The OAT Trail: The Obesity Anti-Coagulation Thromboprophylaxis Trial. (The OAT-RCT)

May 14, 2026 updated by: University College Dublin

A Prospective, Randomised Controlled Trial Comparing Weight-based Tinzaparin Versus Weight-based Enoxaparin for Peri-operative Thromboprophylaxis in Patients Undergoing Bariatric Surgery: Evaluation of Anti-Xa Levels and Clinical Outcomes.

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.

Study Overview

Status

Not yet recruiting

Conditions

Intervention / Treatment

Detailed Description

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.

Study Type

Interventional

Enrollment (Estimated)

180

Phase

  • Not Applicable

Contacts and Locations

This section provides the contact details for those conducting the study, and information on where this study is being conducted.

Study Contact

Study Contact Backup

Study Locations

  • Ireland
      • Dublin, Ireland
        • St Vincent's University Hospital
        • Contact:
          • Czara A Kennedy, BMBS, MRCSI, MSc
          • Phone Number: 0035312214000
          • Email: czkennedy@svhg.ie
        • Contact:

Participation Criteria

Researchers look for people who fit a certain description, called eligibility criteria. Some examples of these criteria are a person's general health condition or prior treatments.

Eligibility Criteria

Ages Eligible for Study

  • Adult
  • Older Adult

Accepts Healthy Volunteers

No

Description

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

Study Plan

This section provides details of the study plan, including how the study is designed and what the study is measuring.

How is the study designed?

Design Details

  • Primary Purpose: Basic Science
  • Allocation: Randomized
  • Interventional Model: Parallel Assignment
  • Masking: Single

Arms and Interventions

Participant Group / Arm
Intervention / Treatment
Experimental: Weight-based Tinzaparin
Tinzaparin (Innohep®, LEO Pharma) administered subcutaneously once daily at a dose of 50 anti-Xa IU/kg total body weight, commencing on post-operative day 1 and continuing for 21 days. Administered via subcutaneous injection into the abdominal wall. Anti-Xa level measured at 4 hours (±30 minutes) after the third consecutive dose on post-operative day 2.
Diagnostic test: Anti-Xa Activity Monitoring
A single venous blood sample (~5 mL, citrated tube) drawn at 4 hours (±30 minutes) after the third consecutive LMWH dose on post-operative day 2, concurrent with routine post-operative bloods. Anti-Xa activity measured using a CE-marked in vitro diagnostic assay at the SVUH Haematology Laboratory. Results are not available in real time and do not influence clinical management. Target prophylactic range: 0.2-0.4 IU/mL per ASMBS 2021 guidance.
Other Names:
  • Anti-factor Xa assay; chromogenic anti-Xa assay
Drug: Tinzaparin (Leo)
Tinzaparin sodium administered subcutaneously once daily at 50 anti-Xa IU/kg total body weight for 21 days post-operatively, commencing on post-operative day 1. Used for pharmacological VTE prophylaxis following laparoscopic bariatric surgery.
Other Names:
  • Innohep
Active Comparator: Weight-stratified Enoxaparin
Enoxaparin (Clexane®, Sanofi) administered subcutaneously twice daily, commencing on post-operative day 1 and continuing for 21 days. Dose: 40 mg twice daily for patients weighing ≤150 kg; 60 mg twice daily for patients weighing >150 kg. Administered via subcutaneous injection into the abdominal wall. Anti-Xa level measured at 4 hours (±30 minutes) after the third consecutive dose on post-operative day 2.
Diagnostic test: Anti-Xa Activity Monitoring
A single venous blood sample (~5 mL, citrated tube) drawn at 4 hours (±30 minutes) after the third consecutive LMWH dose on post-operative day 2, concurrent with routine post-operative bloods. Anti-Xa activity measured using a CE-marked in vitro diagnostic assay at the SVUH Haematology Laboratory. Results are not available in real time and do not influence clinical management. Target prophylactic range: 0.2-0.4 IU/mL per ASMBS 2021 guidance.
Other Names:
  • Anti-factor Xa assay; chromogenic anti-Xa assay
Drug: enoxaparin
Enoxaparin sodium administered subcutaneously twice daily (40 mg for weight ≤150 kg; 60 mg for weight >150 kg) for 21 days post-operatively, commencing on post-operative day 1. Used for pharmacological VTE prophylaxis following laparoscopic bariatric surgery.
Other Names:
  • Clexane

What is the study measuring?

