lymphocytes, an aggregate of monocytes, eosinophils and basophils, and an aggregate of neutrophils can only be accomplished by using at least two suitable surfactants and by rigorously controlling the surfactant concentration. Sakata also teaches that the preferred pH range of the lysing reagent is from 5.0 to 8.0. If the pH value is 3.0 or less, damage to the leukocytes increases 4 thus rendering measurement of leukocytes difficult, and if the pH is 9.0 or more, the stability of hemoglobin deteriorates over time.

U.S. Patent No. 5,612,223 (to Kim) discloses a cyanide-free method and reagent for determining hemoglobin in a whole blood sample. The reagent compπses a ligand selected from the group consisting of imidazole and derivatives, N-hydroxyacetamide, H-hydroxylamine, pyridine, oxazole, thiazole, pyrazole, pyrimidine, purine, quinoline and isoquinoline, and a surfactant with a strong erythrolytic capability selected from the group consisting of lauryl dimethylamine oxide and octylphenoxy polyethoxyethanol. The analysis method is fast, less than

10 seconds. However, the reagent only performs under an extreme alkaline condition, pH from 11 to 14. In addition, no capability of counting leukocytes or differentiating leukocyte subpopulations is taught by Kim.

Stephanos et al., "Iron ligand recognition by monomeric hemoglobins", Biochimica et Biophysica A a, 1295 (1996) pages 209-

221 , disclose a list of organic ligands that bind to hemoglobin monomers.

U.S. Patent No. 4,853,338 (to Benezra et al.) teaches a method of determining hemoglobin concentration in a blood sample. The method comprises the steps of (1 ) combining a sample with a reagent composition including an ionic surfactant at a concentration of from about 2 to 4%, wherein the reagent composition has a pH of from 11.3 to about 13.7 and is free of ionic cyanide to form a reaction mixture; (2) measuring the absorbance. This method is used for measuring hemoglobin concentration only. No leukocyte counting or differentiation of leukocyte subpopulations is taught by Benezra et al.

U.S. Patent No. 5,250,437 (to Toda et al.) teaches a single reagent for determination of hemoglobin and analysis of leukocytes. The reagent is free of cyanide and contains (1) a combination of at least one quaternary ammonium salt and another surfactant including quaternary ammonium salt, nonionic surfactant, pyridinium salt, amphoteric 5

surfactant and a cationic surfactant having a formula of R-|R₂ R₃N⁺- CH₂phenyl wherein Ri, R₂ and R₃ are alkyl groups; and (2) at least one oxidant capable of oxidizing heme in hemoglobin. The reagent is capable of dividing leukocytes into at least two fractions. This method requires denaturing hemoglobin and oxidation of heme group.

U.S. Patent No. 4,529,705 (to Larsen) teaches a reagent for combined diluting and lysing a blood sample for use in electronic enumeration of leukocytes and determination of hemoglobin concentration. The reagent comprises an aqueous solution of: (1 ) a quaternary ammonium salt detergent for lysing red blood cells in the sample; (2) at least one salt of an anion selected from the group consisting of sulfate, carbonate, formate, and acetate for preventing aggregation of platelets in the blood sample; and (3) an alkali metal cyanide for conversion of hemoglobin to a chromogen. This method provides convenience of combined diluting and lysing a blood sample using a single reagent for enumerating leukocytes and measuring hemoglobin concentration. However, it requires the presence of cyanide for hemoglobin measurement.

As described above, current hemoglobin measurement reagents and methods either suffer from toxicity of the reagent, or lack of ability to accomplish multiple diagnostic analyses such as hemoglobin measurement and counting the number of leukocytes or differential counting of leukocyte subpopulations in a single automated step.

A need arises for a new cyanide-free hemoglobin measurement method and a multifunctional reagent which is capable to accomplish multiple diagnostic analyses in a single automated step.

SUMMARY OF THE INVENTION

In view of the foregoing discussions, an object of this invention is to provide a new cyanide-free lytic reagent composition for combined diluting and lysing a blood sample for a determination of hemoglobin concentration and counting leukocytes in a blood sample.

The cyanide-free lytic reagent composition comprises an aqueous solution of a quaternary ammonium salt or a pyridinium salt in a sufficient amount to be able to lyse erythrocytes and release hemoglobin; and an organic ligand in a sufficient amount to form a stable chromogen with hemoglobin selected from the group consisting of triazole and its derivatives, tetrazole and its derivatives, alkaline metal salts of oxonic acid, melamine, aniline-2-sulfonic acid, quinaldic acid, 2-amino-1 ,3,4- thiadiazole, triazine and its derivatives, urazole, DL-pipecolinic acid, isonicotinamide, anthranilonitrile, 6-aza-2-thiothymine, 3-(2-thienyl)acrylic acid, benzoic acid and alkali metal and ammonium salts of benzoic acid, and pyrazine and its derivatives; and a salt in an amount sufficient to adjust conductivity of the lytic reagent for impedance measurement. This invention also provides a method of measuring hemoglobin concentration and counting leukocytes using the new cyanide-free lytic reagent composition for combined diluting and lysing a blood cell sample. The method comprises steps of (1 ) mixing a blood sample with the new cyanide-free lytic reagent composition to lyse erythrocytes and to form a stable hemoglobin chromogen; (2) measuring absorbance of the formed hemoglobin chromogen at a predetermined wavelength; (3) calculating total hemoglobin concentration of the sample from the measured absorbance; (4) counting the number of leukocytes in an automated blood analyzer utilizing DC impedance measurement; and (5) reporting the number of leukocytes for said blood sample.

As will be better appreciated from the ensuing Detailed Description of Preferred Embodiments, the invention is particularly advantageous as compared to the prior art in that it provides a new cyanide-free lytic reagent composition for combined diluting and lysing a blood cell sample 7 for hemoglobin measurement and leukocyte counting without pre-dilution of a blood sample.

