Original Article

Influence of Storage Time and Temperature on Haemato-logical Measurands using a Sysmex NE 8000 AnalyserEinfluß von Lagerungszeit und -temperatur von hämatologischen Parametern beiVerwendung eines Sysmex NE 8000 Analysers

M. Heins1, Britta Schossow2, L. Greiner3, W. Heil2

Summary: The influence of storage lime and tempe-rature on haematological analytes was investigatedwith EDTA samples from 20 healthy subjects for stor-age at room temperature and samples from another 20healthy volunteers for storage at 6 °C in the refrigera-tor. Haemoglobin (Hb), haematocrit, red blood cellcount (RBC), leukocytes, mean corpuscular haemoglo-bin (MCH), mean corpuscular haemoglobin concentra-tion (MCHC), mean corpuscular volume (MCV), pla-telets, neutrophils, eosinophils, basophils, lym-phocytes and monocytes were measured using a Sys-mex NE 8000 haematology analyser at 0, 2, 4, 24, 48,72, 96 and 168 hours. Instability was defined as achange of more than 1/12 of the difference betweenthe upper and lower reference limits according to theRecommendations of the German Medical Associa-tion.At room temperature leukocytes and platelets werestable for 168 h, Hb, RBC and MCH for 96 h, haema-tocrit and MCV for 4 h and MCHC for less than 2 h.In the refrigerator stability was shown by RBC, leuko-cytes and MCH for 168 h, Hb and platelets for 96 h,haematocrit for 72 h and MCHC and MCV for only24 h.At room temperature stability for eosinophils and lym-phocytes was observed for 168 h, basophils for 24 h,neutrophils for 2 h and monocytes for less than 2 h. Inthe refrigerator, maximum storage time of lympho-cytes was reduced to 24 h, basophils to 2 h and neu-trophils as well as monocytes to less than 2 h.

Keywords: Instability; storage time; storage tempera-ture; haematological analytes; blood cell count; whiteblood cell count.

Zusammenfassung: Der Einfluß von Lagerungs-dauer und -temperatur auf hämatologische Analytewurde anhand EDTA-Proben von 20 Probanden, diebei Raumtemperatur aufbewahrt und von weiteren 20Probanden, die bei 6 °C in Kühlschrank gelagertwaren, untersucht. Hämoglobin (Hb), Hämatokrit (Hk),Erythrozytenzahl (RBC), Leukozytenzahl (WBC),mittleres corpusculäres Hämoglobin (MCH), mittlerecorpusculäre Hämoglobinkonzentration (MCHC), mitt-leres corpusculäres Volumen (MCV), Thrombozyten,Neutrophile, Eosinophile, Basophile Granulozyten,Lymphozyten und Monozyten wurden an einem Sys-mex NE 8000 Hämatologie Analyser nach 0, 2, 4, 24,48, 72, 96 und 168 Stunden analysiert. Instabilitätwurde definiert als eine Änderung von mehr als 1/12der Differenz zwischen der oberen und unteren Refe-renzintervallsgrenze entsprechend den Richtlinien derDeutschen Ärztekammer.Bei Raumtemperatur waren Leukozyten und Thrombo-zyten 168 h, Hb, Erythrozyten, und MCH 96 h? Hä-matokrit und MCV 4 h sowie MCHC weniger als 2 hstabil. Im Kühlschrank lag die Stabilität von Erythro-zyten, Leukozyten und MCH bei 168 h, Hb undThrombozyten bei 96 h, Hk bei 72 h und MCHC undMCV bei nur 24 h. Bei.Raumtemperatur betrug die be-obachtete Stabilität für Eosinophile und Lymphozyten168 h, für Basophile 24 h, Neutrophile 2 h und Mono-zyten weniger als 2 h. Im Kühlschrank vermindertesich die Stabilität der Lymphozyten auf 24 h, die derBasophilen auf 2 h und der Neutrophilen auf wenigerals 2 h und Monozyten auf weniger als 2 h.

Schlüsselwörter: Stabilität; Lagerungsdauer; Lage-rungstemperatur; hämatologische Analyte; kleinesBlutbild; Differentialblutbild.

