An improved apparatus and method for performing activated clotting time tests, including their use for evaluating platelet functionality of a blood sample. The apparatus includes a plurality of test cells. Each of the cells comprises a heparin-inactivating agent, an anticoagulant agent, and a clotting reagent. At least one of the test cells further comprises a platelet activating agent. The clotting time is determined for each of the aliquot portions, and the relative clotting times of the aliquot portions in the cells are determinative of the platelet functionality of the sample. The method comprises the steps of combining a heparin-inactivating agent, an anticoagulant agent, a clotting reagent, a platelet activating agent, and the sample of blood to be tested to form a test mixture. The platelets of the sample are activated by agitating the test mixture, and the activated clotting time test is terminated upon detecting a predetermined change in a property of the test mixture. The activated clotting time of the sample of blood is calculated based on the elapsed time.