Bariatric SurgeryThe OAT-RCTNCT07559643
The OAT Trail: The Obesity Anti-Coagulation Thromboprophylaxis Trial.
A study testing Anti-Xa Activity Monitoring, Tinzaparin (Leo) and enoxaparin in people with Bariatric Surgery and Anti-Xa Activity, run by University College Dublin at 1 site in Ireland.
Trial summary
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. Read more about this study
- Who is it for?
- Ages 18 years and older
- Conditions the study is for
- Bariatric Surgery, Anti-Xa Activity, VTE (Venous Thromboembolism)
- What is being tested?
- Anti-Xa Activity Monitoring, Tinzaparin (Leo) and enoxaparin
- Type of treatment
- Injection or infusion
- Could I receive a placebo?
- No, everyone receives an active treatment
- How many people
- 180 participants
- Where
- 1 siteIreland
- Study dates
- 1 Aug 2026to 1 Dec 2026
- Phase
- Interventional
- Are there results?
- Not yet: the study is under way
Key requirements
Who can take part, as listed by the sponsor. Only the study team can confirm whether you are eligible, so take this list to your doctor.
- Ages 18 years and older
- All sexes
- No healthy volunteers
- Adult, Older adult
Who may be able to join
- 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
Who cannot join
- 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
Trial locations and contacts
1 site in 1 country. Contact the study team or a site near you to ask about taking part.
Ireland1 site
St Vincent's University Hospital
Dublin
Czara A Kennedy, BMBS, MRCSI, MSc, 0035312214000, czkennedy@svhg.ie
Naomi J Fearon, MD, FRCSI, 0035312214000, naomifearon@svhg.ie
About this study
Official title: 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.
Read more
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.
Read the 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.
- Being tested
- Anti-Xa Activity Monitoring, Tinzaparin (Leo), enoxaparin
- Keywords
- Anti-Xa, Metabolic Bariatric Surgery, Obesity, VTE, DVT
- Medical subject headings
- Venous Thromboembolism, Obesity, Overweight
What participants receive
What participants receive. In a randomised study each person is assigned to one group by chance.
Anti-Xa Activity MonitoringDiagnostic test
Also called Anti-factor Xa assay; chromogenic anti-Xa assay
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.
Tinzaparin (Leo)Drug
Also called Innohep
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.
enoxaparinDrug
Also called Clexane
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.
| Study group | Role | What they receive |
|---|---|---|
| Weight-based Tinzaparin | Experimental | 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. Receives: Diagnostic Test: Anti-Xa Activity Monitoring; Drug: Tinzaparin (Leo) |
| Weight-stratified Enoxaparin | Active comparator | 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. Receives: Diagnostic Test: Anti-Xa Activity Monitoring; Drug: enoxaparin |
What is being measured
The primary outcome is the main question the study is designed to answer.
Proportion of participants achieving prophylactic anti-Xa levels
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.
Measured: Post-operative day 2 (4 hours after third LMWH dose)
Secondary outcomes (8)
Incidence of clinically significant bleeding events
Incidence of minor and major bleeding events within 30 days of surgery, classified according to International Society on Thrombosis and Haemostasis (ISTH) criteria.
Measured: 30 days post-operatively
Incidence of venous thromboembolism
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).
Measured: 30 days post-operatively
Proportion achieving sub-prophylactic anti-Xa levels
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.
Measured: Post-operative day 2 (4 hours after third LMWH dose)
Proportion achieving supra-prophylactic anti-Xa levels
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.
Measured: Post-operative day 2 (4 hours after third LMWH dose)
Correlation between anti-Xa level and body habitus
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).
Measured: Post-operative day 2
Adverse drug reactions
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.
Measured: 30 days post-operatively
Patient-reported treatment satisfaction
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]
Measured: 6 weeks post-operatively
Patient-reported treatment adherence and injection site tolerability
Self-reported adherence (missed doses and reasons) and injection site tolerability assessed via supplementary questionnaire items at the 6-week post-operative clinic visit.
Measured: 6 weeks post-operatively
Timeline
From the start of the study to its completion; the mark is today. Estimated dates are the sponsor’s plan.
- First posted30 Apr 2026
- Study start1 Aug 2026 (estimated)
- Primary completion1 Aug 2026 (estimated)
- Study completion1 Dec 2026 (estimated)
Resources
- Official record on ClinicalTrials.govThe registry entry this page is built from, with its full change history.