The invention will be better understood from the ensuing description of preferred embodiments, reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1a and 1b are the absorption spectra of whole blood samples processed according to the procedure described in Example 1 using the lytic reagent compositions of Example 1 of the present invention (formula 1b and 1c).

Figure 2 shows a series of absorption spectra of a blood sample processed according to the procedure of Example 1 using the lytic reagent composition of formula 1a. Total of twelve spectra were acquired in twelve hours with a one hour interval.

Figures 3a and 3b show the spectra of a whole blood sample treated according to the procedure described in Example 2, using the lytic reagent composition of Example 2, and standard phosphate buffered saline and a commercial blood diluent, COULTER ISOTON^® III, as diluents.

Figures 4a through 4d show the leukocyte subpopulation distribution histograms of four whole blood samples obtained by using the lytic reagent compositions of Example 4 of the present invention on a commercial blood analyzer, COULTER COUNTER^® Model S-Plus IV. Figures 5a and 5b show the correlations between the hemoglobin concentration and the number of leukocytes obtained on an automated commercial blood analyzer, COULTER^® STKS using a conventional lytic reagent, COULTER^® LYSE S^® III cliff, and the results obtained using a lytic reagent composition of Example 3 (formula 3a) of the present invention on the same instrument. 8

Figures 6a and 6b show the correlations between the hemoglobin concentration and the number of leukocytes obtained on an automated commercial blood analyzer, COULTER^® STKS using COULTER^® LYSE S^® III diff lytic reagent and the results obtained using a lytic reagent composition of Example 3 (formula 3b) of the present invention on the same instrument.

Figures 7a through 7e show the correlations between the number of leukocytes, the leukocyte differential count and hemoglobin concentration obtained on a COULTER COUNTER^® Model S-Plus IV and the results obtained using a lytic reagent composition (formula 4a) and the diluent of Example 4 of the present invention on the same commercial instrument.

Figures 8a and 8b are the absorption spectra of two whole blood samples processed according to the procedure described in Example 5 using a lytic reagent and the diluent compositions of Example 5 of the present invention (formula 5a and 5b).

Figures 9a and 9b show a spectrum and a leukocyte subpopulation distribution histogram of a whole blood sample treated according to the procedure described in Example 6, using a lytic reagent and a diluent composition of Example 6.

Figures 10a and 10b show a spectrum and a leukocyte subpopulation distribution histogram of a whole blood sample using a lytic reagent (formula 7a) and procedure described in Example 7.

Figures 11a and 11 b show a spectrum and a leukocyte subpopulation distribution histogram of a whole blood using a lytic reagent (formula 7b) and procedure described in Example 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In general, to measure total hemoglobin concentration in a blood sample photometrically it is necessary to lyse erythrocytes and release 9 hemoglobin using a lytic reagent, then to convert the hemoglobin to a stable chromogen which is able to be detected and measured by UV spectroscopy at a predetermined wavelength. For the measurement to be quantitative and accurate, the formed chromogen needs to be stable, at least stable in the time frame of the measurement. Lysis of erythrocytes can be accomplished by acid lysing, osmotic lysing and utilizing various natural and synthetic surfactants. The released hemoglobin comprises various forms, such as deoxyhemoglobin, oxyhemoglobin, methemoglobin, carboxyhemoglobin and etc. The most efficient method to convert hemoglobin to a stable chromogen is to provide a ligand which has a high affinity to the heme iron to form a stable hemoglobin complex. This has been successfully demonstrated by the cyanmethemoglobin method, wherein the cyanide anion has an extremely high affinity to the heme iron. The terms of hemoglobin complex and hemoglobin chromogen are used interchangeably in this context. Usually, in the absence of a high affinity ligand, the formed hemoglobin chromogen is not very stable. It's absorption varies, and in most cases decays with time. Under this condition, the method of analysis is unreliable even if the kinetics of the decomposition reaction is well monitored and corrected because the chromogen could be very sensitive to the environment such as temperature and sample preparation conditions, etc. When an appropriate hemoglobin ligand is provided, the hemoglobin conversion can be quantitative, and the reliability of the analysis method is ensured by the stability of the formed hemoglobin complex.

Selection of a ligand depends on the analysis to be accomplished, for instance, for hemoglobin measurement only, or for a multiple diagnostic analysis such as counting the number of leukocytes or differential counting of leukocyte subpopulations in combination with the hemoglobin measurement. A ligand which is perfect for hemoglobin 10

measurement may not be suitable for the later applications if the ligand is not compatible to the other analyses. An example taught by Sakata (in U.S. 5,242,832) is that SLS used for lysing erythrocytes and forming Hgb- SLS chromogen can not be used for leukocyte measurement. A first embodiment of the present invention is directed to a cyanide-free lytic reagent composition for measuring the total hemoglobin concentration present in a blood sample, or in combination with a simultaneous counting of the number of leukocytes or differential counting of leukocyte subpopulations. The cyanide-free lytic reagent composition comprises an aqueous solution of:

(I) at least one surfactant in a sufficient amount to be able to lyse erythrocytes and release hemoglobin selected from the group consisting of quaternary ammonium salts, represented by the following molecular structure:

R,

\

N x^"

*3

wherein Ri is an alkyl, alkenyl or alkynyl group having 10 to 18 carbon atoms; R₂, R₃ and R are alkyl groups having 1 to 4 carbon atoms and X^" is chloride or bromide anion; pyridinium salts represented by the following molecular structure:

^■(CH₂)_n CH3 X^" // wherein n is an integer from 7 to 12 and X^" is an anionic group; alkyl sulfonic acid, or alkaline metal salts of alkyl sulfonates; organic phosphate esters, or alkaline metal salts of organic phosphate esters; 11

