Blood clots in the legs or lungs (called venous thromboembolism or VTE) are one of the most serious complications after weight loss surgery. Most blood clots occur after patients go home from hospital, within the first 30 days after surgery. To prevent blood clots, all patients having weight loss surgery receive a daily blood-thinning injection for 21 days after their operation.

Two blood-thinning injections are currently used at St Vincent's University Hospital for this purpose: enoxaparin (Clexane®) and tinzaparin (Innohep®). Both belong to a group of medicines called low molecular weight heparins (LMWHs). Patients with obesity process these medicines differently to the general population, and previous studies from our hospital have shown that fewer than 53% of patients achieve adequate blood-thinning levels with either injection when measured by a blood test called an anti-Xa level.

Patients will be randomly assigned (like a coin toss) to receive either tinzaparin or enoxaparin for 21 days after their surgery. Both injections are already in routine use at this hospital. A single extra blood sample will be taken on the second day after surgery to measure the anti-Xa level, which tells us whether the injection is providing adequate protection against blood clots. This blood sample will be taken at the same time as routine post-operative blood tests so that no additional blood draws are required.

The study will also look at rates of blood clots and bleeding events within 30 days of surgery, and will ask patients to complete a short questionnaire at their six-week follow-up appointment about their experience with the injection.

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

Individual participant data will not be shared. This study is conducted under the governance of the Health Research Regulations 2018 (Data Protection Act 2018, Section 36(2)) and in compliance with GDPR 2016/679. St Vincent's University Hospital, Dublin, Ireland is the sole Data Controller. Participant data are pseudonymised but contain clinically sensitive information; data sharing beyond the research team was not specified in the ethics application or participant consent documentation approved by the St Vincent's Healthcare Group Research Ethics Committee (Ref: RS26-029). Aggregate and summary data will be made available through peer-reviewed publication of study results.

This is an open-label trial. The laboratory scientist performing anti-Xa analysis and the study statistician performing the primary outcome analysis are blinded to treatment allocation. The statistician will remain blinded until the primary analysis is complete.