- Publications (16)Papers the sponsor has linked to this study.
- Other trials for Bariatric SurgeryStudies of the same condition, open and under way.
- What the trial phases meanHow a treatment moves from first tests in people to approval.
More details
For doctors and researchers: how the study is designed, who runs it, and its registry IDs.
Study design
- Study type
- Interventional
- Phase
- Phase N/A
- Primary purpose
- Basic science
- Allocation
- Randomised
- Intervention model
- Parallel groups
- Model description
- Single-centre, prospective, parallel-group, open-label, randomised controlled superiority trial comparing weight-based tinzaparin (Arm A) versus weight-stratified enoxaparin (Arm B) for 21 days post-operatively in patients undergoing laparoscopic bariatric surgery. The primary endpoint is anti-Xa activity measured on post-operative day 2.
- Masking
- Single blind
- Who is masked
- Outcomes assessor
- Enrolment
- 180 participants (estimated)
Sponsor and investigators
- Lead sponsor
- University College Dublin
- Sponsor type
- University, hospital or other
- Responsible party
- Sponsor
- Investigators
- Helen M Heneghan, PhD, FRCSI, Principal investigator, University College Dublin
IDs, dates and publications
- ClinicalTrials.gov ID
- NCT07559643
- Sponsor's protocol ID
- RS26-029
- Acronym
- The OAT-RCT
- Status last verified
- Apr 2026
- First submitted
- 23 Apr 2026
- Last updated
- 18 May 2026
- Results posted
- No
- Data monitoring committee
- No
- FDA-regulated drug
- No
- FDA-regulated device
- No
- Individual data sharing
- No: 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.
Publications
Winegar DA, Sherif B, Pate V, DeMaria EJ. Venous thromboembolism after bariatric surgery performed by Bariatric Surgery Center of Excellence Participants: analysis of the Bariatric Outcomes Longitudinal Database. Surg Obes Relat Dis. 2011 Mar-Apr;7(2):181-8. doi: 10.1016/j.soard.2010.12.008. Epub 2010 Dec 29.
PubMed 21421182Chan AW, Tetzlaff JM, Gotzsche PC, Altman DG, Mann H, Berlin JA, Dickersin K, Hrobjartsson A, Schulz KF, Parulekar WR, Krleza-Jeric K, Laupacis A, Moher D. SPIRIT 2013 explanation and elaboration: guidance for protocols of clinical trials. BMJ. 2013 Jan 8;346:e7586. doi: 10.1136/bmj.e7586.
PubMed 23303884World Medical Association. World Medical Association Declaration of Helsinki: ethical principles for medical research involving human subjects. JAMA. 2013 Nov 27;310(20):2191-4. doi: 10.1001/jama.2013.281053. No abstract available.
PubMed 24141714Schulz KF, Altman DG, Moher D; CONSORT Group. CONSORT 2010 statement: updated guidelines for reporting parallel group randomised trials. BMJ. 2010 Mar 23;340:c332. doi: 10.1136/bmj.c332.
PubMed 20332509Atkinson MJ, Sinha A, Hass SL, Colman SS, Kumar RN, Brod M, Rowland CR. Validation of a general measure of treatment satisfaction, the Treatment Satisfaction Questionnaire for Medication (TSQM), using a national panel study of chronic disease. Health Qual Life Outcomes. 2004 Feb 26;2:12. doi: 10.1186/1477-7525-2-12.
PubMed 14987333Schulz KF, Grimes DA. Allocation concealment in randomised trials: defending against deciphering. Lancet. 2002 Feb 16;359(9306):614-8. doi: 10.1016/S0140-6736(02)07750-4.
PubMed 11867132Nutescu EA, Spinler SA, Wittkowsky A, Dager WE. Low-molecular-weight heparins in renal impairment and obesity: available evidence and clinical practice recommendations across medical and surgical settings. Ann Pharmacother. 2009 Jun;43(6):1064-83. doi: 10.1345/aph.1L194. Epub 2009 May 19.
PubMed 19458109Aminian A, Vosburg RW, Altieri MS, Hinojosa MW, Khorgami Z; American Society for Metabolic and Bariatric Surgery Clinical Issues Committee. The American Society for Metabolic and Bariatric Surgery (ASMBS) updated position statement on perioperative venous thromboembolism prophylaxis in bariatric surgery. Surg Obes Relat Dis. 2022 Feb;18(2):165-174. doi: 10.1016/j.soard.2021.10.023. Epub 2021 Nov 10. No abstract available.
