A method for screening EDTA(K3) blood samples for platelet function is described. The general availability of whole blood platelet counters in the clinical laboratory suggested their use to measure platelet aggregation. To simplify the collection and storage of blood samples, a method utilizing EDTA collected blood is described. Addition of calcium and citrate restores platelet function to blood samples even when stored at room temperature for several hours. Thus, the blood sample used to assay RBC, WBC and platelet parameters may also be employed in platelet aggregation. A comparison of this method with aggregometry on 120 subjects indicated a similar response to arachidonate, collagen, ristocetin and ADP.