The evaluation of a low platelet count from a peripheral blood smear

In a clinical hematology laboratory, the counting of platelets is increasingly being performed by electronic counting machines. The accuracy and precision of these machines is reported to be excellent for a normal platelet count in the range of 140,000 to 440,000 platelets per microliter. However, when an abnormally low platelet count of 70,000 platelets per microliter or less is obtained, it is good clinical practice to verify the count by another method before this value is reported to the physician. A traditional method to confirm any platelet count is to estimate the number of platelets from a stained peripheral blood smear. Since there is no known published report of a statistical comparison between these two methods in the low platelet count range, it seemed reasonable to determine whether the slide method of estimating low platelet counts is a valid procedure to verify machine counts. Platelet counts of 103 thrombocytopenic patients were evaluated by electronic machine counts, manual counts, and slide estimations of peripheral blood smears. Either platelet rich plasma or whole blood as the specimen source were first counted with the electronic counter. Whenever an electronic count of 70,000 platelets per microliter or less was obtained, a manual count and a slide estimation were also performed. The results obtained from the three different methodologies were compared by linear regression fits and corresponding correlation coefficients were calculated. The data seemed to indicate that the slide estimation of platelets can be a useful tool to verify a low platelet count.