Institut für Laboratoriumsmedizin, Mikrobiologie, Haemostaseolo-qie und Transfusionsmedizin, Marienhospital Osnabrück?Zentrallabor der Klinikum Wuppertal GmbH, Universität Wit-ten/Herdecke3Medizinische Klinik A der Klinikum Wuppertal GmbH, UniversitätWitten/HerdeckeAddress for correspondence: Dr. med. Michael Heins, Institutfür Laboratoriumsmedizin, Mikrobiologie, Haemostaseologie undTransfusionsmedizin, Marienhospital Osnabrück, Johannisfreiheit2-4, D-40479 Osnabrück.Fax: ++49-0541-326-4181; E-mail: Michael.Heins@t-online.deEingegangen: 6. Mai 1999/Angenommen: 29.'Januar 2000

routine laboratory diagnostic determination ofB haemoglobin, the blood cell and white blood cell dif-ferential count are some of the most requested ana-lytes. Counting of blood .cells using counting chamberand blood smear microscopy following Pappenheim orWright staining have been superseded by automatedmethods. The latter was introduced by Joe and Wal-lace-Coulter, who developed one of the first blood cellcounter at the end of the 1940s. In the 1970s blood

236 J Lab Med 2000; 24 (5): 236-242 © 2000 Blackwell Wissenschafts-Verlag, Berlin

M. Heins et al.: Influence of Storage Time and Temperature on Haematological Measurands using a Sysmex NE 8000 Analyser

smear microscopy was automated by pattern recogni-tion systems. High costs and limited throughput werethe disadvantages of this technique. The developmentof Thorn in the 1970s using alternating current to dis-criminate leukocytes has led to further improvement ofhaematology analyser systems. First introduced wasthe 3 cell differential count and nowadays analysersgiving 5 cell differential count are readily available.While with pattern recognition analysers, it was recog-nised that samples had. to be stained within four hours,the new analysers should not have this limitation [1].

The aim of our study was to investigate whether theSysmex NE 8000 analyser is less sensitive to pre-ana-lytical variation.

Materials and methodsSamplesSamples were collected at 8.00 a.m. (time 0) from 40healthy subjects into potassium ethylene diamintetracetic acid Monovette tubes (K3EDTA, Sarstedt,Nümbrecht). Subsequently, samples were analysed at0, 2, 4, 24, 48, 72, 96 and 168 hours. Blood samplesfrom 20 subjects were stored in the refrigerator at 6 °Cand from the remaining 20 at room temperature (ap-proximately 21 °C).

Analytical MethodsAnalyses were performed using an automated SysmexNE-8000 analyser (Sysmex, Hamburg, Germany) witha five part differential screening device. White cell dif-ferentiation was performed by application of radio fre-quency and direct current detection methodology. Be-fore differential analysis, white blood cells were chem-ically treated causing a limited change in cell mor-phology while allowing retention of cell membranesand some cytoplasm. Subsequent analysis by radio fre-quency detection provided information on the cell nu-cleus (size and density), whereas traditional electronicresistance (direct current detection) was used to mea-sure the size of the whole cell. Cells from each samplewere electronically distributed by size and, that madeup the leukocytes scattergram, with the X-axis show-ing direct current detection and Y-axis radio frequencydetection. The scattergram obtained in this manner isshown in figure 1.

Complex algorithms were used to determine the op-timum discriminator placement for separation of eachcell population. Initially, the leukocytes were separat-ed into total granulocytes, lymphocytes, and mono-cytes by size and density characteristics as displayedon the scattergram. Eosinophils and basophils were de-lineated by means of separate temperature controlleddetection blocks after undergoing chemical treatmentwith two distinct reagents.

The RBC count, haematocrit, and platelets were de-termined by making a 1:500 dilution of EDTA wholeblood sample with isotonic diluent. Hydrodynamic fo-cusing resulted in the single file movement of RBCs

and platelets past a counting aperture. Mean corpuscu-lar volume (MCV) was measured electronically basedon the principle of voltage changes generated by RBCspassing through the counting aperture. Haemoglobinwas determined by sodium lauryl sulfate (SLS) method.

Haematocrit, mean corpuscular haemoglobin(MCH), and mean corpuscular haemoglobin concen-tration (MCHC) were calculated.