(II) an organic ligand in a sufficient amount to form a stable chromogen with hemoglobin selected from the group consisting of:

(a) triazole such as 1,2,3-triazole and 1,2,4-triazole, and triazole derivatives such as 1 ,2,4-triazoie-3-thiol, 1 ,2,4-triazole sodium derivative, triazole dicarboxylic acid and heteracyclic derivatives of triazole;

(b) tetrazole and its derivatives such as 5-amino tetrazole;

(c) alkaline metal salts of oxonic acid having the following formula:

. '

HO N C OM

wherein M is an alkaline metal cation; (d) melamine

H,N N H₂

(e) aniline-2-sulfonic acid

-NH₂

^■~SQ3H

(f) quinaldic acid

COOH

(g) 2-amino-1 ,3,4-thiadiazole 12

N N

// \\

(h) triazine and its derivatives having the following formula:

wherein R^ R₂ and R₃ are -H, -OH, -SH, -COOH and heteracyclic derivatives of triazine; (i) urazole

N — N o- N -o

G) DL-pipecolinic acid

COOH

(k) isonicotinamide

CONH₂

^^

(I) anthranilonitrile

.NH,

CN 13

(m) 6-aza-2-thiothymine

CH₃

N

-N

HS N

(n) adenine

NH₂

^■N

N

^

N ~N

(o) 3-(2-thienyl)acrylic acid

'/ \\

-CH ==CHCOOH

(p) benzoic acid, and alkali metal or ammonium salts of benzoic acid having the following formula:

-COOR

wherein R is -H, NH₄ ⁺ or alkali metal cations;

(q) pyrazine and its derivatives having the following formula:

^

14 wherein R₁₍ R₂ R₃ and R₄ are -H, -CN, -OH, -SH, -COOH or - CONH₂.

As known by those skilled in the art, a derivative of an organic ligand in above description means a derivative molecule carrying the functional moiety of the parent compound, such as triazole ring, tetrazole ring and triazine ring.

The pH of the lytic reagent composition ranges from 1 to 13. Among the surfactants, quaternary ammonium salts are more preferred. The concentration of the surfactant in the lytic reagent composition needs to be in an amount sufficient to lyse erythrocytes and release hemoglobin while preserving leukocyte nuclear volumes. To count leukocytes, one does not need to keep the leukocyte membrane intact. In general, using the surfactants described above in the lytic reagent composition of the present invention when erythrocytes are completely lysed and destructed, leukocyte membranes are partially lysed. It is the remaining nuclei of the leukocytes that enable the counting of leukocyte numbers and differentiation of leukocyte subpopulations into lymphocytes, monocytes and granulocytes using the DC impedance method. The concentration of the surfactants in the lytic reagent composition ranges from about 2 g/L to about 250 g/L, preferably, from about 4 g/L to about 80 g/L.

The concentration of the organic ligand in the lytic reagent composition needs to be sufficient to form a stable hemoglobin chromogen. The concentration varies with the ligand type, depending on the affinity of the ligand to the hemoglobin. In general, if the amount of ligand in the lytic reagent composition is not sufficient, the formed hemoglobin chromogen could be unstable. The concentration of the organic ligand in the lytic reagent composition of this embodiment has been found to be effective in a broad range from about 1 g/L to about 30 g/L, preferably from about 2 g/L to about 15 g/L. 15

The concentrations of the chemical ingredients in the lytic reagent composition are the concentrations under conditions in which the hemoglobin and leukocyte measurements are accomplished with the use of a suitable blood diluent for the convenience of using the conventional blood analyzers. However, the concentrations of the chemical ingredients can be changed depending upon the volume ratio between the lytic reagent composition and the diluent.

Optional additives can also be included in the lytic reagent composition in concentrations that their presence is compatible with the primary functional components of the lytic reagent composition. Among these additives are preservatives which have anti-oxidant properties, to increase the shelf-life of the composition, and which have anti-microbial properties.

The first embodiment of the present invention is also directed to a method of using the above described cyanide-free lytic reagent composition for measuring the total hemoglobin concentration present in a blood sample, or in combination with a simultaneous counting of the number of leukocytes or differential counting of leukocyte subpopulations.

An anti-coagulated blood sample is diluted by a suitable blood diluent, then a sufficient amount of lytic reagent composition described above is mixed with the diluted sample by manual or mechanical mixing. The dilution ratio of the blood is from about 125:1 to about 500:1 , total reagent volume versus blood. The sample mixture is measured photometrically, 8 to 60 seconds after the addition of the lytic reagent composition, either on a UV spectrometer or on an automated blood analyzer equipped with a UV detector at a predetermined absorption wavelength for total hemoglobin measurement. The sample mixture can also be introduced to a blood analyzer equipped with a UV detector and a DC impedance measurement device to measure the hemoglobin concentration of the blood sample and to count the number of leukocytes, 16 or to further differentiate the leukocyte subpopulations based on the population distribution histogram obtained. In the later case, the leukocytes are differentiated into three subpopulations, including lymphocytes, monocytes and granulocytes. The detection methods used for the leukocyte counting by a blood analyzer equipped with a DC impedance measurement device are generally described in U.S. Patent No. 2,656,508 (to Wallace H. Coulter) which is hereby incorporated by reference in its entirety. The method of differentiating leukocyte subpopulations utilizing DC impedance measurement is described in U.S. Patent Nos. 4,485,175 and 4,528,274.

