Removal by white cell‐reduction filters of activated platelets expressing CD62

White cell (WBC)-reduction filters that remove more than 99 percent of the WBCs from platelet concentrates are rapidly being introduced into routine use. Using activation-dependent monoclonal antibodies and flow cytometry, platelet activation was evaluated before and after WBC reduction in 10 platelet concentrates prepared manually from whole blood obtained from five male and five female regular volunteer blood donors. In general no significant increases were found in platelet activation markers after WBC reduction using filters. However, if platelets were activated during preparation, increased numbers of platelets were found expressing the activation marker CD62, and this correlated with the decrease in the platelet count after WBC reduction. These observations may explain increased platelet loss following WBC reduction in some platelet components.