Anti-Xa Activity After Enoxaparin Prophylaxis In Hospitalized Patients Weighing Less Than Fifty-Five Kilograms

Abstract Introduction Low-molecular-weight heparins are commonly used for the prophylaxis of thromboembolic disease. In contrast to therapeutic doses, recommended prophylactic doses are fixed (i.e., 40 mg/day enoxaparin). Dosing of patients with extreme body weights has not been well studied, especially dosing of low weight patients. Objectives To establish the anti-Xa activity that results from 40 mg/day enoxaparin in inpatients with body weight ≤ 55 kg. Patients/Methods Cross-sectional study including inpatients older than 18 years, with body weight ≤ 55 kg, and whose treating physician found indication for 40 mg/day enoxaparin. We excluded patients with renal failure and those using oral anticoagulants. Anti-Xa activity was measured 3 hours after the second dose of enoxaparin. Statistical analyses were conducted to determine the effect of body weight on anti-Xa levels. Results Average age was 72.5 years (interquartile range, 30) and median body weight was 49.7 kg (interquartile range, 7). Twenty-five percent of patients weighed ≤ 45 kg, 37.5% weighed 46–50 kg, and 37.5% weighed 51–55 kg. The mean anti-Xa activity was 0.54 ± 0.18 IU/ml, and 60% of the patients exhibited activity ≥ 0.5 IU/ml. Weight and anti-Xa activity inversely correlated (Spearman’s rho = − 0.428, p = 0.001). Patients weighing ≤ 45 kg exhibited higher anti-Xa activity (0.61 ± 0.18 IU/ml, p = 0.008) than heavier patients and an odds ratio of 8 for anti-Xa level ≥ 0.5 IU/ml (95% CI: 1.42-45.06). Conclusions Anti-factor Xa activity rises significantly when body weight decreases. Patients of low weight, especially those weighing < 45 kg, exhibited an anti-Xa activity higher than the desired range for thromboembolic prophylaxis.