The organic ligands described above, for instance, triazole and melamine, not only form stable chromogens with hemoglobin, but also stabilize the leukocytes in the treated sample mixture. This leukocyte stabilization effect prevents the leukocytes from over lysing and nuclear shrinkage, facilitates the separation of the leukocyte subpopulations based on their volumes, and makes the differentiation of leukocytes into three subpopulations possible with a broad pH range. Example 4 illustrates such successful examples using leukocyte stabilizing ligands in the lytic reagent compositions. On the other hand, some of the organic ligands described above, for instance, aniline-2-sulfonic acid and quinaldic acid, exhibit an intermediate to strong impact on the size of leukocyte subpopulations. When these ligands are used for hemoglobin measurement, leukocyte subpopulations collapse into one or two clusters, rendering the leukocyte differentiation difficult. However, even with these ligands the leukocyte nuclear size remains far above the cellular debris of the sample after treated by the lytic reagent composition and above the detection threshold of the conventional DC impedance measurement devices, so that an accurate leukocyte count can be achieved conveniently on 17 commercial blood analyzers as illustrated by the applications in Example 3.

In general, the lytic reagent composition of the first embodiment comprising the organic ligands and surfactants described above is capable of measuring total hemoglobin concentration of a blood sample in a very broad pH range. Preferably, a combination of a quaternary ammonium salt or salts with any of the organic ligands is used to provide the measurement of total hemoglobin concentration and counting the number of leukocytes in a blood sample simultaneously by a DC impedance measurement method. Most preferably, a combination of a quaternary ammonium salt or salts with an organic ligand which is compatible with leukocyte differential analysis is used to provide the measurement of total hemoglobin concentration, counting the number of leukocytes and differential counting the leukocyte subpopulations of a blood sample simultaneously. In the most preferred mode, the leukocytes are differentiated into three subpopulations including lymphocytes, monocytes and granulocytes.

The cyanide-free lytic reagent composition and the method of using the same provide several advantages over the methods of hemoglobin measurement of the prior art. The present invention allows for an accurate measurement of hemoglobin concentration of a blood sample in the absence of cyanide along with a determination of total leukocyte numbers or differentiation of leukocyte subpopulations into lymphocytes, monocytes and granulocytes. The lytic reagent composition converts the hemoglobin present in a blood sample rapidly to a stable chromogen, from about 8 seconds to 60 seconds depending on the organic ligand and diluent used, allowing for rapid automated analysis. The hemoglobin chromogen once formed is stable during the time of measurement. 18

Figure 2 shows a series of absorption spectra of a blood sample processed according to the procedure of Example 1 using the lytic reagent composition of formula 1a containing tetrazole as the hemoglobin ligand, and COULTER^® ISOTON^® II (a commercial blood diluent) as the diluent. Figure 2 illustrates a total of twelve spectra acquired from 12 seconds to 12 hours after the addition of the lytic reagent composition with an interval of one hour. The spectra of the Hgb-tetrazole chromogen are very stable and exhibit no shift or decay during twelve hours data collection time. The nature of a specific hemoglobin chromogen depends on the organic ligand used in the lytic reagent composition. Most chromogens formed by treating the blood sample with the lytic reagent composition using the organic ligands described above have their maximum absorptions between about 510 nm and about 560 nm. Therefore, the chromogens can be measured by most commercial blood analyzers with incorporation of the absorption coefficient of the specific chromogen.

The lytic reagent composition of the first embodiment can be used with many suitable blood diluent. Figure 3 shows the spectra of a whole blood sample treated by the procedure described above using the lytic reagent composition containing tetrazole (Example 2), and using standard phosphate buffered saline and a commercial blood diluent, COULTER^® ISOTON^® III, as the diluents. With both diluents, basically the same hemoglobin chromogen forms, which can be measured at 540 nm. Unlike the previous reagents, the lytic reagent composition described above has a broad pH range, from 1 to 13. This broadens the scope of chemicals which can be used as a hemoglobin ligand. For instance, potassium salt of oxonic acid forms a stable chromogen with hemoglobin which has a strong absorption at 538 nm. However, potassium salt of oxonic acid is only readily soluble in water at low pH, 19 less than pH 3. The previous neutral and alkaline reagents exclude using this chemical for hemoglobin measurement. Sakata teaches in U.S. Patent No. 5,242,832 that If the pH value is 3.0 or less, damage to the leukocytes increases thus rendering measurement of the leukocytes difficult. Figure 4 shows leukocyte subpopulation distribution histograms obtained following the procedure described above using a lytic reagent composition of Example 4, which contains 0.5% of potassium salt of oxonic acid and has a pH of 2.3, and COULTER^® ISOTON^® III as the diluent. As illustrated by Figure 4c and 4d, the leukocytes are clearly differentiated into lymphocytes, monocytes and granulocytes. Figure 6b shows an excellent linear correlation between the number of leukocytes obtained on a commercial automated blood analyzer, COULTER^® STKS, and the results obtained using one of the lytic reagent compositions of the Example 3 (formula 3b) on the same instrument, wherein pH of the lytic reagent composition is only 1.67.

The lytic reagent composition of the first embodiment and the method of using the same provide an accurate hemoglobin measurement, accurate count of leukocytes and differential counting of leukocyte subpopulations. Figure 5a and 6a demonstrate an excellent linear correlation between the hemoglobin concentrations obtained on a COULTER^® STKS using a conventional lytic reagent and the hemoglobin concentrations obtained using the lytic reagent compositions of Example 3 (formula 3a and 3b). Figure 5b and 6b illustrate excellent correlation between the leukocyte count obtained on a COULTER^® STKS, and the results obtained using formula 3a and 3b on the same instrument.

The lytic reagent composition of this embodiment provides an accurate hemoglobin measurement in the presence of common interference materials. Totally 72 whole blood samples were analyzed using the formula 3a of Example 3 on a COULTER^® STKS. 70% of the samples are clinical samples including various diseases such as sickle 20 cell crisis and hepatitis C. It is known that these abnormal bloods contain abnormal hemoglobins and interference materials for hemoglobin measurement. However, the results of measurement using the lytic reagent composition and the method of the present invention correlate excellently with the conventional cyanide-Hgb method, as illustrated by Figures 5 and 6, demonstrating that the total hemoglobin concentration of various clinical samples can be measured by using the lytic reagent composition of the present invention.

