Interpretation

Results are reported as:

1. Heparin-induced thrombocytopenia (HIT) enzyme-linked immunosorbent assay (ELISA) optical density (OD)

2. Heparin inhibition (%)

3. Interpretation.

Typical patterns of results and interpretations are depicted in the following table. Interpretive comments will also accompany test reports, when indicated.

Table. Results and Interpretation

A negative result of testing for human platelet factor 4 (H/PF4) antibodies has about a 90% negative predictive value for exclusion of clinical type II HIT (HIT-II).

As up to 10% of patients with clinical HIT may have a negative H/PF4 antibody ELISA result, a negative H/PF4 antibody ELISA result does not exclude the diagnosis of HIT when clinical suspicion remains high. A functional assay for HIT antibodies (eg, heparin-dependent platelet aggregation or serotonin release assay) may be helpful in these circumstances. Call 800-533-1710 for ordering information.

A positive result is indicative of the presence of H/PF4 complex antibodies. However, this test's specificity is as low as 20% to 50% for clinical diagnosis of HIT, depending on the patient population studied. For example, up to 50% of surgical patients and up to 20% of medical patients treated with heparin may develop H/PF4 antibodies as measured by ELISA, and only a small proportion (1%-5%) develop clinical HIT. Accordingly, this test does not confirm the diagnosis of HIT-II. The diagnosis must be made in conjunction with clinical findings, including evaluation for other potential causes of thrombocytopenia.

The presence of H/PF4 antibodies likely increases the risk of clinical HIT, with risk probably partly dependent on associated medical and surgical conditions, but currently there is little data about relative risk of HIT in various populations with positive tests for H/PF4 antibodies.