Maximum 24‐hour platelet count fall: Metric for improving the diagnosis of heparin‐induced thrombocytopenia among patients with intermediate probability 4Ts scores

BACKGROUND: Most patients with suspected heparin-induced thrombocytopenia (HIT) and an intermediate probability 4Ts score do not have HIT. We aimed to develop a metric based on the rate of platelet count fall to aid in discriminating HIT status among patients with an intermediate 4Ts score. METHODS: We derived a measure of the maximum 24-hour percentage decrease in platelet count (Fall_max ) in a cohort of patients with suspected HIT and an intermediate 4Ts score at the University of Pennsylvania. We validated this metric in a prospectively collected cohort of patients with suspected HIT and an intermediate 4Ts score from four hospitals in Hamilton, Ontario. RESULTS: One hundred fifty-eight and 139 patients were included in the analysis from the derivation and validation cohorts, respectively. Fall_max was significantly higher in HIT-positive patients in the derivation cohort (49.6% versus 38.6%, P = .009) and validation cohort (43.5% versus 29.3%, P = .027). The area under the receiver operating characteristic curve was 0.68 (95% confidence interval [CI] 0.57-0.78) and 0.71 (0.59-0.83) in the two cohorts, respectively. At Fall_max  ≥ 30%, sensitivity and specificity were 95.5% and 29.4% in the derivation cohort and 80.0% and 52.7% in the validation cohort. CONCLUSIONS: Among patients with suspected HIT and an intermediate 4Ts score, Fall_max aided in discriminating HIT-negative from HIT-positive patients. Using a measure that accounts for the rate of platelet count fall may help to avoid unnecessary suspension of heparin and treatment with an alternative anticoagulant in HIT-negative patients with an intermediate probability 4Ts score, though further evaluation is warranted.