Correlation Of Heparin Confirmatory Test (HCT) With Optical Density (OD) and Serotonin Release Assay (SRA) In The Diagnosis Of Heparin Induced Thrombocytopenia (HIT)

Heparin-induced thrombocytopenia (HIT) is a potentially fatal, thrombotic complication of heparin therapy mediated by antibodies to complexes between platelet factor 4 (PF4) and heparin. Accurate and rapid diagnosis with prompt commencement of therapy are imperative as delays in treatment are associated with an increasing risk of thrombosis, amputation, or death. On the flip side, initiation of therapy with direct thrombin inhibitors without laboratory confirmation carries a significant risk of bleeding. Two types of laboratory tests are available for detection of these antibodies: a widely available immunoassay (ELISA), which is very sensitive to the presence of anti-heparin/PF4 antibodies, but is less specific to the clinical syndrome of HIT because of detection of non-pathological antibodies. The Serotonin Release Assay (SRA) is a functional assay that is now considered the gold standard for confirmatory diagnosis of HIT due to its high specificity. However, the downside of SRA is the cost involved, limited availability and a turnaround time of 5-7 days. As such, a heparin confirmatory test (HCT) with excess heparin has been in use since mid 2011 on positive ELISA samples in our laboratory to improve test specificity. This test is more cost and time efficient, with a turnover time of no more than 48 hours. As noted in prior studies, inhibition of a positive ELISA result by 50% or more in the presence of excess heparin is considered confirmatory of heparin-dependent antibodies. Likewise a negative confirmatory test is defined as a decrease of 50% or less in antibody binding in the presence of heparin.

a) Correlation of Heparin Confirmatory test (HCT) with strength of HIT ELISA, vis-à-vis optical density (OD) of 0.4 - 0.99 and OD of >/= 1.0.

b) Correlation of HCT results with SRA, to see if the latter can be replaced by the heparin confirmatory test.

A retrospective chart review of adult patients hospitalized at our institution with suspected HIT from July 2011 until January 2013 was done. There were 101 such patients. All patients who had a positive HIT ELISA, then had HCT as per our standard lab practice, with an SRA test done for diagnosis/confirmation of HIT, as per standard clinical practice. Historically, the major strength of SRA assay is its specificity. The optical density on HIT ELISA and SRA results were then compared with the Heparin Confirmatory test to establish clinical significance.

Of the 101 patients tested for HIT ELISA, 49 were positive. HCT and SRA were performed on all 49 samples, 1 out of which was reported as indeterminate. Hence 48 samples were used for primary analysis, comparing HCT to the OD as well as the SRA results. Out of 48 patients, 6 had positive SRA with Heparin inhibition of >50% (sensitivity 6/6 = 100%). Remaining 42 patients had negative SRA, 7 out of which had Heparin inhibition of <50% (specificity 7/42 = 16.6%). All 7 patients with a negative HCT had a negative SRA, making the negative predictive value of the HCT 100%; however positive predictive value was only 14.6% (6/41). There was no correlation between the OD and Heparin Confirmation test.

Although there is data suggesting that there might be some value to the Heparin Confirmation test, we were unable to show a significant correlation between HCT and OD or between HCT and SRA. The prospect of having a cost effective and rapid assay for laboratory confirmation of HIT will always be a relevant need. We feel that a larger, prospective study should be conducted to definitively assess the relationship between HCT and SRA.

No relevant conflicts of interest to declare.