Statistical AnalysisThe Friedman test was used to analyse the influence ofstorage time and temperature on the haematologicalanalytes. Results were plotted as median of the per-centage deviation from the initial value with the 10th,25th, 75th and 90th percentiles for both storage condi-tions. In the figures the bottom and the top of a boxaround the median represent the variable's inner quar-tile range (25th-75ih percentiles). The antennae(whiskers) indicated 10th and 90th percentiles. Instabil-ity was defined as a significant change (by Friedmantest, p<0.05) of the median of greater than 1/12 of thedifference between the upper and lower reference lim-its (Table 1) according to the Recommendations of theGerman Medical Association [2]. Statistical analyseswere performed using SPSS for Windows and GB-StatVersion 6.5.

ResultsAccording to the Recommendations of the GermanMedical Association [2] analytical limits were calcu-lated using 1/12 of the difference between the upperand lower reference limits (Table 1). These limits andalternatively the 1/6 difference were used to calculatethe stability of analytes (Table 2).

For the blood cell count with samples stored atroom temperature, leukocytes and platelets were stablefor 168 h, Hb, RBC and MCH for 96 h. haematocritand MCV for 4 h and MCHC for less than 2 hours. Inthe refrigerator, maximum storage time was enhancedto 168 h for RBC and MCH, to 72 h for haematocritand to 24 h for MCV and MCHC. Only the maximumstorage time of platelets was reduced to 96 h (Table 2).

Doubling of the analytical limits led to an enhancedmaximum storage time of 168 h for Hb, RBC, MCHand platelets at both storage temperatures. Maximumstorage time of haematocrit and MCV was increased to96 h and MCHC to 72 h with storage in the refrigera-tor while maximum storage time of MCHC was onlyincreased to 4 h and that for MCV remained un-changed at room temperature (Table 2).

For Hb, RBC, leukocytes, MCH and platelets themedian did not alter significantly with storage up to96 h at both temperatures. For haematocrit and MCVthe median increased significantly after 4 h at roomtemperature but remained unchanged for 72 h and 24 hat 6 °G (Figures 2a and b). For MCHC, the median de-creased significantly before 2 h at room temperatureand after 24 h at 6 °C (Figure 2c).

J Lab Med 2000; 24 (5): 236-242 237

M. Heins et al.: Influence of Storage Time and Temperature on Haematological Measurands using a Sysmex NE 8000 Analyser

Table 1 Calculation

Analyte

HaemoglobinHaematocritRBCLeukocytesMCHMCHCMCVPlatelet countNeutrophilsEosinophilsBasophilsLymphocytesMonocytes

of the stability

Unit

[g/di][%]

[106/ìÉ][ÊÑ/ìÉ]

[pg][g/di]

[fl][KF/pl]

[%][%][%][%][%]

limits according to

Reference range14.0-17.5

40-524,5-5.94.4-11.3 .28-3333-3680-96

150-3505075

2-40-1

25-402-8

the reference ranges.

Difference

3.5121.46.95316

2002521156

1/1 2 of difference

0.291

0.120.580.420.251.3316.7 .2.080.170.081.250.50

Significance limit

1.9%2.2%2.3%7.3%1.4%0.7%1.5%6.7%3.3%5.6%

16.7%3.8%

10.0%

Table 2 Storagetween the upper

time according to the Guidelines of the German Medical Associationand lower reference ranges) or 1/6 of the difference.

1/6 of differenceAnalyte

HaemoglobinHaematocritRBC·LeukocytesMCHMCHCMCVPlatelet countNeutrophilsEosinophilsBasophilsLymphocytesMonocytes

RT

168 h4 h***

168h168h168h

4h***4h***

168h4h***

168h24h***

168h<2 h ***

Friedman test was statistically significant *: p<0.05,

6°C

168h96 h ***

168h168h168h72 h ***96h***

168h2h***

168h96 h ***

168h<2 h***

**: p<0.01,**

RT

96 h***4h***

96 h***168h96h*

(1/12 of the difference be-

1/12 of difference6°C

96 h"*72 h***

168 h168 h168h

< 2 h *** 24 h***4h***

168 h2h***

• 168h24 h***

168h<2 h***

*: p<0.001 . RT: Room

24 n**»96 h***<2 h***

168h2h***

24 h***<2 h***

temperature.

RF

lUMATUftCORANUIQCYTES

DC

Figure 1 Sysmex NE 8000 scattergram with radio frequency (RF)on the vertical axis and direct current (DC) detection along thehorizontal axis [15].

