Laboratory testing for the antibodies that cause heparin-induced thrombocytopenia: how much class do we need?

Heparin-induced thrombocytopenia (HIT) is usually caused by platelet-activating antibodies of immunoglobulin G class that recognize platelet factor-4 (PF4) bound to heparin or certain other polyanions. Commercial enzyme immunoassays (EIAs) for PF4/polyanion-reactive antibodies detect two immunoglobulin classes (IgA and IgM) besides IgG. To investigate whether the additional detection of these antibody classes improves or worsens assay operating characteristics, we compared the sensitivity and specificity of EIAs that detect these 3 immunoglobulin classes individually with that of a commercial EIA (Genetic Testing Institute, GTI), as well as a platelet-activation assay, the serotonin-release assay (SRA). We compared the operating characteristics of these 5 assays by evaluating 448 patients, in 14 of whom clinical HIT developed, who received either unfractionated or low molecular weight heparin in prospective studies that included systematic platelet-count monitoring and serologic evaluation for anti-PF4/polyanion antibodies. We found that the SRA and IgG and commercial EIAs had similar high sensitivity for HIT; however, diagnostic specificity (for unfractionated and low molecular weight heparin, respectively) varied considerably, as follows: SRA (95.1%, 97.2%) > IgG EIA (89.0%, 93.7%) > GTI EIA (74.2%, 87.6%). Additional detection of IgA and IgM antibodies by the GTI EIA worsened test specificity by detecting numerous nonpathogenic antibodies. Moreover, the frequency and magnitude of IgA and IgM antibody formation in non-HIT immune responses did not differ from that exhibited by patients in whom clinical HIT developed. We conclude that an EIA that detects anti-PF4/polyanion antibodies of only the IgG class has greater diagnostic usefulness in revealing clinical HIT than does an assay that also detects IgA and IgM class antibodies.