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

Abstract

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.

Keywords: blood platelets; clinical laboratory techniques; heparin; immunoassay; thrombocytopenia.

Figure 1

Clinical course and laboratory test results in a patient with LIA/CLIA score of 5 points but negative testing for platelet‐activating antibodies. The patient was scored as high probability for HIT (6 points per the 4Ts scoring system) although the possibility of non‐HIT critical illness‐associated consumptive thrombocytopenia cannot be completely excluded. The four red arrows correspond to the four components of the 4Ts scoring system, and the associated text provides information that supports the score that was given. For one of the EIAs—the EIA‐IgG(GTI)—the high heparin maneuver was performed, with 89% inhibition of reactivity seen at high heparin concentrations. BMI, body mass index; CABG × 6, coronary artery bypass grafting (six grafts); CLIA, chemiluminescence immunoassay; CRRT, continuous renal replacement therapy; DVT, deep vein thrombosis; EIA‐IgA (McM), IgA‐specific in‐house PF4/heparin EIA of the McMaster Platelet Immunology Laboratory; EIA‐IgG (GTI), commercial (from GTI) IgG‐specific PF4/PVS EIA; EIA‐IgG (McM), IgG‐specific in‐house PF4/heparin EIA of the McMaster Platelet Immunology Laboratory; EIA‐IgGAM (GTI), commercial (from GTI) polyspecific PF4/PVS EIA that detects antibodies of IgG, IgA, and/or IgM classes; EIA‐IgM (McM), IgM‐specific in‐house PF4/heparin EIA of the McMaster Platelet Immunology Laboratory; FP, frozen plasma (units); HIT, heparin‐induced thrombocytopenia; LIA, latex immunoturbidimetric assay; NSTEMI, non‐ST‐elevation myocardial infarction; PF4/H‐SRA, PF4/heparin serotonin‐release assay; PF4‐SRA, PF4‐enhanced serotonin‐release assay; Plt, platelet transfusion (adult dose); q12 h, every 12 hours; RCC, red cell concentrate; SC, subcutaneous; SRA, serotonin‐release assay; U, units; UFH, unfractionated heparin