Heparin Induced Thrombocytopenia: A Laboratory Comparison of the IgG Specific EIA Vs Polyspecific EIA

Introduction:Heparin-induced Thrombocytopenia (HIT) is a potentially life/limb threatening immune mediated pro-thrombotic syndrome triggered by heparin/platelet factor 4 (PF4) complexes. Based on clinical probability, if indicated, initial laboratory testing for HIT is typically based on enzyme immunoassay (EIA) to detect anti-PF4/heparin(H) antibodies with or without supplemental functional assays, the serotonin release assay (SRA) is considered to be the gold standard. EIAs detect either polyspecific (IgG, IgA, IgM) or IgG specific antibodies, the latter are recommended by ISTH/SSC, and should have optimal performance characteristics (sensitivity/specificity/negative and positive predictive values (NPV/PPV)). Each performing laboratory, however, needs to establish in-house performance characteristics of the assay.

Objective:To compare laboratory performance characteristics of IgG specific and polyspecific PF4/H EIAs for a positive SRA.

Materials and methods: Between 2013 to 2017, 284 serum samples submitted to our laboratory for clinical HIT EIA testing were studied. IgG specific and polyspecific (Immucor GTI Diagnostics, Inc. Waukesha, WI) assays were performed per the manufacturer's instructions with the exception of performance on a liquid handling instrument rather than manually. The manufacturer assigned optical density (OD) cutoff of ≥0.400 was defined as positive. The SRA, ordered separately by clinicians, were sent out to reference laboratories; criteria for a positive result was ≥20% serotonin release with low dose heparin and <20% release with high dose of heparin. The data were entered into Excel spreadsheet, de-identified and analyzed by Analyse-it (Analyse-it Software, LTD, Leeds, U.K.). We also correlated the SRA positivity with the EIA ODs.

Results:Of the 284 serum samples, 112 were positive with IgG specific EIA, 68 (60.7%) of those also had a positive SRA. 172 were negative with IgG specific EIA, 2 (1.2%) of those had a positive SRA. Of the 148 were positive with polyspecific EIA, 69 (46.6%) also had a positive SRA. 136 were negative with polyspecific EIA, 1 (0.7%) had a positive SRA. The performance characteristics are shown in table 1. Compared with the polyspecific EIA, the IgG specific EIA had superior specificity, PPV and positive likelihood ratio (LR); the NPVs and negative LR of both assays were similar. The percent of patients having a positive SRA at different OD cutoffs is illustrated in Table 2.

Conclusion: In aggregate the data suggest that the IgG specific EIA had superior performance characteristics in detecting HIT antibodies. Given the high likelihood of a positive SRA for OD >2.0, in the appropriate clinical context, supplemental SRA testing may not be essential. Since the data were de-identified, assay performance for the clinical diagnosis of HIT was not possible.

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