Anti-Xa activity by weight in critically ill patients receiving unfractionated heparin for venous thromboembolism prophylaxis

Purpose: This study compared anti-Xa activity in critically ill patients receiving UFH for VTE prophylaxis between two weight groups (<100 kg vs ≥100 kg). Methods: This prospective, observational study included critically ill patients on UFH 5000 or 7500 units every 8 h. A peak and trough anti-Xa activity assay was ordered for each patient at steady state. Goal peak anti-Xa activity was 0.1–0.3 units/mL. Results: From March 2017 to June 2018, 75 patients were enrolled with 44 in the <100 kg group and 31 in the ≥100 kg group. There was no significant difference in the percentage of patients with peak anti-Xa activity within goal range between patients <100 kg and ≥ 100 kg (55.3% vs 35.7%, p = 0.12). The odds ratio for achieving peak anti-Xa activity within goal range as weight-based dose increased was 1.03 (95% CI 0.99–1.07). No differences were found in trough anti-Xa activity, VTE, bleeding, length of stay, or death. Conclusions: Though only one-third of patients ≥100 kg had peak anti-Xa activity within goal range, no significant difference was found between the weight groups. Additional prospective studies with adequate sample sizes are warranted to further investigate appropriate weight-based dosing of UFH in critically ill patients.