Automated Optical Counting of Blood Platelets

We have evaluated the use of an optical particle counter to perform automated platelet counts on whole blood. The erythrocytes were lysed by dilution of whole blood with 2 M urea and the remaining platelets and leukocytes were enumerated by a darkfield microscope optical system that detects light diffracted by them. A suspension of fixed human platelets available commercially was highly satisfactory for standardization. The method gave accurate and reproducible platelet counts, comparable with those of electronic particle counting on venous blood and substantially more reliable platelet counts on thrombocytopenic and finger-puncture blood samples. We believe that errors resulting from the electronic method were caused by technical difficulties of sample handling and not to an intrinsic error in electronic counting. By using the automated optical method we found no significant difference between the platelet counts of capillary and venous blood, although capillary platelet counts had twice the variability of venous counts. The optical technique has important advantages over electronic platelet counting, and its superiority appears to be due to the ability to count platelets in diluted whole blood rather than in plasma. It should prove especially useful in performing the large numbers of platelet counts on thrombocytopenic and finger-puncture blood samples that are increasingly important for management of patients receiving chemotherapy.