Anticoagulation in Obesity

Obesity raises the risk of venous thromboembolism and changes anticoagulant distribution and clearance, but fixed-dose and weight-based regimens still work in most patients. The main problems are under-dosing of prophylaxis, uncertain data in very large patients and altered absorption after bariatric surgery.

Definition. Class 3 obesity is a body mass index of 40 or over, or a weight over 120 kg. Dose by actual body weight, not ideal weight, for low molecular weight heparin.

Principle. Use drugs with predictable response and avoid arbitrary dose caps.

• Treatment of thrombosis and atrial fibrillation: apixaban or rivaroxaban at the usual doses (apixaban 10 mg twice daily for 7 days then 5 mg twice daily; rivaroxaban 15 mg twice daily for 21 days then 20 mg once daily with food) are acceptable at any weight. Dabigatran and edoxaban have limited data over 120 kg, so they are not first choice.
• Enoxaparin treatment: 1 mg/kg subcutaneously twice daily on actual weight with no upper cap. Dalteparin 200 units/kg once daily.
• Enoxaparin prophylaxis: 40 mg twice daily if body mass index is 40 or over (or 0.5 mg/kg twice daily). Unfractionated heparin prophylaxis 7500 units subcutaneously every 8 hours is an alternative.
• Warfarin: target INR 2.0 to 3.0 with the usual starting dose; INR control is similar to other weights.
• After bariatric surgery (gastric bypass): absorption of rivaroxaban and other direct oral anticoagulants may be reduced, so warfarin or low molecular weight heparin is preferred for high-risk indications.

Anti-Xa monitoring. For weight over 150 kg, creatinine clearance under 30 mL/min or prolonged use, check peak anti-Xa activity 4 hours after the third or fourth dose (0.6 to 1.0 units/mL with twice-daily dosing, 1.0 to 2.0 units/mL with once-daily dosing).

Kidney function. Use actual weight in the Cockcroft-Gault equation for direct oral anticoagulant decisions only with care, because it overestimates clearance; many clinicians use adjusted body weight (ideal weight plus 0.4 times the excess).

Pregnancy. Weight-banded enoxaparin: 91 to 130 kg 60 mg daily; 131 to 170 kg 80 mg daily; over 170 kg 0.6 mg/kg daily.

Surgery. Obesity with surgery: enoxaparin 40 mg twice daily with mechanical prophylaxis for the duration of high-risk immobility.

Special groups. Older obese patients need kidney function checks at least every 6 months. In East Africa, direct oral anticoagulants may not be available, so weight-based low molecular weight heparin or warfarin is used.

Monitoring. Weight and creatinine at baseline and every 3 to 6 months, full blood count yearly, and INR at the usual intervals on warfarin.

Referral and escalation. Seek haematology advice for weight over 150 kg with clot recurrence, bariatric surgery with anticoagulation, or doubt about anti-Xa results.

Stopping. Stop on the same indication-based schedule as other patients. Re-weigh at each review and adjust doses after weight change of more than 10%.