Antibody Test Validates Diagnosis of Heparin-Induced Thrombocytopenia

An immunoassay has been validated as diagnostic for heparin-induced thrombocytopenia (HIT), the most important and most frequent drug-induced, immune-mediated type of platelet deficiency.

The enzyme-linked immunosorbent assay (ELISA) detects antibodies in the blood to the heparin/platelet factor 4 (PF4) complexes. It is now known that antibodies associated with Type II HIT recognize sites on "platelet factor 4” (PF4) that are created when PF4 is complexed with heparin or another linear polyanionic compound such as polyvinyl sulfonate (PVS).

The PF4 Enhanced Solid Phase ELISA is manufactured by GTI diagnostics, (Waukesha, WI, USA), who recommends the use of a high-dose heparin confirmatory procedure to improve the specificity of the ELISA. In this particular assay, inhibition of a positive ELISA result by 50% or more in the presence of excess heparin (100 U/mL) is considered confirmatory of heparin-dependent antibodies.

In a retrospective study carried out at the Duke University Medical Center (Durham, NC, USA), 115 patients with anti-heparin/PF4 antibodies, detected by the ELISA, were classified as clinically HIT-positive or HIT-negative, followed by confirmation with excess heparin. Heparin-induced thrombocytopenia, was defined as at least a 30% decline in the platelet count, with a platelet count increase after heparin cessation; timing of platelet counts fall between 4 and 14 days after heparin exposure or within 24 to 48 hours if heparin exposures was recent (within the last 100 days); and lack of other, predominant causes of thrombocytopenia. A multivariate logistic regression model was fitted to estimate relationships between patient characteristics, laboratory findings, and clinical HIT status. This model was validated on an independent sample of 97 patients with anti-heparin/PF4 antibodies.

The optical density (OD) value of anti-heparin/PF4 antibodies detected by ELISA correlated with both a clinical diagnosis of HIT and a higher incidence of heparin-induced thrombocytopenia and thrombosis (HITT).

The authors of the study concluded that the confirmatory assay with excess heparin is a valuable adjunct in the laboratory diagnosis of HIT. With a multivariate statistical model, the probability of being clinically HIT positive can be estimated for an individual patient with both the maximal anti-heparin/PF4 OD value and the confirmatory assay result. Accurate predictions of the probabilities of HIT will enable clinicians to initiate appropriate therapy rapidly, thereby reducing complications of HIT. The results of the study were published in the September 2010 issue of the journal Blood.

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