Primary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Proportion of participants achieving prophylactic anti-Xa levels
Time Frame: Post-operative day 2 (4 hours after third LMWH dose)
Proportion of participants achieving a prophylactic anti-Xa level of 0.2-0.4 IU/mL, measured at 4 hours (±30 minutes) after the third consecutive LMWH dose on post-operative day 2, as specified by the ASMBS 2021 position statement. Measured using a CE-marked chromogenic anti-Xa assay at the SVUH Haematology Laboratory.
Post-operative day 2 (4 hours after third LMWH dose)

Secondary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Incidence of clinically significant bleeding events
Time Frame: 30 days post-operatively
Incidence of minor and major bleeding events within 30 days of surgery, classified according to International Society on Thrombosis and Haemostasis (ISTH) criteria.
30 days post-operatively
Incidence of venous thromboembolism
Time Frame: 30 days post-operatively
Incidence of confirmed VTE events including deep vein thrombosis (DVT) and/or pulmonary embolism (PE) within 30 days of surgery, diagnosed by standard clinical imaging (duplex ultrasound or CT pulmonary angiography).
30 days post-operatively
Proportion achieving sub-prophylactic anti-Xa levels
Time Frame: Post-operative day 2 (4 hours after third LMWH dose)
Proportion of participants in each arm achieving a sub-prophylactic anti-Xa level of less than 0.2 IU/mL, measured at 4 hours (±30 minutes) after the third consecutive LMWH dose on post-operative day 2.
Post-operative day 2 (4 hours after third LMWH dose)
Proportion achieving supra-prophylactic anti-Xa levels
Time Frame: Post-operative day 2 (4 hours after third LMWH dose)
Proportion of participants in each arm achieving a supra-prophylactic anti-Xa level of greater than 0.4 IU/mL, measured at 4 hours (±30 minutes) after the third consecutive LMWH dose on post-operative day 2.
Post-operative day 2 (4 hours after third LMWH dose)
Correlation between anti-Xa level and body habitus
Time Frame: Post-operative day 2
Pearson or Spearman correlation coefficients between anti-Xa level and BMI, total body weight, and procedure type (sleeve gastrectomy versus Roux-en-Y gastric bypass).
Post-operative day 2
Adverse drug reactions
Time Frame: 30 days post-operatively
Incidence and nature of adverse drug reactions attributable to either LMWH, including injection site reactions (pain, bruising, erythema, haematoma), heparin-induced thrombocytopaenia, and other drug-related adverse events.
30 days post-operatively
Patient-reported treatment adherence and injection site tolerability
Time Frame: 6 weeks post-operatively
Self-reported adherence (missed doses and reasons) and injection site tolerability assessed via supplementary questionnaire items at the 6-week post-operative clinic visit.
6 weeks post-operatively
Patient-reported treatment satisfaction
Time Frame: 6 weeks post-operatively

Treatment satisfaction assessed using the validated Treatment Satisfaction Questionnaire for Medication (9-item version; TSQM-9), generating scores across three domains: Effectiveness, Convenience, and Global Satisfaction. Each domain is scored on a scale of 0 to 100, where higher scores indicate greater treatment satisfaction. Completed at the standard 6-week post-operative clinic visit.

[Time Frame: 6 weeks post-operatively]
6 weeks post-operatively

Collaborators and Investigators

This is where you will find people and organizations involved with this study.

Sponsor

University College Dublin

Investigators

  • Principal Investigator: Helen M Heneghan, PhD, FRCSI, University College Dublin

Publications and helpful links

The person responsible for entering information about the study voluntarily provides these publications. These may be about anything related to the study.

General Publications

Study record dates

These dates track the progress of study record and summary results submissions to ClinicalTrials.gov. Study records and reported results are reviewed by the National Library of Medicine (NLM) to make sure they meet specific quality control standards before being posted on the public website.

Study Major Dates

Study Start (Estimated)

August 1, 2026

Primary Completion (Estimated)

August 1, 2026

Study Completion (Estimated)

December 1, 2026

Study Registration Dates

First Submitted

April 23, 2026

First Submitted That Met QC Criteria

April 23, 2026

First Posted (Actual)

April 30, 2026

Study Record Updates

Last Update Posted (Actual)

May 18, 2026

Last Update Submitted That Met QC Criteria

May 14, 2026

Last Verified

April 1, 2026

More Information

Terms related to this study

Keywords

Additional Relevant MeSH Terms

Other Study ID Numbers

  • RS26-029

Plan for Individual participant data (IPD)

Plan to Share Individual Participant Data (IPD)?

NO

IPD Plan Description

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.

Drug and device information, study documents

Studies a U.S. FDA-regulated drug product

No

Studies a U.S. FDA-regulated device product

No

product manufactured in and exported from the U.S.

No

This information was retrieved directly from the website clinicaltrials.gov without any changes. If you have any requests to change, remove or update your study details, please contact register@clinicaltrials.gov. As soon as a change is implemented on clinicaltrials.gov, this will be updated automatically on our website as well.

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