Effect of heparin on immunological heparin-induced thrombocytopenia-IgG chemiluminescence testing

Heparin-induced thrombocytopenia (HIT) requires clinical evaluation and laboratory testing using immunological assays with confirmation by functional assays such as serotonin release assay (SRA). The HIT-IgG chemiluminescence immunoassay (CLIA) is a rapid immunological assay; we aimed to assess the effect of heparin on this assay. The effect of low-dose (0.1 U/ml) and high-dose (100 U/ml) heparin on the CLIA HIT-IgG assay was evaluated, these concentrations aligning to those used for SRA. A cohort of available pretested patient material (plasma or serum) comprised samples negative for HIT-IgG (i.e., <1.0 U/ml) (n = 22), or positive for HIT-IgG (i.e., ≥1.0 U/ml) ( n  = 25), with the latter split into 'low positive' (1.0-<10 U/ml; n  = 16) and 'high positive' (≥10 U/ml; n  = 9). Low-dose heparin, used for initiating serotonin release in SRA, had no appreciable effect on the CLIA HIT-IgG assay for any HIT-IgG group. High-dose heparin, used for inhibiting serotonin release in SRA, had a variable effect on HIT-IgG testing, generally reducing detected HIT-IgG levels in high positive samples, and mostly reducing detected HIT-IgG levels in low positive samples. However, some samples from negative and low positive groups showed counterintuitive increases in HIT-IgG signal. Heparin levels may influence HIT-IgG test values in various ways. Although high positive samples showed an expected decrease in signal with high-dose heparin, some low positive and negative samples showed unexpected increases in signal. Whether these findings improve identification of pathological HIT, or of variable HIT subtypes, requires further investigation.