Combination of two complementary automated rapid assays for diagnosis of heparin‐induced thrombocytopenia (HIT)

BACKGROUND: HIT diagnosis typically uses complementary diagnostic assays (eg, a PF4‐dependent enzyme‐immunoassay [EIA] and a platelet activation assay such as the serotonin‐release assay [SRA]). OBJECTIVES: To determine whether the combination of two automated assays—a latex immunoturbidimetric assay (LIA) that evaluates competitive inhibition of a HIT‐like monoclonal antibody and a chemiluminescence immunoassay (CLIA) for detecting anti‐PF4/heparin IgG—optimizes diagnostic sensitivity while also yielding good specificity, particularly at high assay reactivities. PATIENTS/METHODS: We determined operating characteristics using combined LIA/CLIA results from a HIT observational trial (n = 430; derivation cohort) and 147 consecutive patients with HIT (n = 147; supplementary derivation cohort). We also evaluated 678 consecutive samples referred for HIT testing (replication cohort). LIA/CLIA reactivities were scored individually as “negative” (<1.00 U/mL, 0 points), “weak” (1.00‐4.99 U/mL, 1 point), “moderate” (5.00‐15.99 U/mL, 2 points) and “strong” (≥16.00 U/mL, 3 points), thus contributing up to 6 points (maximum) when LIA/CLIA results were combined. We also examined whether higher LIA/CLIA scores predicted presence of platelet‐activating antibodies by conventional and modified (PF4‐ or PF4/heparin‐enhanced) SRA. RESULTS: Combined LIA/CLIA testing yielded high diagnostic sensitivity (~99%) similar to EIA. Interpretation of LIA/CLIA results using the 6‐point scale indicated progressively greater likelihood for the presence of platelet‐activating antibodies with increasing scores (semi‐quantitative reactivity). A LIA/CLIA score ≥ 4 points predicted the presence of platelet‐activating antibodies by SRA or PF4‐enhanced SRA with high probability (~98%). CONCLUSION: Combined LIA/CLIA testing optimizes diagnostic sensitivity, with progressively greater probability of detecting platelet‐activating antibodies with higher assay reactivity that reaches 98% when both automated assays yield moderate or strong results.