An interventional study of Anti-Xa Activity Monitoring and Tinzaparin (Leo) in Bariatric Surgery, Anti-Xa Activity and VTE (Venous Thromboembolism), sponsored by University College Dublin. Not yet recruiting at 1 site in Ireland. Open to participants aged 18 Years and older. Per ClinicalTrials.gov, last updated 2026-05-18.
Sponsored by University College Dublin · Not applicable, Interventional, and Basic science
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.
712 studies on the registry are indexed under Venous Thromboembolism; 146 are open to participants now.
This study's planned enrollment of 180 is below the median of 240 across 401 interventional studies indexed under Venous Thromboembolism.
Browse Venous Thromboembolism studies →University College Dublin is the lead sponsor of 113 studies on the registry; 32 are open to participants now.
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Go through the criteria one by one — yes, no or not sure — and take the list to your doctor or the study team. Kept in this browser only.
Exclusion Criteria:
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Weight in kilograms divided by height in metres squared. Example: 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) ≈ 22.9.
What counts as overweight or obesity differs between countries (some use lower cut-offs), so go by the range the study states.
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
An allergy, or another reaction of the immune system, to a substance. “Hypersensitivity to the study drug” means a known allergic reaction to it — not every side effect a medicine can have.
A reason a treatment, test or procedure should not be given to a particular person, such as an allergy, another illness or a medicine they take.
GFR (glomerular filtration rate) is how much blood the kidneys filter each minute. It is usually estimated from a blood test — eGFR, by formulas such as CKD-EPI or MDRD — and some studies ask for it to be measured directly instead. Usually given in mL/min/1.73 m².
Check the method, the units and the minimum the study states. A value of 90 or above is in the normal-or-high range, but kidney damage can show in other tests too.
BMI (body mass index)
Weight in kilograms divided by height in metres squared. Example: 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) ≈ 22.9.
What counts as overweight or obesity differs between countries (some use lower cut-offs), so go by the range the study states.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Informed consent
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Hypersensitivity
An allergy, or another reaction of the immune system, to a substance. “Hypersensitivity to the study drug” means a known allergic reaction to it — not every side effect a medicine can have.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Contraindication
A reason a treatment, test or procedure should not be given to a particular person, such as an allergy, another illness or a medicine they take.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
GFR and eGFR (kidney filtering)
GFR (glomerular filtration rate) is how much blood the kidneys filter each minute. It is usually estimated from a blood test — eGFR, by formulas such as CKD-EPI or MDRD — and some studies ask for it to be measured directly instead. Usually given in mL/min/1.73 m².
Check the method, the units and the minimum the study states. A value of 90 or above is in the normal-or-high range, but kidney damage can show in other tests too.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
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 · Drug: Tinzaparin (Leo)
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 · Drug: enoxaparin
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.
Also known as: Anti-factor Xa assay; chromogenic anti-Xa assay
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.
Also known as: Innohep
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.
Also known as: Clexane
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.
Time frame: Post-operative day 2 (4 hours after third LMWH dose)
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.
Time frame: 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).
Time frame: 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.
Time frame: 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.
Time frame: 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).
Time frame: 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.
Time frame: 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. \[Time Frame: 6 weeks post-operatively\]
Time frame: 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.
Time frame: 6 weeks post-operatively
Plan to share: 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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Studies of the drug tested here: Tinzaparin