Low‐molecular‐weight heparins are the most commonly used for thromboembolic disease prophylaxis, probably because of their security profile and once‐daily administration. Contrary to therapeutic doses, prophylactic recommended doses are fixed (40 mg once a day for enoxaparin). Dosing in extreme body weights has little evidence, especially in patients with low weight. The aim of the study was to establish if the recommended dose of enoxaparin (40 mg once a day) in patients who weighed less than 55 kg produces antifactor Xa activity over desired ranges for thromboembolic prophylaxis.


This was a transversal study with prospective recruitment. Sample size was estimated in 53 patients. Inclusion criteria were: patients older than 18 years, body weight ≤ 55 kg, hospitalized in medical or surgical services in the Hospital Clínico Pontificia Universidad Católlica de Chile, and with indication of thromboembolic prophylaxis with enoxaparin 40 mg once a day by the treating physician. Exclusion criteria were: renal failure (creatinine clearance < 30 mL/min estimated with Cockroft‐Gault formula), amyloidosis, and concomitant use of oral anticoagulants. Antifactor Xa activity was measured 3‐4 hours after the second or third dose of enoxaparin. We estimated the proportion of patients with antifactor Xa activity over 0.5 unit/mL and the average of antifactor Xa activity.


The average age of patients was 65.4 ± 20.3 years, the average weight was 47.7 kg (26–54.8 kg), and 86.7% of patients were female. The average antifactor Xa activity was 0.54 ± 0.18 units/mL, and the proportion of patients with values over 0.5 units/mL was 60%. Weight and antifactor Xa activity had an inverse correlation, with a Pearson coefficient of 20.497. In subgroup analysis, patients < 50 kg of weight had antifactor Xa activity of 0.61 ± 0.18 units/ mL, whereas those who weighed > 50 kg had an antifactor Xa activity of 0.47 ± 0.16 unit/mL (P = 0.019).


Antifactor Xa activity rises significantly when body weight decreases. Patients with low weight had antifactor Xa activity over the desired range for thromboembolic prophylaxis, especially in those under 50 kg. Further investigation is needed to determine if these data have clinical significance and if prophylactic doses should be adjusted for body weight


L. Rojas ‐ none; A. Aizman, none; D. Ernst, none; M. Paz Acuña, none; P. Moya, none; R. Mellado, none; F. Garrido, none; J. Cerda ‐ none