The dynamic range of immunoassays for heparin-induced thrombocytopenia

BACKGROUND Following the current guidelines, immunoassays for the diagnosis of heparin-induced thrombocytopenia (HIT) are interpreted dichotomously, with test results categorized as either positive or negative. However, the extent to which test results hold diagnostic significance across the entire dynamic range remains unclear. OBJECTIVES We utilized data from the prospective towards precise and rapid diagnosis of heparin-induced thrombocytopenia study, comprising 1393 consecutive patients with suspected HIT, to assess the diagnostic significance of 2 heparin/platelet factor 4 immunoassay test results across their respective dynamic ranges: HemoSil Acustar HIT IgG (chemiluminescence immunoassay [CLIA]) and Lifecodes PF4 immunoglobulin G (enzyme-linked immunosorbent assay [ELISA]). METHODS HIT diagnosis was determined by a washed platelet heparin-induced platelet activation assay. For each measurement point in the dataset, we computed likelihood ratios (LRs), sensitivities, and specificities. To provide posttest probabilities for individual test results, we calculated interval-specific LRs and integrated them into a web-based calculator. RESULTS The prevalence of HIT was 8.5% (n = 119). An LR of ≥10 was first achieved at 0.3% of the dynamic range (0.4 U/mL; CLIA) and then at 16% (0.64 optical density; ELISA). An LR of ≥100 was present at 9.4% (12 U/mL; CLIA) and 75.0% (3.0 optical density; ELISA). The slope of the linear regression line (LR ∼ dynamic range) was 9.5 (CLIA) and 0.9 (ELISA). CONCLUSION Despite both immunoassays showing an association between results and diagnostic significance, the strength of the association varies by assay. CLIA has a larger increase per measurement unit. Posttest probabilities for individual patients can be estimated using a web-based calculator: https://pcd-research.shinyapps.io/BayesianCalculator/.