An interlaboratory study of a candidate reference method for platelet counting

A multinational interlaboratory task force explored the important variables of platelet reference counting and developed a candidate flow cytometric r eference method based on the RBC/platelet ratio. A multicenter comparison w as performed to determine whether the method met the necessary criteria and was precise enough to be recommended as a new reference method. Each labor atory analyzed serial dilutions of normal speciemens, stabilized material, and at least 60 patient specimens with a range of platelet counts from 1 to 400 X 10(3)/muL (1 -400 X 10(9)/L). Pooled analysis of the serial dilution s showed that RBC-platelet and RBC-RBC coincidence events became negligible at sufficiently high dilutions (ie, >1:1,000). All laboratories demonstrat ed excellent intra-assay and acceptable interlaboratory precision. Two anti bodies (CD61 and CD41) were used for identifying platelets and individually gave acceptable results, but in a minority of samples, staining difference s were observed. The optimum method thus uses a double-labeling procedure w ith a final dilution factor of 1:1,000. The study demonstrated that this me thod meets the criteria for a reference platelet count.