[Automated hematology analysers and spurious counts Part 2. Leukocyte count and differential].

Using hematology analysers, white blood cell (WBC) counts and differentials (either three or five parameters) may be ascertained after Red Blood Cell (RBC) lysis and analysis using either impedance and/or optical (laser) technology. Cells or particles not destroyed by lytic agents are enumerated as WBC: abnormal particles may be observed on WBC differential scattergrams, if performed, appearing as a variable number of dots, which location may help to ascertain the nature of the abnormality. Spuriously low WBC counts are rare, mainly related to agglutination in the presence of ethylenediamine tetra-acetic acid. Cryoglobulins, lipids, insufficiently lysed RBC, erythroblasts and platelet aggregates are common situations increasing WBC counts. So far, many current high performance analysers clearly identify and enumerate erythroblasts now. In normal patients and in reactive disorders automated differential provides true and accurate results. However, failure to enumerate accurately basophilic granulocytes and monocytes is not uncommon. Using myeloperoxidase cytochemistry to ascertain differential may lead to slide review if the enzyme expression is low or absent. Low number of abnormal cells (blasts, lymphoma cells, dysplastic granulocytes) may be missed, more frequently if leukopenia is present. In many but not all instances flagging and/or an abnormal WBC differential scattergram will alert the operator. Although these flags are sensitive enough to allow the identification of several spurious counts, only the most sophisticated analysers have optimal flagging, whereas more simple ones, especially those without a WBC differential scattergram, do not demonstrate the same sensitivity for the detection of abnormal results.