Evaluation of appropriate serologic testing for suspected heparin-induced thrombocytopenia

The predictive value of clinical "4-T's" scoring in patient selection for serologic testing to confirm suspected heparin-induced thrombocytopenia (HIT) was evaluated.

In a chart review-based study at a large hospital, all adult inpatients who underwent enzyme-linked immunosorbent assay (ELISA) testing for HIT-antibody detection during a two-year period were identified. Scoring of the 4 T's (degree of thrombocytopenia, timing of symptom onset, presence of thrombosis or other sequelae, and other potential causes) was retrospectively performed on a random sample of cases (n = 70) by three pharmacist investigators. Based on the 4-T's scores, the probability of HIT confirmation via ELISA testing was classified as low, intermediate, or high; by comparing those classifications with documented ELISA results, the positive predictive value of 4-T's scoring was calculated. Interrater variability in 4-T's scoring was also assessed.

4-T's scoring of the 70 randomly selected cases indicated low, intermediate, and high probabilities of HIT confirmation via ELISA testing in 52, 17, and 1 case, respectively. Negative ELISA results were documented in the records of 37 patients in the low-probability group; the 4-T's scoring system was calculated to have an adjusted negative predictive value of 77.3%. The calculation of Gwet's agreement coefficient indicated substantial agreement in 4-T's scoring by the three raters.

The study results suggest that among patients whose 4-T's scores indicate a low probability of HIT, the results of subsequent ELISA testing for HIT antibodies are likely to be negative in about 8 of 10 cases.