Cryoglobulins Interfere with Platelet Counts by Optical and Impedance Methods but not with the CD61 Immunoplatelet Count

Automated complete blood counts (CBCs) use supplementary warnings (flags) to provide alerts for the existence of hematologic abnormalities (1). We observed on several occasions platelet-count interference by cryoglobulins, where the impedance and optical results were in error to a similar extent and did not trigger a delta alert. Platelets usually are counted by impedance analysis or by optical procedures (refractive and laser light-scattering properties). The CELL-DYN CD4000 system (Abbott Laboratories) simultaneously provides both and offers a further optional approach using immunoplatelet analysis (2)(3). In the latter method, a fluorescein isothiocyanate-labeled monoclonal antibody (CD61; Clone RUU-PL 7F12, IgG1 subclass) binds the glycoprotein (Gp)IIIa part of the GpIIb/IIIa receptor subunit (3). This glycoprotein is expressed by all human platelets, both resting and activated, but not by erythrocytes or leukocytes. The immunochemical platelet count is largely unaffected by sample interferences and allows determination of platelet counts to <1 × 109/L (4). The analytical performance is virtually identical to that reported for alternative flow cytometric immunomethods (5). We recently encountered four patients with cryoglobulinemia in whom erroneously high platelet counts were obtained by both optical and impedance platelet counts. Patient 1 had lymphoplasmacytoid lymphoma. Because the CD4000 analyzer flagged the presence of variant lymphocytes, a peripheral blood smear was examined. …