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

Condition(s) studied

Investigational drug(s) / intervention(s)

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.

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.

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.

Study 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. 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.

Eligibility

Primary outcome measure(s)

• Proportion of participants achieving prophylactic anti-Xa levels — 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.

Trial sites (1)

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