Interpreting HIT antibody results

When evaluating patients for heparin induced thrombocytopenia (HIT) we commonly order two tests: antibodies to heparin-platelet factor 4 complex and a serotonin release assay (SRA). Both tests are sensitive but the antibodies are nonspecific. Invariably the antibodies come back first. If they come back positive you're faced with a dilemma: should you go ahead and treat or wait for the more specific SRA result? According to a study in the Journal of Thrombosis and Haemostasis the magnitude of positivity of the antibody result, expressed as optical density, correlates well with strong SRA positivity, thus predicting true HIT:

Results: For patient sera investigated for HIT antibodies, a weak-positive result (0.40–less than 1.00 OD units) in either EIA indicated a low probability (less than or epual to 5%) of a strong-positive SRA; the risk increased to approximately 90% with an OD greater than or equal to 2.00 units. Quantifying the EIA–SRA relationship for 1553 referred patient sera, we found that for every increase of 0.50 OD units in the EIA–IgG, the risk of a strong-positive SRA result increased by OR = 6.39 [95% confidence interval (CI), 5.13, 7.95; P less than 0.0001]. For every increase of 1.00 OD units in the EIA–IgG, the risk increased by OR = 40.81 (95% CI, 26.35, 63.20; P less than 0.0001). Conclusions: The probability of HIT antibodies (strong-positive SRA result) inferred by a positive PF4-dependent EIA varies considerably in relation to the magnitude of the EIA result, expressed as OD values. In our laboratory, the probability of HIT antibodies being present reached greater than or equal to 50% only when the OD level was greater than or equal to 1.40units.

In the concluding paragraph the term “HIT antibodies” means truly functional antibodies, thus predictive of HIT.