Fixed-Dose Enoxaparin After Bariatric Surgery: The Influence of Body Weight on Peak Anti-Xa Levels

Introduction: There is lack of data on the pharmacodynamics of low-molecular-weight heparins in obese patients. Background: The aims of this study are to investigate the correlation between anti-factor Xa (anti-Xa) levels and body weight with fixed-dose enoxaparin after bariatric surgery and to investigate the percentage of patients that reach the desired prophylactic range for anti-Xa levels. Methods: Blood for anti-Xa peak levels measurement was drawn 3–5 h after administration of enoxaparin at the planned visit 8–16 days after surgery. Patients were included in three categories: <110 kg (group 1), 110–150 kg (group 2), and >150 kg (group 3). Results: Fifty-one patients were included (43.9 ± 9.9 years, 75 % women). Mean anti-Xa level was 0.37 ± 0.14 IU/ml. This level was the highest in group 1 (0.47 ± 0.13 IU/ml) and lowest in group 3 (0.23 ± 0.07). No subprophylactic (<0.2 IU/ml) anti-Xa levels were detected in group 1, whereas this was observed in 38 % in patients in group 3. Supraprophylactic levels (>0.5 IU/ml) were most often present in group 1 (36 %). With multivariable regression analysis, body weight (β −0.720 (95 % confidence interval −.717; −.993), p < 0.001) was an independent predictor of anti-Xa levels, whereas lean body was not independently associated. This was confirmed in a non-linear mixed effects analysis of the data. Conclusions: Patients with excessive body weight may not be adequately treated with fixed-dose enoxaparin thromboprophylaxis while patients with lower body weight may have an increased bleeding risk. Body weight is a better predictor of anti-Xa levels compared to lean body weight.