The Clinical Utility of the Heparin Neutralization Assay in the Diagnosis of Heparin-Induced Thrombocytopenia

Abstract

Heparin-induced thrombocytopenia (HIT) remains diagnostically challenging. Immunoassays including PF4/heparin enzyme-linked immunosorbent assay (ELISA) have high sensitivity but low specificity. Whether the heparin neutralization assay (HNA) improves the diagnostic accuracy of the PF4/heparin ELISA for HIT is uncertain. In this study, to assess its clinical utility and evaluate whether it improves the diagnostic accuracy for HIT, we implemented HNA in conjunction with PF4/heparin ELISA over a 1-year period. A total of 1194 patient samples were submitted to the laboratory for testing from December 2015 to November 2016. Heparin neutralization assay alone is a poor predictor for HIT, but it has high negative predictive value (NPV): Cases with %inhibition <70% are always negative for serotonin release assay. It improves the diagnostic positive predictive value (PPV) of ELISA without compromising sensitivity: ELISA optical density (OD) !1.4 alone has a sensitivity of 88% (14/16) and a PPV of 61% (14/23); with HNA %inhibition !70%, the sensitivity remains 88% (14/16) and PPV is 82% (14/17). 4Ts score correlates with ELISA OD and predicts HIT; the predictive accuracy of 4Ts score is further improved by HNA. Interestingly, HNA %inhibition of <70% correlates with low 4Ts scores. Based on its high NPV, HNA has the potential to facilitate more timely and accurate HIT diagnosis.

A novel diagnostic algorithm for heparin‐induced thrombocytopenia

“…The 4T score calculation was performed independently by 2 different reviewers based upon a retrospective chart review as described previously . The reviewers were blinded to the results from PF4‐heparin ELISA, HNA, and SRA.…”

“…Anti‐PF4/heparin, IgG‐specific ELISA (Immucor‐GTI diagnostics; Norcross) was performed according to the manufacturer's instructions as described previously . OD values equal to or >0.40 were considered as positive.…”

“… We previously showed that HNA %inhibition <70% essentially excludes HIT …”

“…Both heparin‐PF4 ELISA and the serotonin‐release assay (SRA) are widely used for HIT diagnosis. The PF4‐heparin ELISA is sensitive but not specific; while the SRA is more specific and considered as the “gold standard” test for HIT testing, it is only available in a few reference laboratories so the turnaround time is several days. Therefore, replacing SRA with an alternative assay that is easy‐to‐perform or a simple accurate diagnostic algorithm has been a goal for clinical laboratories …”

“…We previously have shown that HNA has excellent negative predictive value (NPV) and can improve the diagnostic specificity for HIT . It has been also shown that the 4T score has good pretest NPV but variable positive prediction value (PPV) for HIT . However, it is uncertain whether a novel algorithm can be developed that has sufficiently high diagnostic accuracy such that the SRA is no longer necessary.…”