For the white cell differentiation count eosinophiiswere stable for 168 h when stored at both tempera-tures. At room temperature, lymphocytes showed amaximum storage time of 168 h, basophils of 24 h,neutrophils of 2 h and monocytes of less than 2 hours.During storage in the refrigerator the maximum stor-age time of basophils and neutrophils decreased to 2 hand less than 2 h, respectively (Table 2).

In doubling the stability limits, maximum storagetime of neutrophils was increased to 4 h at room tem-perature and 2 h at 6 °C, basophils to 96 h and lym-phocytes to 168 h at 6 °C.

For neutrophils the median cell count was stable upto 2 h at room temperature and less than 2 h at 6 °C(Figure 3a) while for basophils median was stable upto 24 h at room temperature and to only 2 h at 6 °C

238 J Lab Med 2000; 24 (5): 236-242

M. Heins et al.: Influence of Storage Time and Temperature on Haematological Measurands using a Sysmex NE 8000 Analyser

Figure 2a-c Median deviation duringstorage of haematocrit, MCV andMCHC at room temperature and in therefrigerator.

130

120

110

100

Haematocrit [% of initial values]

0 2 4 24 48 72 96168 0 2 4 24 48 72 96168time[h] A

135130

125120115

110105100

95

Room temperature

MCV [% of initial values]

Refrigerator

105

100

95

90

85

80

0 2 4 24 48 72 96168 0 2 4, 24 48 72 96 168time[h]

Room temperature Refrigerator

MCHC [% of initial values]

TTJJo

75 —^^——^^^^^—^^^^^—^—^^——-—^^—0 2 4 24 48 72 96 168 0 2 4 24 48 72 96 168

time [h]Room temperature Refrigerator

J Lab Med 2000; 24 (5): 236-242 239

M. Heins et al. Influence of Storage Time and Temperature on Haematological Measurands using a Sysmex NE 8000 Analyser

Figure 3a-c Median deviation duringstorage of neutrophils, basophils andmonocytes at room temperature and inthe refrigerator.Horizontal dotted lines: stability limits;dot: median; bottom and top of shadedbox: 25th and 75th percentiles; bottomand top of bar: 10th and 90th per-centiles.

1101009080706050403020

Scutrophils (% of initial values]

0 2 4 24 48 72 96 168 0 2 4 24 48 72 96 168time [h]

Room temperature Refrigerator

1400

1200

1000

800

600

400

200

Basophils [% of initial values]

0 2 4 2 4 4 8 7 2 9 6 1 6 8 0 2 4 2 4 4 8 7 2 9 6 1 6 8 TTJtime [h] 13

Room temperature Refrigerator

1400

1200

1000

800

600

400

200

vlonocytes [% of initial values]

0 2 4 24 48 72 96 168 0 2 4 24 48 72 96 168time [h]

Room temperature Refrigerator

240 J Lab Med 2000; 24 (5): 236-242

M, Heins et al.: Influence of Storage Time and Temperature on Haematological Measurands using a Sysmex NE 8000 Analyser

(Figure 3b). Monocytes showed a dramatic increase inthe median cell count after 2 h at both temperatures(Figure 3c).

Discussion

Some 300 years ago the number of erythrocytes in ahuman body was calculated by Antony van Leeuwen-hoek and 150 years later microscopic blood cell cham-ber counting was developed. The most outstanding ad-vance in clinical haematology since then was the auto-mated blood cell count introduced by Wallace and JoeCoulter 50 years ago [1]. Subsequently these analyserswere further improved to examine additionally differ-ential white ceil count. Stability of haematological an-alytes is not just a current problem with modernanalyser systems, stability of differential white cellcount analysed by blood smear microscopy was con-sidered more than 25 years ago [3]. As the number ofcells counted is considerably higher, the imprecision ofautomated methods is lower in comparison to micro-scopic chamber counting and to blood smear analysis.There are two limits for analytical imprecision: 1/12 ofthe difference between the upper and lower referencelimits derived from the Recommendations of the Ger-man Medical Association [2] or 1/4 of the within-per-son diurnal variation according to the National Com-mittee for Clinical Laboratory Standards (NCCLS) [4].Studies on diurnal change of blood count analytes indi-cated that the latter did not meet the clinical require-ments [5]. Therefore, some authors suggested the useof 3 times the short-term variation data [6]. However,we felt that the limits for imprecision and stability de-rived from the recommendation of the German MedicalAssociation for each analyte better reflect clinical re-quirements than from intra-individual diurnal variation.Even new analysers did not meet the analytical goal of1/12 of the difference of reference limits for all ana-lytes [7-10], hence we used a wider limit of 1/6 the dif-ference for comparison (Tables 1 and 2).

