Effect of fondaparinux prophylaxis on anti-factor Xa concentrations in patients with morbid obesity

Anti-factor Xa values in morbidly obese patients receiving standard doses of fondaparinux sodium for the prevention of venous thromboembolism (VTE) were analyzed in a retrospective chart evaluation. The administration of low-molecular-weight heparins to obese patients (body mass index [BMI] of ≥30 kg/m(2)) at the dose recommended for VTE prophylaxis has been reported to result in increased thromboembolic events and decreased anti-factor Xa levels, and some evidence indicates that weight-based dosing adjustments may be appropriate. To study this phenomenon among morbidly obese patients (BMI of ≥40 kg/m(2)), a review of the charts of 45 adult patients for whom steady-state anti-factor Xa laboratory values were obtained after at least four fondaparinux injections was conducted; in all instances, fondaparinux sodium was given at the standard dose (2.5 mg once daily). Of the total of 47 anti-factor Xa values analyzed, 22 (47%) were below the study institution's target peak range (0.3-0.5 mg/L), 20 values (43%) were within the range, and 5 (11%) were above the range. No documented thromboembolic events occurred during hospitalization in the cases evaluated. A stepwise linear regression analysis of selected demographic and clinical variables indicated that better renal function, male sex, increased BMI, and fewer fondaparinux doses were associated with a greater likelihood of diminished anti-factor Xa activity in the cases evaluated. Anti-factor Xa concentrations in morbidly obese patients receiving fondaparinux sodium 2.5 mg subcutaneously daily for VTE prophylaxis were within or above the target range in 53% of the instances evaluated.