Platelet Counting by the RBC/Platelet Ratio Method

The International Council for Standardization in Haematology (ICSH) and the International Society of Laboratory Hematology (ISLH) recommend the counting of specifically labeled platelets relative to the RBCs with a fluorescence flow cytometer, together with an accurate RBC count determined with a semiautomated, single-channel aperture-impedance counter as a reference method for the enumeration of platelets. Fresh EDTA-anticoagulated venous blood specimens are measured within 4 hours of the draw. The specimen is prediluted (1:20) and the platelets labeled with two monoclonal antibodies specific to a cluster of differentiation common to all platelets. A final 1:1,000 dilution is made and at least 50,000 events with a minimum of 1,000 platelet events are counted with a flow cytometer to determine the RBC/platelet ratio. The platelet count is then calculated from this ratio and the RBC concentration of the original blood specimen.