Immunoplatelet counting: a proposed new reference procedure

Given the high degree of interoperator error and poor precision of manual p latelet counting, it has recently been proposed that an immunoplatelet coun ting method could become the new reference procedure. Platelets are identif ied immunologically with a suitable monoclonal antibody, and the platelet c ount is derived from the ratio of fluorescent platelet events to collected red blood cell (RBC) events that are also counted by a reliable and calibra ted standard impedance counter (RBC ratio). In this study, we have set up a rapid and simple method for immunoplatelet counting and simultaneously com pared the RBC ratio with the bead ratio derived from two different preparat ions of commercial calibration beads (Trucount and FlowCount beads). Compar ison of the level of imprecision of the RBC ratio with either the manual co unt or bead ratios revealed a superior coefficient of variation of < 5% eve n in samples with a platelet count <20 x 10(9)/I. The RBC ratio correlated extremely well with the existing manual phase reference method (r(2) = 0.93 ) and especially well with three different commercial impedance counters an d a dual-angle optical counter (r(2) = 0.98-0.99). However, at <100 x 10(9) /I, the correlation of the RBC ratio with the dual-angle optical count (ADV IA 120) (r(2) = 0.96) was superior to all impedance counters. This suggests that automated optical counting methods may be more accurate al;determinin g platelet counts in thrombocytopenic samples. As the RBC ratio Is rapid, c heap and relatively easy to perform, we propose that this method could repl ace the manual count as a new international reference method.