PubMed 34896011Garcia DA, Baglin TP, Weitz JI, Samama MM. Parenteral anticoagulants: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012 Feb;141(2 Suppl):e24S-e43S. doi: 10.1378/chest.11-2291.
PubMed 22315264Borch KH, Braekkan SK, Mathiesen EB, Njolstad I, Wilsgaard T, Stormer J, Hansen JB. Abdominal obesity is essential for the risk of venous thromboembolism in the metabolic syndrome: the Tromso study. J Thromb Haemost. 2009 May;7(5):739-45. doi: 10.1111/j.1538-7836.2008.03234.x. Epub 2008 Nov 24.
PubMed 19036065Ageno W, Becattini C, Brighton T, Selby R, Kamphuisen PW. Cardiovascular risk factors and venous thromboembolism: a meta-analysis. Circulation. 2008 Jan 1;117(1):93-102. doi: 10.1161/CIRCULATIONAHA.107.709204. Epub 2007 Dec 17.
PubMed 18086925Birkmeyer NJ, Share D, Baser O, Carlin AM, Finks JF, Pesta CM, Genaw JA, Birkmeyer JD; Michigan Bariatric Surgery Collaborative. Preoperative placement of inferior vena cava filters and outcomes after gastric bypass surgery. Ann Surg. 2010 Aug;252(2):313-8. doi: 10.1097/SLA.0b013e3181e61e4f.
PubMed 20622663Mechanick JI, Apovian C, Brethauer S, Timothy Garvey W, Joffe AM, Kim J, Kushner RF, Lindquist R, Pessah-Pollack R, Seger J, Urman RD, Adams S, Cleek JB, Correa R, Figaro MK, Flanders K, Grams J, Hurley DL, Kothari S, Seger MV, Still CD. Clinical Practice Guidelines for the Perioperative Nutrition, Metabolic, and Nonsurgical Support of Patients Undergoing Bariatric Procedures - 2019 Update: Cosponsored by American Association of Clinical Endocrinologists/American College of Endocrinology, The Obesity Society, American Society for Metabolic and Bariatric Surgery, Obesity Medicine Association, and American Society of Anesthesiologists. Obesity (Silver Spring). 2020 Apr;28(4):O1-O58. doi: 10.1002/oby.22719.
PubMed 32202076Bray GA, Fruhbeck G, Ryan DH, Wilding JP. Management of obesity. Lancet. 2016 May 7;387(10031):1947-56. doi: 10.1016/S0140-6736(16)00271-3. Epub 2016 Feb 10.
PubMed 26868660Adams TD, Davidson LE, Litwin SE, Kim J, Kolotkin RL, Nanjee MN, Gutierrez JM, Frogley SJ, Ibele AR, Brinton EA, Hopkins PN, McKinlay R, Simper SC, Hunt SC. Weight and Metabolic Outcomes 12 Years after Gastric Bypass. N Engl J Med. 2017 Sep 21;377(12):1143-1155. doi: 10.1056/NEJMoa1700459.
PubMed 28930514
Study at a glance
From the registry record, last updated by the sponsor on 18 May 2026. Recruitment status is as the sponsor reported it: confirm with the study team.
- Official title
- 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.
- ClinicalTrials.gov ID
- NCT07559643
- Acronym / sponsor ID
- The OAT-RCT / RS26-029
- Keywords
- Anti-Xa, Metabolic Bariatric Surgery, Obesity, VTE, DVT
- Study type
- Interventional
- Recruitment status
- Not yet recruiting
- Enrolment
- 180 participants (estimated)
- Lead sponsor
- University College DublinUniversity, hospital or other
- Study start
- 1 Aug 2026 (estimated)
- Primary completion
- 1 Aug 2026 (estimated)
- Study completion
- 1 Dec 2026 (estimated)
- First posted
- 30 Apr 2026 (actual)
- Results
- Not posted in the source record
Clinical summary
The research question as the sponsor registered it. This describes what the study plans to test, not what it found.
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.