Our results showed that the stability of the bloodcell count was, with the exception of platelet count,higher when samples were stored in the refrigeratorcompared to room temperature especially that of MCVand derived parameters such as haematocrit andMCHC. Even doubling the stability limits had only alimited effect on storage at room temperature. Storagein the refrigerator generally increased the maximumstorage time of most analytes'to 24 h for the 1/12 lim-its and to 72 h for the 1/6 limits. The poor stability ofMCV and the derived parameters of haematocrit andMCHC may be due to adenosine triphosphate deple-tion and change in the membrane lipids of red bloodcells which may lead to dramatic changes in rigidityand volume of red cells. So cell shrinkage due topreservation or osmotic changes which is detectedwith centrifugal haematocrit methods (better men-tioned as packed cell volume), is missed in automatedanalysers (6, 11).

Our results were in accordance with those of Cohleand co-workers [12]. They found no statistically sig-nificant change in analytes with storage at 4°C for72 h and blood stored at room temperature showed asignificant increase in MCV, haematocrit and MCHCin 24 hours. Hallwell et al. describe a maximum stor-age time of 72 h for all parameters at 4 °C and roomtemperature (9). But in 1975, Lawrence et al. alreadynote an increase of MCV after storage of bloodovernight at room temperature compared with storageat 4 °C [13]. They made further improvements to sta-bility by using acid citrate dextrose (ACD) as preserv-ative instead of EDTA. Springer and co-workers addedfresh EDTA blood 1:1· to a blood preservative calledCyto-Chex for the evaluation of control material. Itwas a urea-based reagent which preserved blood in anon-cross-linking, non-formalin manner. It increasedthe maximum storage time of the blood cell count tomore than 25 days [6].

For the white cell differential count, with samplestorage at room temperature eosinophils and lympho-cytes had the longest maximum storage time of 168 h,whereas neutrophils and monocytes of only 2 h andless than 2 h, respectively. Storage in the refrigeratordecreased the time of lymphocytes to 24 h, basophilsfrom 24 h to 2 h, and neutrophils to less than 2 h. Thepoor stability may be due to the inability of theanalyser to differentiate the neutrophil and monocytecell population in the scattergram (Figure 1). Similarresults were described by Barrels and Schoorl [14].They observed an apparent shift from granulocytes to-wards monocytes due to granulocyte activation anddegranulation. Additionally, blood smear microscopyindicated a change in leukocyte morphology towardsgreater homogeneity of the nuclei, greater vacuolisa-tion in the cytoplasm and more ragged and less welldefined cytoplasmic outlines. Flow cytometry analysisof CD62, 63 and 67 antigen expression showed a de-struction of azurophil granulocytes in the neutrophils.Depending on the algorithms, the patient or healthysubject population, other studies found a higher stabil-ity with storage in the refrigerator [14, 15]. In addi-tion, sample carryover of neutrophils were erroneous-ly recognised as monocytes in the subsequent sample[16]. These phenomena may yield correlation coeffi-cients of 0.1 [17] and 0.8 [15] between the monocytecount from the Sysmex analyser compared to bloodsmear microscopy. Other analysers showed a high ormoderate correlation with bloo'd smear microscopy ofmonocytes, also the correlation of the latter with flowcytometry of CD 14 and CD45 labelled cells was high[8, 18, 19].

Instability due to pre-analytical variation is not onlyencountered with automated haematology analysersystems but also with blood smear microscopy. This isbecause cell morphology changes within 3 hours ofstorage. After such time, an increase of band neu-trophils was observed and after 24 hours a decrease ofneutrophils and an increase of lymphocytes werefound [3].

J Lab Med 2000; 24 (5): 236-242 241

M. Heins et al.: Influence of Storage Time and Temperature on Haematological Measurancls using a Sysmex NE 8000 Analyser

In conclusion, our results showed that the generalstability limits according to the German Medical As-sociation were not met by all the parameters of bloodcell and white cell differential count. Taking this intoconsideration and the comparison data of doubling thelimits, we recommend that samples for haematologicalinvestigation should be stored at room temperature and.analysed within 4 h. Results for monocytes and neu-trophils should be interpreted with caution. For the fu-ture, perhaps the use of new stabilisers such as CytoChex or the use of more reliable detection methods formonocytes with for example CD 14 specific immuno-magnetic beads [16] could enhance the stability.

