title: Anti-Xa activity after enoxaparin prophylaxis in hospitalized patients weighing less than fifty-five kilograms.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  To establish the anti-Xa activity that results from 40 mg/day enoxaparin in inpatients with body weight ≤ 55 kg.
  
  
  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±0.18IU/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 ≥0.5IU/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.
authors: School of Medicine, Faculty of Medicine, Department of Internal Medicine, Pontificia Universidad Católica de Chile. Electronic address: lrojas@med.puc.cl.
authors: L
authors: L
authors: Rojas
authors: School of Medicine, Faculty of Medicine, Department of Internal Medicine, Pontificia Universidad Católica de Chile.
authors: A
authors: A
authors: Aizman
authors: Resident of Internal Medicine, School of Medicine, Faculty of Medicine, Pontificia Universidad Católica de Chile.
authors: D
authors: D
authors: Ernst
authors: Resident of Internal Medicine, School of Medicine, Faculty of Medicine, Pontificia Universidad Católica de Chile.
authors: M P
authors: MP
authors: Acuña
authors: Student of Chemistry and Pharmacy, Faculty of Chemistry and Pharmacy, Pontificia Universidad Católica de Chile.
authors: P
authors: P
authors: Moya
authors: Faculty of Chemistry and Pharmacy, Pontificia Universidad Católica de Chile.
authors: R
authors: R
authors: Mellado
authors: School of Medicine, Faculty of Medicine, Department of Public Health, Pontificia Universidad Católica de Chile.
authors: J
authors: J
authors: Cerda
mesh_terms: D000368:Aged; D000925:Anticoagulants / Q000627:therapeutic use*; D001835:Body Weight / Q000187:drug effects*; D003430:Cross-Sectional Studies; D017984:Enoxaparin / Q000627:therapeutic use*; D015951:Factor Xa / Q000378:metabolism*; D005260:Female; D006760:Hospitalization; D006801:Humans; D008297:Male; D011446:Prospective Studies; D054556:Venous Thromboembolism / Q000097:blood / Q000188:drug therapy / Q000517:prevention & control*
chemical_list: D000925:Anticoagulants; D017984:Enoxaparin; D015951:Factor Xa
keywords: ANOVA; Body weight; IQR; Inpatients; LMWH; Low molecular weight heparin; Low molecular weight heparins; Prophylaxis; SD; VTE; Venous thromboembolism; Weight loss; one-way analysis of variance; standard deviation; the median and interquartile range