Heparin-induced thrombocytopenia

Thrombocytopenia is a known adverse reaction occurring in some of the patients receiving heparin. Two types of heparin-induced thrombocytopenia (HIT) have been described. HIT type I is a mild thrombocytopenia probably caused by a direct proaggregating effect of heparin and occurs during the first few days of heparin treatment. No specific treatment is necessary. HIT type II is a severe thrombocytopenia mediated by an immunologic mechanism where antibodies against heparin/platelet factor 4 (PF4) complexes play a major role. Thrombocytopenia usually commences 4-14 days after the onset of heparin administration. The incidence of HIT type II is below 3% and even lower when low-molecular-weight heparin is used. The possible occurrence of life-threatening thrombembolic events may complicate the course of HIT type II. Diagnosis of HIT type II by clinical features alone is often difficult. A few laboratory tests are pertinent for diagnosing HIT type II including the C-14-serotonin assay, the heparin-induced platelet activation test and the heparin/PF4 ELISA. Immediate cessation of heparin administration is essential in the treatment of patients with HIT type II, if need be even without waiting for the result of the antibody search test. Several alternatives of anticoagulation for patients with HIT type II have been investigated in the past. Danaparoid-sodium as well as recombinant hirudin have shown promising results when used for this purpose.