Algorithm for workup of thrombocytopenia based on the observation of the peripheral blood film. The thrombocytopenia of ITP is by definition an isolated hematologic abnormality, although anemia of pregnancy or iron deficiency may also be present. Other than an occasional large form, platelets should appear normal. Consistently large and/or hypogranular platelets may suggest congenital thrombocytopenia. Uniformly small platelets are typically found in Wiskott-Aldrich syndrome. The presence of targeted red blood cells, schistocytes, macrocytosis, or spherocytes may be clues to liver disease, thrombotic microangiopathy, nutritional deficiencies, or autoimmune hemolysis. A direct antiglobulin test is necessary to rule out complicating autoimmune hemolysis (Evans syndrome). DITP indicates drug-induced thrombocytopenia; HIT, heparin-induced thrombocytopenia; and GT, gestational thrombocytopenia.