Background Enoxaparin is commonly used for thromboembolic disease prophylaxis probably because of its safety profile and once-daily administration. In contrast to therapeutic doses, the prophylactic recommended dose is fixed (40 mg once a day for enoxaparin). There is little evidence for suitable dosing in extreme body weights, especially in low-weight patients.

Purpose To establish whether the recommended dose of Enoxaparin (40 mg/day) in patients weighing less than 55 kilogrammes produces anti-factor Xa activity over the desired ranges for thromboembolic prophylaxis.

Materials and Methods Cross sectional study. Sample size estimated in 53 patients. Inclusion criteria: over 18 years, body weight equal or less than 55 kilogrammes, hospitalised in medical wards and with an indication of thromboembolic prophylaxis with enoxaparin 40 mg/day by the treating physician. Exclusion criteria: renal failure and concomitant use of oral anticoagulants. Anti-factor Xa activity was measured 3 hours after the third dose of enoxaparin. We estimated the proportion of patients with anti-factor Xa activity over 0.5 u/ml and the average anti-factor Xa activity.

Results Average age was 65.4 ± 20.3 years and average weight 47.7 kilogrammes (26 to 54). The average anti-factor Xa activity was 0.54 ± 0.18 u/ml and the proportion of patients with values over 0.5 u/ml was 60%. Weight and anti-factor Xa activity were inversely correlated, with a Pearson coefficient of −0.497. In subgroup analysis, patients weighing less than 50 kilogrammes had anti-factor Xa activity of 0.61 u/ml, while those with weight over 50 kilogrammes had an anti-factor Xa activity of 0.47 u/ml (p = 0.019).

Conclusions Anti-factor Xa activity rises significantly when body weight decreases. Patients with low weight had an anti-factor Xa activity over the desired range for thromboembolic prophylaxis, especially in those under 50 Kilograms. Further study is needed to determine if these data are clinically significant and whether prophylactic doses should be adjusted for body weight.

No conflict of interest.