A second embodiment of the present invention is directed to an alternative mode of utilizing the hemoglobin stabilization functionality of the organic ligands by adding the organic ligand into a blood diluent instead of the lytic reagent composition. According the method described above, a blood sample is prediluted by the blood diluent before mixed with the lytic reagent. When a diluent contains an organic ligand described above, the hemoglobin molecules in the sample mixture after released from the erythrocytes by a lytic reagent will be in immediate contact with the organic ligand to form a stable hemoglobin chromogen. Therefore, the alternative mode serves the same purpose for hemoglobin measurement of a blood sample. However, it provides options to reagent designers to choose the most appropriate mode based on their specific need, for instance, the compatibility of an organic ligand to other tests that the lytic reagent or the diluent are required to perform beside the hemoglobin measurement, or an organic ligand's compatibility to other chemical ingredients in either lytic reagent or the diluent. Example 5 illustrates such an application of the organic ligands in blood diluent to form stable hemoglobin chromogens, wherein the lytic reagent contains only the surfactant. Figure 9a shows a spectrum of a blood sample treated according to the procedure of Example 6 using a diluent containing triazole as the hemoglobin ligand. The characteristics of the chromogen is the same to the one obtained using a lytic reagent 21 composition containing the same ligand as shown in Figure 1a. Figure 9b shows a leukocyte distribution histogram of a blood sample obtained by using the diluent of Example 6 on a COULTER COUNTER^® Model S- Plus IV, wherein the lytic reagent contains only surfactant. This example shows the compatibility of the organic ligand to the leukocyte differential analysis when it is used in the alternative mode.

The third embodiment of the present invention employs a single lytic reagent composition for combined diluting and lysing a blood cell sample for hemoglobin measurement as well as leukocyte analysis, without pre-dilution by a blood diluent. In this approach, the concentrations of the chemical ingredients of the lytic reagent composition of the first embodiment are adjusted to achieve sufficient lysing of erythrocytes and to form a stable hemoglobin chromogen. For leukocyte counting or differential counting of leukocyte subpopulations the conductivity of the single lytic reagent composition is adjusted to enable its use for impedance measurements. The conductivity is adjusted by addition of a sufficient amount of a salt or salts. Suitable salts for the single lytic reagent composition of the present invention include alkaline metal salts of various anions. Suitable examples include alkaline metal salts of sulfate, chloride, formate, acetate, citrate and bicarbonate. The amount of salt needs to be sufficient to adjust the conductivity of the lytic reagent to a level that may be measured by an automatic blood analyzer. Using the single lytic reagent, leukocytes can be differentiated into at least two subpopulations, preferably three subpopulations, together with a determination of hemoglobin concentration and counting of total leukocytes in a blood cell sample. As shown in Figure 10b and 11 b, two leukocyte subpopulations are obtained with the use of the single lytic reagents of Example 7, which are lymphoid and myeloid populations. 22

The surfactants for the single lytic reagent include quaternary ammonium salts and pyridinium salts, which are described in detail in the first embodiment. Preferably, quaternary ammonium salts are used for the single lytic reagent. The concentrations of surfactants and organic ligands in the single lytic reagent are generally lower than those in a lytic reagent composition used with a diluent. The surfactant concentration ranges from about 0.3 g/L to about 200 g/L. The concentration of the organic ligands ranges from about 0.1 g/L to about 30 g/L. The pH range of the single lytic reagent is slightly different from a lytic reagent composition used with a diluent. The pH in the single lytic reagent is from about 3 to about 13, and preferably from about 3.5 to about 11.

Example 7 shows two working examples of the single reagent approach.

The single reagent method provides a simple one-step sample preparation and it reduces the cost of instrumentation. The following examples are illustrative of the invention and are in no way to be interpreted as limiting the scope of the invention, as defined in the claims. It will be understood that various other ingredients and proportions may be employed, in accordance with the proceeding disclosure.

Example 1

The reagents of the following compositions were prepared.

Formula 1a tetrazole 5.0 g tetradecyltrimethylammonium bromide 15.0 g distilled water adjusted to 1 liter pH 277

Formula 1b triazole 10.0 g dodecyltrimethylammonium chloride (50% solution) 35.0 ml tetradecyltrimethylammonium bromide 3.5 g distilled water adjusted to 1 liter 23 pH 6.19

Formula 1c quinaldic acid 5.0 g dodecyltrimethylammonium chloride (50% solution) 50.0 ml distilled water adjusted to 1 liter pH 2.55

Formula 1d melamine 5.0 g tetradecyltrimethylammonium bromide 3.6 g dodecyltrimethylammonium chloride (50% solution) 36.0 ml distilled water adjusted to 1 liter pH 4.55

Formula 1e tetrazole 5.0 g

Chemfac NB-104

(complex phosphate ester made by Chemax, Inc.) 30.0 g distilled water adjusted to 1 liter pH 7.11

11.6 μl of a whole blood sample was diluted by 2500 μl of ISOTON^® II, then 403 μl one of the above lytic reagent compositions was mixed manually with the prediluted sample. The absorption spectrum of the sample was measured immediately on a Beckman DU 7500 spectrophotometer. Figure 1a and 1 b show the spectra of the blood samples treated according to above procedure using formula 1 b and 1c. Figure 2 shows a total of twelve spectra obtained from a blood sample processed according to above procedure using formula 1a. The spectra were acquired from 12 seconds to 12 hours after the addition of the lytic reagent composition with an interval of one hour.

