Heparin-induced thrombocytopenia.

Thrombocytopenia is a common adverse effect of heparin therapy which may occur with bovine or porcine heparin when it is given either intravenously or subcutaneously. There are two main clinical types: a common, mild thrombocytopenia of early onset, probably due to the platelet proaggregating effect of heparin itself and a severe delayed-onset thrombocytopenia caused by an immune mechanism. Patients with mild thrombocytopenia due to heparin are usually asymptomatic. However, patients with severe thrombocytopenia may develop thromboembolic complications which often result in disastrous sequelae such as limb gangrene necessitating amputation or death. The thromboembolic complications may be attributed to an IgG heparin-dependent platelet antibody which induces thromboxane production and platelet aggregation. The diagnosis of heparin-induced thrombocytopenia is made clinically and may be confirmed by the demonstration of the heparin-dependent antibody in vitro by platelet aggregometry or [14C] serotonin release. In patients with delayed-onset severe thrombocytopenia, heparin should be stopped and warfarin commenced. Antiplatelet drugs and/or dextran may also be added. Recently low molecular weight heparin has been used with some success. Conversely, heparin may be continued in patients with asymptomatic mild thrombocytopenia provided the patients' condition and the platelet counts are closely monitored.