We studied whether laboratory confirmation of heparin-induced thrombocytopenia (HIT) can be improved after antigen clearance by determining free antibody and combining the results of an antigenic and a biologic assay. Blood samples taken over 40 days in 14 patients with HIT with thromboembolism underwent fluorescence-linked immunofiltration and the carbon 14-serotonin release assays. Of the 14 patients, 11 showed positive results in both assays at day 1 after stopping heparin. The 3 patients with negative results seroconverted in one or both assays during the subsequent 7 days. Combining the positive results of the assays increased the sensitivity from 85% at day 1 to 100% at day 7. Assay results became negative in all patients within 40 days. The platelet count normalized between days 2 and 9 after withdrawal of heparin. It is assumed that the free antibody can be detected after withdrawal of heparin and after clearance of the platelet factor 4-heparin complex in patients with HIT.