Example 2 A reagent of the following composition was prepared. 24 tetrazole 5.0 g dodecyltrimethylammonium chloride (50% solution) 36.0 ml tetradecylammonium bromide 3.6 g distilled water 1 liter

PH 273

11.6 μl of a whole blood sample was diluted by 2500 μl of a blood diluent, then 403 μl of the above formula was mixed manually with the prediluted sample. The absorption spectrum of the sample was measured immediately on a Beckman DU 7500 spectrophotometer. Five commercial blood diluents were used for the hemoglobin measurement, i.e., COULTER^® ISOTON^® III, COULTER^® ISOTON^® II, Technicon H'™ Systems RBC DIL (a commercial product of Technicon Instruments Corp.), the standard phosphate buffered saline and the standard sodium chloride saline. The chromogen formed has a maximum absorption peak at about 540 nm with a shoulder at about 565 nm. The maximum absorption of the chromogen is only slightly different when different diluents used for the hemoglobin measurement, i.e., 540 nm with the ISOTON^® III, 539 nm with the ISOTON^® II, the Technicon H^,TM Systems RBC DIL, and the standard phosphate buffered saline and 538 nm with the standard sodium chloride saline. The chromogen formed immediately after addition of the above lytic reagent compositions, from 8 seconds to about 35 seconds depending on the blood diluent used. Figure 3 shows the spectra obtained following above procedure, using the standard phosphate buffered saline and ISOTON^® III as the diluents.

Example 3

The reagents of the following compositions were prepared.

Formula 3a tetrazole 5.0 g tetradecyltrimethylammonium bromide 15.0 g distilled water adjusted to 1 liter 25 pH 12.06

Formula 3b aniline-2-sulfonic acid 5.0 g tetradecyltrimethylammonium bromide 10.0 g cetyltrimethylammonium bromide 4.0 g distilled water 1 liter

PH 1.67

Formula 3c quinaldic acid 5.0 g dodecyltrimethylammonium chloride (50% solution) 40.0 ml distilled water 1 liter

PH 2.58

About 70 blood samples, whose hemoglobin concentration ranged from 6 to 17 g/dL and leukocyte count ranged from 1,000/μL to 40,000/μL, were analyzed on a calibrated COULTER^® STKS instrument under standard instrument configuration except the lytic reagent, the LYSE S^® III diff being replaced by the above formulations. For each formula, about half of the samples are clinical samples including various diseases. In the case of formula 3a, 70% of samples are clinical samples. Figure 5 shows the correlations between the hemoglobin concentration and the number of leukocytes (reported as WBC in 10³/μL) obtained by using the reference reagent, LYSE S^® III diff, and the results obtained using the formula 3a on the same instrument. Figure 6 shows the correlations between the hemoglobin concentration and the number of leukocytes obtained by using the LYSE S^® III diff lytic reagent and the results obtained using the formula 3b on the same instrument. Figure 5 and 6 demonstrate excellent linear correlations between the lytic reagent compositions of the present invention and the conventional cyanide-containing lysing reagent in hemoglobin concentration and WBC measurements. 26

Example 4

The reagents of the following compositions were prepared. Lvtic reagent

Formula 4a triazole 10.0 g dodecyltrimethylammonium chloride (50% solution) 35.0 ml tetradecyltrimethylammonium bromide 3.5 g distilled water adjusted to 1 liter pH 6.39

Formula 4b oxonic acid potassium salt 5.0 g dodecyltrimethylammonium chloride (50% solution) 35.0 ml tetradecyltrimethylammonium bromide 3.5 g distilled water adjusted to 1 liter pH was adjusted to 2.3 by HCI

Diluent sodium sulfate 9.7 g sodium chloride 4.0 g distilled water adjusted to 1 liter pH adjusted to 7.14 by 1 % NaOH

85 blood samples, whose hemoglobin concentration ranged from 6 to about 17 g/dL and leukocyte count ranged from 1 ,500 /μL to 45,000/μL, were analyzed on a calibrated COULTER COUNTER^® Model S-Plus IV under standard instrument configuration using reference reagents, LYSE S^® III diff and ISOTON^® III. Then the same samples were analyzed again on the same instrument except that the lytic reagent, the LYSE S^® III diff, was replaced by the above lytic reagent compositions and the ISOTON^® III was replaced by the above diluent. Figure 4 shows the leukocyte subpopulation distribution histograms from four of these blood samples obtained on the COULTER COUNTER^® Model S-Plus IV. Figure 4a and 4b were obtained using Formula 4a and 27 the above diluent, wherein Figure 4a showing a histogram of a normal blood and Figure 4b showing a histogram of a blood sample having elevated monocytes. Figure 4c and 4d were obtained using Formula 4b and ISOTON^® III, wherein Figure 4c showing a histogram of a normal blood and Figure 4d showing a histogram of a clinical sample having more than 90% of granulocytes. Figure 7a to 7e show the correlations between the number of leukocytes (reported as WBC in 10³/μL, Figure 7a), lymphocyte% (Figure 7b), monocyte% (Figure 7c), granulocyte% (Figure 7d) and hemoglobin concentration (Figure 7e) obtained by using the reference reagents and those obtained using Formula 4a and the above diluent. The correlation coefficients, as well as the slopes and the intercepts of the regression lines demonstrate excellent linear correlations for hemoglobin concentration, WBC, lymphocyte% and granulocyte%, and a good correlation for monocyte%.

Example 5

Lvtic reagent dodecyltrimethylammonium chloride (50% solution) 36.0 ml tetradecyltrimethylammonium bromide 3.6 g distilled water adjusted to 1 liter pH 6.88

Diluent

Formula 5a tetrazole 5 g standard phosphate buffered saline 1 liter pH 3.86

Formula 5b

2-amino-1 ,3,4-thiazole 5.0 g sodium sulfate 9.7 g sodium chloride 4.0 g distilled water adjusted to 1 liter 28 pH 6.04

11.6 μl of a whole blood sample was diluted by 2500 μl one of the above diluent formulae, then 403 μl of above lytic reagent formula was mixed manually with the pre-diluted sample. The absorption spectrum of the sample was measured immediately on a Beckman DU 7500 spectrophotometer. Figure 8a and 8b show the spectra of the blood samples treated according to above procedure using formula 5a and 5b, respectively.

