Prospective Evaluation of Laboratory Tests for the Diagnosis of Heparin-Induced Thrombocytopenia.

The laboratory diagnosis of heparin-induced thrombocytopenia (HIT) relies on the demonstration of antibodies to the heparin-platelet factor 4 (H-PF4) complex. Assays are based on the functional ability of H-PF4 antibodies to activate platelets, or detect the antibody directly by immunological methods. Multiple assays in each category are currently in clinical use and newer, rapid immunological assays are becoming available. The aim of this study was to compare available methods for detecting H-PF4 antibodies in a prospective study of patients with clinically suspected HIT. Functional assessment included serotonin release assay (SRA) and lumi-aggregometry (LA). Immunological assessment included ELISA (GTI), and particle gel immunoassay (PGIA; Diamed and Akers). Circulating platelet microparticles (PMP) were assessed by flow cytometry. Patients were also assessed for the pre-test probability of HIT using the Warkentin 4-T scoring system. 151 patients were enrolled. 54/151 patients (35.8%) had a positive GTI ELISA, while 53/151 (35.1%) and 39/151 (25.8%), respectively, had positive Akers and Diamed PGAI tests. Only 15/149 (10.1%) patients had a positive SRA, while only 5/150 (3.3%) gave a positive result by lumi-aggregometry. There was a strong correlation between the ELISA OD values obtained in serum and plasma using both fresh (r=0.98) and frozen (r=0.99) samples, although slightly more positive results were obtained using serum. Differences were only seen with OD values around the cut-off of 0.4. The majority (77.8%) of H-PF4 antibodies detected by ELISA were neutralized by heparin in the ‘confirmatory’ procedure. Weak antibodies (OD 0.4–0.5) were more likely to be non-neutralizable (5/12; 42%) than strong antibodies (OD>1.0; 4/23; 17%). 47 patients positive by ELISA were retested to determine the predominant immunoglobulin subclass. 15/47 (32%) were positive (OD>0.4) for IgG; 27/47 (57%) for IgM, and 12/47 (25%) for IgA. The Diamed assay more closely correlated with the GTI ELISA than the Akers test (82.1% vs. 56.7%, respectively). The PGIAs were only moderately correlated with each other (64%) with the Akers assay giving more “false positive” results relative to the ELISA. PMP were higher in patients with a positive ELISA (6.2 vs 4.7 × 10⁶/ml) or positive SRA (5.5 vs. 5.1 ×10⁶/ml) but this was not statistically significant due to the wide range of results. Of 119 patients assessed, 87 had a low pre-test probability of HIT (4-T score 0–3), 27 had an intermediate probability (4–5), and 5 had a high probability (6–8). The GTI ELISA was positive in 24, 56 and 80% of low, intermediate and high probability cases. The Akers PGIA was positive in 39, 41 and 40% respectively; the Diamed assay in 21, 33 and 40%, and the SRA in 7, 11 and 40%, respectively. This study was conducted in a patient population biased towards cardiovascular surgery, and confirms previously reported observations that immunoassays are more frequently positive than functional assays. The ELISA correlated better than the PGIA tests with the pre-test probability of HIT, although the Diamed test showed acceptable correlation with the ELISA. In contrast, the Akers assay correlated poorly with the ELISA, often producing positive results when the latter test was negative. We conclude that while the PGIA tests are rapid and convenient, further studies are needed to determine the basis for disparate results relative to the widely used ELISA.