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 design
- Primary purpose
- Basic science
- Allocation
- Randomised
- Intervention model
- Parallel groups
- Masking
- Single blind (outcomes assessor)
- Number of arms
- 2
- Model description
- Single-centre, prospective, parallel-group, open-label, randomised controlled superiority trial comparing weight-based tinzaparin (Arm A) versus weight-stratified enoxaparin (Arm B) for 21 days post-operatively in patients undergoing laparoscopic bariatric surgery. The primary endpoint is anti-Xa activity measured on post-operative day 2.
| Arm | Type | Receives |
|---|---|---|
| Weight-based Tinzaparin | Experimental | Diagnostic Test: Anti-Xa Activity Monitoring, Drug: Tinzaparin (Leo)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. |
| Weight-stratified Enoxaparin | Active comparator | Diagnostic Test: Anti-Xa Activity Monitoring, Drug: enoxaparinEnoxaparin (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. |
Who may qualify
- Ages
- 18 years and older
- Sex
- All
- Healthy volunteers
- Not accepted
- Age groups
- Adult, Older adult
Key inclusion criteria (4 listed)
- 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
Key exclusion criteria (1 listed)
- 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
Eligibility is decided by the study team after screening; this overview does not establish that a patient qualifies. Full inclusion and exclusion criteria.
Interventions
| Intervention | Type | Arms |
|---|---|---|
| Anti-Xa Activity MonitoringAlso: Anti-factor Xa assay; chromogenic anti-Xa assayA 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. | Diagnostic test | Weight-based Tinzaparin, Weight-stratified Enoxaparin |
| Tinzaparin (Leo)Also: InnohepTinzaparin 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. | Drug | Weight-based Tinzaparin |
| enoxaparinAlso: ClexaneEnoxaparin 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. | Drug | Weight-stratified Enoxaparin |
Primary outcome
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.
8 secondary outcomes
Incidence of clinically significant bleeding events
Time frame: 30 days post-operatively
Incidence of venous thromboembolism
Time frame: 30 days post-operatively
Proportion achieving sub-prophylactic anti-Xa levels
Time frame: 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)
Correlation between anti-Xa level and body habitus
Time frame: Post-operative day 2
Adverse drug reactions
Time frame: 30 days post-operatively
Patient-reported treatment satisfaction
Time frame: 6 weeks post-operatively
Patient-reported treatment adherence and injection site tolerability
Time frame: 6 weeks post-operatively
Results and safety
This study is still under way. Results are not available in the source record yet.
Research links
- Official record on ClinicalTrials.govLast updated by the sponsor 18 May 2026.
- Background: PubMed 21421182Winegar DA, Sherif B, Pate V, DeMaria EJ. Venous thromboembolism after bariatric surgery performed by Bariatric Surgery Center of Excellence Participants: analysis of the Bariatric Outcomes Longitudinal Database. Surg Obes Relat Dis. 2011 Mar-Apr;7(2):181-8. doi: 10.1016/j.soard.2010.12.008. Epub 2010 Dec 29.
- Results publication: PubMed 23303884Chan AW, Tetzlaff JM, Gotzsche PC, Altman DG, Mann H, Berlin JA, Dickersin K, Hrobjartsson A, Schulz KF, Parulekar WR, Krleza-Jeric K, Laupacis A, Moher D. SPIRIT 2013 explanation and elaboration: guidance for protocols of clinical trials. BMJ. 2013 Jan 8;346:e7586. doi: 10.1136/bmj.e7586.
- Results publication: PubMed 24141714World Medical Association. World Medical Association Declaration of Helsinki: ethical principles for medical research involving human subjects. JAMA. 2013 Nov 27;310(20):2191-4. doi: 10.1001/jama.2013.281053. No abstract available.
- Results publication: PubMed 20332509Schulz KF, Altman DG, Moher D; CONSORT Group. CONSORT 2010 statement: updated guidelines for reporting parallel group randomised trials. BMJ. 2010 Mar 23;340:c332. doi: 10.1136/bmj.c332.
- Results publication: PubMed 14987333Atkinson MJ, Sinha A, Hass SL, Colman SS, Kumar RN, Brod M, Rowland CR. Validation of a general measure of treatment satisfaction, the Treatment Satisfaction Questionnaire for Medication (TSQM), using a national panel study of chronic disease. Health Qual Life Outcomes. 2004 Feb 26;2:12. doi: 10.1186/1477-7525-2-12.
- Results publication: PubMed 11867132Schulz KF, Grimes DA. Allocation concealment in randomised trials: defending against deciphering. Lancet. 2002 Feb 16;359(9306):614-8. doi: 10.1016/S0140-6736(02)07750-4.
