Clinical significance of a borderline titer in a negative ELISA test for heparin-induced thrombocytopenia

We studied the usefulness of repeated enzyme-linked immunosorbent assay testing for heparin-induced thrombocytopenia (HIT) when the initial test result was negative and sought to determine whether the titer of the initial negative result correlated with the likelihood of obtaining a positive test result in repeated testing. We divided 150 patients who underwent HIT testing into 3 groups (50 patients each): (1) very low titer negative (0.0%-33.3% of the threshold for a positive test); (2) low titer negative (33.4%-66.6% of the threshold); and (3) high titer negative (66.7%-99.9% of the threshold). Among the patients who underwent a repeat test, 5% (1/20) of group 1 patients, 13% (4/32) of group 2 patients, and 43% (13/30) of group 3 patients tested positive in the repeat test (P = .0026). Thus, nearly half of patients with initially negative HIT test results had positive results in the repeat test if the negative titer was 66.7% or more of the threshold. If laboratories report the HIT titer, rather than just negative or positive, the titer might help clinicians predict which patients have HIT despite a negative initial test, and the overall sensitivity for diagnosing HIT might be improved.