Heparin-associated thrombocytopenia: case report and prospective study.

After observing a patient with heparin-induced thrombocytopenia we prospectively recorded the incidence of thrombocytopenia associated with heparin treatment by measuring the platelet count every second day in 166 patients given therapeutic heparin for various thromboembolic disorders, and 51 patients given low-dose heparin prophylaxis. A platelet count below 100 x 10(9)/litre developed in nine patients (5.4%) during or soon after full-dose heparin therapy, and in one patient given low-dose heparin. Careful clinical review suggested that heparin was either the most likely cause of a contributing cause of thrombocytopenia in 5/166 patients (3.0%) receiving therapeutic heparin and none of the patients who received prophylactic heparin. Associated laboratory studies suggest that heparin-initiated platelet aggregation in vivo is a useful marker for heparin-induced thrombocytopenia.