Laboratory Testing for Hit Antibodies: How Much Class Do We Need?.

HIT is usually caused by platelet-activating antibodies of IgG class that bind to neoepitopes on platelet factor 4 (PF4) bound to heparin or certain other polyanions. Commercial enzyme-immunoassays (EIAs), however, measure PF4/polyanion-reactive antibodies of IgA and IgM class, in addition to IgG class antibodies. This raises the question: Does the detection of these IgA and IgM class antibodies improve HIT assay operating characteristics (perhaps because IgA and IgM are a marker for pathogenic HIT-IgG and/or for clinical HIT) or worsen operating characteristics (perhaps via detection of numerous non-HIT sera containing non-pathogenic IgA and/or IgM class antibodies)? We performed systematic serologic studies of 362 stored sera from a clinical trial of heparin therapy (

Table 1. Serologic features of 12 patients with clinical HIT.

In contrast, the EIA-A and EIA-M assays were positive in less than half of the HIT patients. Further, the magnitude of the IgA and IgM anti-PF4/heparin immune response did not differ between the 12 HIT patients, and the 69 non-HIT patients who had any PF4/polyanion immune response, as defined by a positive EIA-GTI without HIT (Table 2).

Table 2. Comparison of the HIT and non-HIT Immune Response for IgA and IgM.

Similar observations were made when we compared the 24 SRA-positive patients (including the 12 HIT patients) against the 57 SRA-negative, EIA-GTI positive patients. CONCLUSION: Detection of PF4/polyanion-reactive IgA and IgM class antibodies worsens the operating characteristics of HIT assays through the detection of numerous non-pathogenic antibodies, without any offsetting advantages in the detection of pathogenic HIT antibodies. Optimal diagnostic laboratory testing for HIT antibodies should include a platelet activation assay and an EIA that detects only IgG class antibodies reactive against PF4/heparin (or PF4/polyanion).