Screening for pseudothrombocytopenia caused by in vitro platelet clumping has been performed in 45,000 subjects attending a general hospital. In our region, the observed prevalence of EDTA-induced pseudothrombocytopenia in blood samples with an initial platelet count below 150 x 10(9)/l was estimated to amount to 0.1%. EDTA-induced pseudothrombocytopenia was confirmed by detection of platelet aggregates by means of microscopic evaluation from the blood smear. In routine investigations, pseudothrombocytopenia could be highly suspected when the Sysmex NE 8000 showed characteristic peculiarities in the white blood cell (WBC) scattergram and histogram. Platelet aggregation is avoided in such cases by the use of citrate as an anticoagulant instead of EDTA. Pseudothrombocytopenia was detected in 46 subjects. As a screening test for pseudothrombocytopenia, increased cut-off values derived from the WBC histogram demonstrated 90% sensitivity and 100% specificity. Automated flagging for platelet clumps, deviations reflecting MPV, or PDW abnormalities revealed lower scores with respect to sensitivity.

Unrecognized anticoagulant-induced platelet (PLT) aggregation, leading to pseudothrombocytopenia and concomitant pseudoleukocytosis, can have serious clinical consequences. It can be readily recognized by inspecting conventional blood smears or the white blood cell histograms generated by modern blood cell counters. Blood specimens from twenty patients with known EDTA-induced platelet aggregation were consecutively drawn into three other anticoagulants (using Vacutainer tubes) and processed in a three-part differential Coulter Counter S Plus IV. PLT aggregation is shown to be generally induced by Li-heparin but much less frequently by citrate solutions. Prevention is almost invariably achieved by acid citrate dextrose (ACD). To that end approximately 2.5 mL of sterile ACD was aseptically injected beforehand through the stoppers of plain 5 mL Vacutainer tubes without breaking the vacuum. After blood drawing, correction for dilution was made by comparing hemoglobin values in EDTA and ACD.

SUMMARY.After some brief remarks on counting chambers, references to the ICSH-recommended haemoglobin-determination are given. The microhaematocrit of normal blood is advocated as a potential routine calibration method. Comments are given on discrepancies between centrifugal and flow haemocytometry haematocrits of abnormal and artificial bloods. Flow haemocytometry instruments are classified into analogue and digital instruments or into electrical and optical instruments. Their hydrodynamic properties are discussed. The principles and problems of electrical and optical cell counting and sizing are dealt with. The importance of the refractive index and of flow-induced cell shape changes for the MCV determinations is stressed. It is argued that MCV and haematocrit values are exaggerated at both low and high values and consequently MCHC is erroneously constant. Various prevailing red cell distribution width (RDW) and platelet distribution width (PDW) definitions bring about considerable confusion. The major features of the counting and sizing of white blood cells and platelets are described.

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