The Effect of Obesity on Anti-Xa Concentrations in Bariatric Patients.

Background: Morbidly obese patients are at increased risk to develop venous thromboembolism (VTE), especially after bariatric surgery. Adequate postoperative thrombosis prophylaxis is of utmost importance. It is assumed that morbidly obese patients need higher doses of low molecular weight heparin (LMWH) compared to normal-weight patients; however, current guidelines based on relative efficacy in obese populations are lacking.

Objectives: First, we will evaluate the relationship between body weight descriptors and anti-Xa activity prospectively. Second, we will determine the dose-linearity of LMWH in morbidly obese patients.

Setting: This study was performed in a general hospital specialized in bariatric surgery.

Methods: Patients were scheduled for a Roux-en-Y gastric bypass with a total bodyweight (TBW) of ≥ 140 kg. Patients (n = 50, 64% female) received a daily postoperative dose of 5700 IU of nadroparin for 4 weeks. Anti-Xa activity was determined 4 h after the last nadroparin administration. To determine the dose linearity, anti-Xa was determined following a preoperative dose of 2850 IU nadroparin in another 50 patients (52%).

Results: TBW of the complete group was 148.5 ± 12.6 kg. Mean anti-Xa activity following 5700 IU nadroparin was 0.19 ± 0.07 IU/mL. Of all patients, 32% had anti-Xa levels below the prophylactic range. Anti-Xa activity inversely correlated with TBW (correlation coefficient - 0.410) and lean body weight (LBW; correlation coefficient - 0.447); 67% of patients with a LBW ≥ 80 kg had insufficient anti-Xa activity concentrations. No VTE events occurred.

Conclusions: In morbidly obese patients, a postoperative dose of 5700 IU of nadroparin resulted in subprophylactic exposure in a significant proportion of patients. Especially in patients with LBW ≥ 80 kg, a higher dose may potentially be required to reach adequate prophylactic anti-Xa levels.