Patients with HIV commonly have coagulation deficiencies. Most of the deficiencies are related to reduced amounts of protein S or to thrombocytopenia. We have identified therapies that may be effective by increasing protein S and C levels. We have developed an assay that can differentiate between thrombocytopenia caused by increased destruction of platelets and that caused by decreased production of platelets. This assay allows the clinician to identify the potential cause of the platelet defect and treat the platelet abnormality by specific therapies; e.g., platelet transfusion, intravenous gamma globulin, and the use of vincristine and recombinant thrombopoeitin.