Coagulation and Transfusion Medicine / IMPROVED LABORATORY CONFIRMATION

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. Thrombocytopenia associated with heparin occurs in a moderate form within a few days of the onset of treatment and spontaneously reverses under continued therapy. If thrombocytopenia occurs 5 to 15 days after the onset of heparin therapy, heparin-induced thrombocytopenia (HIT) is most severe; the platelet count drops to less than 100 ·103/μL (100 ·109/L) or decreases by more than 50% of the pretreatment value, thromboembolism occurs, and the platelet count normalizes only after withdrawal of heparin.1,2 The diagnosis of HIT may be difficult on the basis of clinical symptoms alone, especially in patients with other diseases that may induce thrombocytopenia. Laboratory confirmation of HIT therefore is required using biologic or antigen assays. Biologic assays include the carbon 14–serotonin release assay (14C-SRA),3 the heparin-induced platelet activation assay,4 and the flow cytometric assay.5 Antigen assays determine the platelet factor 4 (PF4)–heparin complex by enzymelinked immunosorbent assay,6 heparin-induced IgG by biotinlabeled PF4,7 and fluorescent-labeled heparin.8,9 The sensitivity of all assays ranges from 80% to 98%. Antigenic and biologic assays have been compared in several studies to define the sensitivity and specificity.10-16 Combination of the results of two assays to enhance the laboratory confirmation of HIT has been suggested.14 An improvement of the sensitivity from 56% to 85% by using a combination of assays has been reported in HIT without thrombosis17 but has not been studied in HIT with thromboembolism. HIT may be understood as an immune-complex disease with development of antibodies within 8 to 10 days of starting heparin treatment. The synthesized antibodies result in formation of antigen-antibody complexes, which facilitate removal of the antigen by the reticuloendothelial system. Free Coagulation and Transfusion Medicine / ORIGINAL ARTICLE Am J Clin Pathol 2001;115:432-438 433 © American Society of Clinical Pathologists antibody can be detected after clearance of the immune complexes.18 Based on these considerations, we hypothesized that in patients with HIT with negative test results at day 1 of blood sampling, the antibody may appear in the first few days after disappearance of the antigen. The time course of PF4-heparin antibodies during 50 days has been reported in 1 case of HIT,19 and another patient was monitored for 2 weeks using the 14C-SRA.20 In the present article, we report on the time course of biologic and antigen findings in patients with HIT using the fluorescence-linked immunofiltration assay (FLIFA)9 and the 14C-SRA.3 Materials and Methods