The OAT Trail: The Obesity Anti-Coagulation Thromboprophylaxis Trial.

Venous thromboembolism (VTE) is a leading cause of morbidity and mortality following bariatric surgery, with a reported incidence of symptomatic DVT of 0.4-3% and PE of 0.3-2%. Over 80% of VTE events occur after hospital discharge, within 30 days of surgery. Obesity is an independent risk factor for VTE through venous stasis, a chronic pro-inflammatory and hypercoagulable state, adipokine dysregulation, and endothelial dysfunction. The pneumoperitoneum and reverse Trendelenburg positioning required for laparoscopic bariatric surgery further amplify venous stasis and thrombotic risk intraoperatively.

Low molecular weight heparin (LMWH) is the cornerstone of pharmacological VTE prophylaxis in bariatric surgery. LMWH exerts its anticoagulant effect via antithrombin-mediated Factor Xa inhibition, and pharmacodynamic adequacy can be assessed by measurement of anti-Xa activity. The American Society for Metabolic and Bariatric Surgery (ASMBS) 2021 position statement recommends a target prophylactic anti-Xa range of 0.2-0.4 IU/mL, measured at 4 hours after the third consecutive dose. The pharmacokinetics of LMWH are significantly altered in patients with severe obesity due to increased volume of distribution, altered renal clearance, elevated Factor Xa activity, and aberrant subcutaneous absorption, reducing the predictability of standard dosing regimens in this population.

Our research group at St Vincent's University Hospital (SVUH) has completed two prospective observational pilot studies. The first (Ethics Ref: RS22-017; n=20) examined weight-stratified enoxaparin dosing and found that 52.2% of patients achieved prophylactic anti-Xa levels. The second (Ethics Ref: RS25-035; n=51) examined weight-based tinzaparin (50 anti-Xa IU/kg once daily) and found that 47.1% achieved prophylactic anti-Xa levels - the first study of tinzaparin pharmacokinetics in a bariatric surgery population. A systematic review and meta-analysis conducted by the research team estimated a pooled prophylactic anti-Xa rate of 0.68 for weight-stratified enoxaparin dosing in the published literature. These data directly informed the design and power calculation of this RCT.

To our knowledge, no adequately powered randomised controlled trial has directly compared anti-Xa activity between tinzaparin and enoxaparin in a bariatric surgery population. The OAT-RCT is designed to address this gap using prospective anti-Xa level monitoring as its primary endpoint.

Design: Single-centre, prospective, parallel-group, open-label, randomised controlled superiority trial at SVUH, Dublin, Ireland.

Randomisation: Participants will be randomised 1:1 to Arm A (weight-based tinzaparin 50 anti-Xa IU/kg once daily) or Arm B (weight-stratified enoxaparin: 40 mg twice daily for weight ≤150 kg; 60 mg twice daily for weight >150 kg) for 21 days post-operatively, commencing on post-operative day 1. Randomisation will be performed using the web-based Sealed Envelope system (www.sealedenvelope.com) with computer-generated block randomisation (block sizes 4 and 6).

Primary endpoint: Proportion of patients achieving a prophylactic anti-Xa level (0.2-0.4 IU/mL) measured at 4 hours (±30 minutes) after the third consecutive LMWH dose on post-operative day 2.

Secondary endpoints: Incidence of clinically significant bleeding events within 30 days (ISTH criteria); incidence of confirmed VTE events (DVT/PE) within 30 days; proportion achieving sub-prophylactic (<0.2 IU/mL) and supra-prophylactic (>0.4 IU/mL) anti-Xa levels; correlations between anti-Xa level and BMI, total body weight, and procedure type; adverse drug reactions; patient-reported treatment satisfaction using the TSQM-9 questionnaire at 6 weeks.

Sample size: 180 participants (90 per arm), based on a formal power calculation using G*Power (z-test: difference between two independent proportions, two-tailed; p1=0.68, p2=0.47; α=0.05; power=0.80; 86 per arm, inflated to 90 per arm to allow for 5% dropout).

Statistical analysis: Primary analysis by intention-to-treat. Chi-squared or Fisher's exact test for the primary endpoint, with between-group difference reported with 95% confidence intervals. Pre-specified subgroup analyses by BMI category, sex, and procedure type. Interim analysis at 50% enrolment reviewed by an independent Data Safety Monitoring Board, with Haybittle-Peto stopping rule (p<0.001).

Ethics: Ethical approval granted by the SVHG Research Ethics Committee (Ref: RS26-029). The study will be conducted in accordance with the Declaration of Helsinki and ICH GCP guidelines.

Inclusion criteria

• Age ≥18 years
• BMI ≥40 kg/m², or BMI ≥35 kg/m² with at least one obesity-related comorbidity (type 2 diabetes, hypertension, obstructive sleep apnoea, dyslipidaemia, or metabolic dysfunction-associated steatotic liver disease (MASLD))
• Scheduled to undergo laparoscopic bariatric surgery (sleeve gastrectomy or gastric bypass) at St Vincent's University Hospital, Dublin, Ireland
• Capacity to provide written informed consent

Exclusion criteria

• Current therapeutic anticoagulation for any indication Known allergy or hypersensitivity to tinzaparin, enoxaparin, heparin, or any heparin-derived product, including documented heparin-induced thrombocytopaenia (HIT) Any other contraindication to LMWH therapy Severe renal impairment (eGFR <30 mL/min/1.73m²) Known haematological disorder or coagulopathy Pregnancy, breastfeeding, or planning pregnancy during the study period Active major bleeding or high bleeding risk at the discretion of the treating clinician Inability to provide written informed consent Participation in another interventional clinical study within 30 days prior to enrolment