- Results publication: PubMed 19458109Nutescu EA, Spinler SA, Wittkowsky A, Dager WE. Low-molecular-weight heparins in renal impairment and obesity: available evidence and clinical practice recommendations across medical and surgical settings. Ann Pharmacother. 2009 Jun;43(6):1064-83. doi: 10.1345/aph.1L194. Epub 2009 May 19.
- Results publication: PubMed 34896011Aminian A, Vosburg RW, Altieri MS, Hinojosa MW, Khorgami Z; American Society for Metabolic and Bariatric Surgery Clinical Issues Committee. The American Society for Metabolic and Bariatric Surgery (ASMBS) updated position statement on perioperative venous thromboembolism prophylaxis in bariatric surgery. Surg Obes Relat Dis. 2022 Feb;18(2):165-174. doi: 10.1016/j.soard.2021.10.023. Epub 2021 Nov 10. No abstract available.
- Results publication: PubMed 22315264Garcia DA, Baglin TP, Weitz JI, Samama MM. Parenteral anticoagulants: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012 Feb;141(2 Suppl):e24S-e43S. doi: 10.1378/chest.11-2291.
- Results publication: PubMed 19036065Borch KH, Braekkan SK, Mathiesen EB, Njolstad I, Wilsgaard T, Stormer J, Hansen JB. Abdominal obesity is essential for the risk of venous thromboembolism in the metabolic syndrome: the Tromso study. J Thromb Haemost. 2009 May;7(5):739-45. doi: 10.1111/j.1538-7836.2008.03234.x. Epub 2008 Nov 24.
- Results publication: PubMed 18086925Ageno W, Becattini C, Brighton T, Selby R, Kamphuisen PW. Cardiovascular risk factors and venous thromboembolism: a meta-analysis. Circulation. 2008 Jan 1;117(1):93-102. doi: 10.1161/CIRCULATIONAHA.107.709204. Epub 2007 Dec 17.
- Results publication: PubMed 20622663Birkmeyer NJ, Share D, Baser O, Carlin AM, Finks JF, Pesta CM, Genaw JA, Birkmeyer JD; Michigan Bariatric Surgery Collaborative. Preoperative placement of inferior vena cava filters and outcomes after gastric bypass surgery. Ann Surg. 2010 Aug;252(2):313-8. doi: 10.1097/SLA.0b013e3181e61e4f.
- Individual participant dataSharing planned: No. 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.
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Referral and contact
AzTrials does not enrol participants or take referrals. Contact the study team or a site listed below; they decide who can take part.
Helen M Heneghan, PhD, FRCSI principal investigator
University College Dublin
1 site in 1 country: see every site and its status.
Questions about this trial
What is this clinical trial studying?
A study testing Anti-Xa Activity Monitoring, Tinzaparin (Leo) and enoxaparin in people with Bariatric Surgery and Anti-Xa Activity, run by University College Dublin at 1 site in Ireland.
The full registered description is in the About this study section and the official record.
Who may be eligible to take part?
The sponsor lists 4 inclusion and 1 exclusion criteria. Eligibility is decided by the study team after screening, so read the full criteria and contact a listed site.
Is this clinical trial currently recruiting?
The record says: opening soon, as of the sponsor's last update on 18 May 2026. Status can change between updates, so confirm with the study contact before making plans.
Where is the study taking place?
The record lists 1 site in Ireland. Whether a site is taking part right now should be confirmed with that site. See every site.
What are the possible benefits and risks?
They depend on the study and the treatment, and no study can promise a benefit. The study team explains the known risks during informed consent, before anyone joins; the record itself does not list them.
Are results available for this study?
This study is still under way. Results are not available in the source record yet.
How can a doctor refer a patient or contact the research team?
Contact Czara A Kennedy, BMBS, MSc, MRCI (00353861933967, czara.kennedy@ucd.ie), or a site near the patient. AzTrials does not enrol participants or take referrals.
What does the primary outcome measure?
Proportion of participants achieving prophylactic anti-Xa levels (time frame: Post-operative day 2 (4 hours after third LMWH dose)). It is what the study was designed to measure, not a finding.
Where can I read the original study information?
On the ClinicalTrials.gov record for NCT07559643, which also lists 16 linked publications.
Does a listing mean the treatment is approved or proven to work?
No. A listing means the study is registered. It does not show that the treatment is safe, works, or is approved by a regulator.
How often is this information updated?
AzTrials copies changed records from ClinicalTrials.gov every night and reloads the whole registry every week. The sponsor last updated this record on 18 May 2026.