References1. Seeger HAT, Poppy U. Zellzähl- und Differenziergeräte. In:Boll I, Heller S, eds. Instand Schriftenreihe Band 7: PraktischeBlutzelldiagnostik. Berlin: Springer, 1990: 115-161.2. Neue Richtlinien der Bundesärztekammer. Qualitätsicherungder quantitativen Bestimmungen im Laboratorium. Dt Ärztebl1988; 11:697-712.3. Hausvvaldt C, Schröder U. Differentialblutbilder im EDTA-Blut. Dtsch Med Wschr 1973; 98: 2391-2397.4. National Committee for Clinical Laboratory Standards. Perfor-mance goals for the internal quality control of multichannel hema-tology analysers. Approved standard. Villaova, Pa: National Com-mittee for Clinical Laboratory Standards; 1996. NCCLS publicationH26-A.5. Richardson-Jones A, Twedt D, Swaim W, Gottfried E. Diurnalchange of blood count analytes in normal subjects. Am J ClinPathol 1996; 106: 723-727.6. Springer W, Prohaska W, Neukammer J| Hope A, von RueckerA. Evaluation of a new reagent for preserving fresh blood samplesand its potential usefulness for internal quality controls of multi-channel hematology analyzers. Am J Clin Pathol 1999; 111: 387-396.

7. Van Wersch JWJ, Bank C. A new development in haematolog-ical cell counting: The Sysmex NE-8000, automated for cell countand physical five-part leukocyte differentiation. J Clin Chem ClinBiochem 1990; 28: 233-240.8. Sachse C. Baudach A. Tille D, Avenarius HJ, Heller S, Ruby C,Henkel E. Direkte mechanische Leukozytendifferenzierung amCobas Argos Hämatologie-Analyzer im Vergleich mit dem BayerDiagnostic HI und dem Coulter STKS. Lab med 1995, 19: 9-21.9. Hallawell R, O'Malley C, Hussein S, Dauer RJ, Tanti M,Wooton AM, McGrath KM. An evaluation of the Sysmex NE-8000hematology analyzer. Am J Clin Pathol 1991; 96: 594-601.10. Butarello M, Bulian P, Temporin V, Rizzotti P. Sysmex SE-9000 hematology analyzer - performance evaluation on leukocytedifferential counts using NCCLS H20-A protocol. Am J Clin Pathol1997; 108:674-686.11. Neumann E. Automation im hämatologischen Laboratorium,In: Huber H, Pastner G, Gabi F, eds. Hämatologie und Immunhä-matologie. Berlin: Springer; 1983: 469-486.12. Cohle SD, Saleem A, Makkaoui DE. Effects of storage of bloodon stability of hematologic parameters. Am J Clin Pathol 1981; 76:67-69.13. Lawrence ACK, Bevington JM, Young M. Storage of blood andthe mean corpuscular volume. Am J Clin Pathol, 1975; 28: 345-349.14. B artel s PCM, Schoorl M. Time dependent increase of differen-tial monocyte count on the Sysmex NE-8000. Clin Lab Haem 1998;20: 165-168.15. Brigden ML, Page NE, Graydon C. Evaluation of the SysmexNE-8000 automated hematology analyzer in a high-volume outpa-tient laboratory. Am J Clin Pathol 1993; 100: 618-625.16. Theodorsen L. Evaluation of monocyte counting with two au-tomated instruments by the use of CD14-specific immunomagneticDynabeads®. Clin Lab Haem 1995; 17: 225-229.17. Devreese K, De Logi E, Francart· C, Heyndrickx B, Leroux-Röels JP & G. Evaluation of the automated hematology analyserSysmex NE-8000. Eur J Clin Chem Clin Biochem 1991; 29: 339-345.18. Warner BA, Reardon DM, Marshall DP. Automated haematol-ogy analysers: a four-way comparison. Med Lab Sei 1990; 47:285-296.19. Goossens W, Van Hove L, Verwilghen RL. Monocyte counting:Discrepances in results obtained with different automated instru-ments. J Clin Pathol 1991;44:220-227.

242 J Lab Med 2000; 24 (5): 236-242