Five studies are important to the diagnosis of bleeding disorders: bleeding time (BT) (Simplate), platelet count, activated partial thromboplastin time (aPTT), prothrombin time (PT), and thrombin time (TT). If the platelet count alone is low, the cause is usually peripheral destruction of platelets, immunothrombocytopenia, or an abnormality of bone marrow production. An abnormal bleeding time alone suggests a platelet aggregation defect that is most likely due to medication. When the aPTT is the only abnormal test and the patient has a definite history of bleeding, one of the hemophiliac states is present. An abnormal PT, with or without an abnormal aPTT but with normal results in the other three tests, indicates an abnormal reduction in the vitamin K-dependent clotting factors (II, VII, IX, X) or factor V. When the TT is abnormal, disseminated intravascular coagulation, the presence of plasma heparin, or a hepatopathy should be suspected.