Example 6

Diluent triazole 5.0 g sodium sulfate 9.7 g sodium chloride 4.0 g distilled water adjusted to 1 liter pH 6.06

10 blood samples were analyzed on a calibrated COULTER

COUNTER^® Model S-Plus IV using the above lytic reagent and diluent. Figure 9a shows a leukocyte subpopulation distribution histogram of a blood sample obtained on the COULTER COUNTER^® Model S-Plus IV. These samples were also treated and measured on the spectrometer according the procedure described in Experiment 5 using the above lytic reagent and diluent. Figure 9a shows a spectrum of the same blood sample used in Figure 9b. Both spectrum and the leukocyte subpopulation distribution histogram have the same characteristics to those obtained when triazole is used in the lytic reagent composition. 29

Example 7

Formula 7a dodecyltrimethylammonium chloride (50% solution) 5.0 ml triazole 3.0 g sodium sulfate 15.9 g distilled water adjusted to 1 liter pH 6.30

Formula 7b dodecyltrimethylammonium chloride (50% solution) 5.0 ml tetrazole 1.0 g sodium sulfate 15.9 g distilled water adjusted to 1 liter pH 3.69

10 μl of a whole blood sample was mixed with 2.5 ml one of above formulae manually. The absorption spectrum of the sample was measured immediately on a Beckman DU 7500 spectrophotometer.

Figure 10a and 11a show the obtained spectra 15 seconds after addition of formula 7a and formula 7b, respectively. Another 12 μl of a whole blood sample was mixed with 3.0 ml one of above formulae automatically on an experimental hematology analyzer. The sample mixture was analyzed by DC impedance measurement 10 seconds after the addition of the lytic reagents. Figure 10b and 11b show the obtained leukocyte subpopulation distribution histograms. As recognized by those skilled in the art, the gain of DC impedance measurement is adjusted to cover all cell ranges depending on the size of leukocytes upon exposing to the lytic reagents.

Claims

30What is claimed is:

1. A cyanide-free lytic reagent composition for combined diluting and lysing a blood cell sample for a determination of hemoglobin concentration and leukocyte counting comprising an aqueous solution of:

(I) a quaternary ammonium salt, represented by the following molecular structure:

^R1.

N X^"

wherein Ri is an alkyl, alkenyl or alkynyl group having 10 to 18 carbon atoms; R₂, R₃ and R₄ are alkyl groups having 1 to 4 carbon atoms and X^" is chloride or bromide anion; or a pyridinium salt represented by the following molecular structure:

(CHa)ΓÇ₧ CE3 X

wherein n is an integer from 7 to 12 and X^" is an anionic group; wherein the quaternary ammonium salt or the pyridinium salt is in a sufficient amount to be able to lyse erythrocytes and release hemoglobin;

(II) an organic ligand in a sufficient amount to form a stable chromogen with hemoglobin selected from the group consisting of: (a) triazole and its derivatives

(b) tetrazole and its derivatives

(c) alkaline metal salts of oxonic acid having the following formula: 31

N ^'

HO N "OM

wherein M is an alkaline metal cation; (d) melamine

NH₂

N

H,N N NH₂

(e) aniline-2-sulfonic acid

.NH₂

SO3H

(f) quinaldic acid

COOH

(g) 2-amino-1 ,3,4-thiadiazole

N ΓÇö N

//

^ΓûáNH₂

(h) triazine and its derivatives having the following formula:

N ^ N

R, N 32

wherein R_{1 (} R₂ and R₃ are -H, -OH, -SH, -COOH and heteracyclic derivatives of triazine; (i) urazole

N ΓÇö N o- N^ -o

(j) DL-pipecolinic acid

COOH

(k) isonicotinamide

CONH₂

^^

(I) anthranilonitrile

.NH,

CN

(m) 6-aza-2-thiothymine

CH₃

N

.

HS N

(n) 3-(2-thienyl)acrylic acid 33

\ /-ΓÇö CH =╬╣ CHCOOH

(o) benzoic acid, or alkali metal or ammonium salts of benzoic acid having the following formula: OOR

wherein R is -H, NH ⁺ or alkali metal cations; and

(p) pyrazine and its derivatives having the following formula:

R₃-. A* R

wherein R^ R₂ R₃ and R₄ are -H, -CN, -OH, -SH, -COOH or - CONH₂; and

(IQ) a salt in an amount sufficient to adjust conductivity of the lytic reagent for impedance measurement.

2. The reagent composition of Claim 1 , wherein the salt used to adjust conductivity of the lytic reagent comprises alkaline metal salts of sulfate, chloride, formate, acetate, citrate and bicarbonate.

3. The reagent composition of Claim 1 , wherein the surfactant comprises a quaternary ammonium salt having a concentration ranging from about 0.3 g/L to about 200 g/L.

4. The reagent composition of Claim 1 , wherein the surfactant comprises a pyridinium salt having a concentration ranging from about 0.5 g/L to about 130 g/L. 34

5. The reagent composition of Claim 1 , wherein the organic ligand has concentrations in a range of about 0.1 g/L to about 30 g/L.

6. A cyanide-free lytic reagent composition for combined diluting and lysing a blood cell sample for a determination of hemoglobin concentration and leukocyte counting comprising an aqueous solution of:

-1 +

^R1.

