Serological investigation of patients with a previous history of heparin-induced thrombocytopenia who are reexposed to heparin.

[heparin-induced thrombocytopenia]

Heparin reexposure despite a history of previous heparin-induced thrombocytopenia (HIT ) can be appropriate if platelet -activating antibodies are no longer detectable . We determined the frequency , timing , and magnitude of the antiplatelet factor 4 (anti-PF 4 )/heparin immune response (by serotonin-release assay [SRA ] and enzyme-immunoassay [EIA ]), and the frequency of recurrent HIT in 20 patients with previous HIT reexposed to heparin 4 .4 years (mean ) post-HIT ; 17 patients were given heparin intraoperatively (without postoperative heparin ) for cardiac /vascular surgery . One patient developed recurrent HIT beginning 7 days after cardiac surgery , with newly regenerated HIT antibodies exhibiting strong heparin-independent platelet -activating properties . Intraoperative heparin induced EIA seroconversion in 11 /17 (65 %) patients (immunoglobulin G [IgG ]>IgA >IgM ) and SRA seroconversion in 8 /17 (47 %), whereas none of 3 medical patients reexposed to heparin developed seroconversion . Anti-PF 4 /heparin IgG became detectable at day 7 (median ), ie , no sooner than observed in typical-onset HIT . The high proportion of SRA positivity among EIA-seroconverting patients (8 /11 [73 %]) suggests that patients with previous HIT may be especially predisposed to forming recurrent antibodies with platelet -activating properties . We conclude that among patients with a previous history of HIT who are reexposed to intraoperative (but not postoperative ) heparin , the risk of recurrent HIT appears to be low , but is possible if antibodies with strong heparin-independent platelet -activating properties are formed .