N

R; ^{^}R,

wherein R, is an alkyl, alkenyl or alkynyl group having 10 to 18 carbon atoms; R₂, R₃ and R₄ are alkyl groups having 1 to 4 carbon atoms and X^" is chloride or bromide anion; wherein the quaternary ammonium salt is in a sufficient amount to be able to lyse erythrocytes and release hemoglobin;

(II) triazole and its derivatives in a sufficient amount to form a stable hemoglobin chromogen; and

(III) a salt in an amount sufficient to adjust conductivity of the lytic reagent for impedance measurement.

7. A cyanide-free lytic reagent composition for combined diluting and lysing a blood cell sample for a determination of hemoglobin concentration and leukocyte counting comprising an aqueous solution of:

(I) a quaternary ammonium salt, represented by the following molecular structure: 35

-╬╣ +

\ ^! X^"

* N.

wherein Ri is an alkyl, alkenyl or alkynyl group having 10 to 18 carbon atoms; R₂, R₃ and R₄ are alkyl groups having 1 to 4 carbon atoms and X^" is chloride or bromide anion; wherein the quaternary ammonium salt is in a sufficient amount to be able to lyse erythrocytes and release hemoglobin;

(II) tetrazole and its derivatives in a sufficient amount to form a stable hemoglobin chromogen; and

8. A method of measuring hemoglobin concentration and counting leukocytes using a cyanide-free lytic reagent composition for combined diluting and lysing a blood cell sample comprising steps of:

(1 ) mixing a blood sample with a cyanide-free lytic reagent composition to lyse erythrocytes and to form a stable hemoglobin chromogen, wherein the lytic reagent composition comprising an aqueous solution of: (I) a quaternary ammonium salt, represented by the following molecular structure:

R, R,

N 36 wherein Ri is an alkyl, alkenyl or alkynyl group having 10 to 18 carbon atoms; R₂, R₃ and R are alkyl groups having 1 to 4 carbon atoms and X^" is chloride or bromide anion; or a pyridinium salt represented by the following molecular structure:

(CH2)_n CHs X^"

(a) triazole and its derivatives

(b) tetrazole and its derivatives

(c) alkaline metal salts of oxonic acid having the following formula:

N '

HO N C OM

wherein M is an alkaline metal cation; (d) melamine

N

H₂N N TJH,

(e) aniline-2-sulfonic acid 37

NH₂

^ ^

SO₃H

(f) quinaldic acid

COOH

(g) 2-amino-1 ,3,4-thiadiazole ΓÇö N

/^{/ <} NH,

(h) triazine and its derivatives having the following formula:

R₃

wherein R_1f R₂ and R₃ are -H, -OH, -SH, -COOH and heteracyclic derivatives of triazine; (i) urazole

N ΓÇö N

O O

N

G) DL-pipecolinic acid

COOH 38

(k) isonicotinamide

CONH₂

^¹

(I) anthranilonitrile

-NH₂

CN

(m) 6-aza-2-thiothymine

CH₃

N

.

HS N

(n) 3-(2-thienyl)acrylic acid

CH =CHCOOH

(o) benzoic acid, or alkali metal or ammonium salts of benzoic acid having the following formula:

.COOR

wherein R is -H, NH₄ ⁺or alkali metal cations; and

(p) pyrazine and its derivatives having the following formula: 39

J.

wherein R_{1 f} R₂ R₃ and R₄ are -H, -CN, -OH, -SH, -COOH or - CONH₂; and

(III) a salt in an amount sufficient to adjust conductivity of the lytic reagent for impedance measurement;

(2) measuring absorbance of the formed hemoglobin chromogen at a predetermined wavelength;

(3) calculating total hemoglobin concentration of said sample from the measured absorbance;

(4) counting the number of leukocytes in an automated blood analyzer utilizing DC impedance measurement; and

(5) reporting the number of leukocytes for said blood sample.

9. The method of Claim 8, wherein the absorbance of the formed hemoglobin chromogen has a maximal absorbance between about 510 nm and about 560 nm.

10. The method of Claim 8, wherein the lytic reagent composition comprises an aqueous solution of:

N X^"

R RΓÇ₧

wherein Ri is an alkyl, alkenyl or alkynyl group having 10 to 18 carbon atoms; R₂, R₃ and R₄ are alkyl groups having 1 to 4 carbon atoms 40

and X^" is chloride or bromide anion; wherein the quaternary ammonium salt is in a sufficient amount to lyse erythrocytes and release hemoglobin for a determination of hemoglobin concentration and preserve nuclei of leukocyte subpopulations for a differential analysis of leukocytes; and (II) the organic ligand in a sufficient amount to form a stable chromogen with hemoglobin is selected from the group consisting of :

(a) triazole and its derivatives

(b) tetrazole and its derivatives

(c) melamine

N

H,N

N

(d) alkaline metal salts of oxonic acid having following formula:

N '

HO N X C! OM

wherein M is an alkaline metal cation;

(e) 2-amino-1 ,3,4-thiadiazole

N ΓÇö N \_S/ ^NH₂

(f) urazole ΓÇö N

┬░^{^} V ^┬░ 41

(g) DL-pipecolinic acid

COOH

(h) isonicotinamide

CONH₂

^¹

(i) pyrazine and its derivatives having following formula:

R .N. .R

R4-

wherein R_{1 t} R₂, R₃ and R₄ are -H, -CN, -OH, -SH, -COOH or - CONH₂;

(m) a salt in an amount sufficient to adjust conductivity of the lytic reagent for impedance measurement.

11. The method of Claim 10, wherein the quaternary ammonium salt has a concentration ranging from about 0.3 g/L to about 20 g/L.

12. The method of Claim 10 further comprising steps of: differentiating leukocyte subpopulations according to a population distribution histogram; and reporting the number of leukocyte subpopulations. 42

13. The method of Claim 10, wherein the organic ligand is triazole.

14. The method of Claim 10, wherein the organic ligand is tetrazole.

15. The method of Claim 12, wherein the leukocytes are differentiated into at least two subpopulations.