An obscure case of using apixaban anti-Xa levels in a morbidly obese patient who was nil per os with enterocutaneous fistula

Apixaban anti-Xa levels have been introduced to monitor apixaban activity. Presented is a fundamental use of anti-Xa monitoring in questionable absorption in enterocutaneous fistula. A 57-year-old morbidly obese male (150 kg, BMI 42.5) presented to the emergency department with deep venous thromboses and pulmonary embolisms. He also had high-output enterocutaneous fistula managed with an abdominal collection device, total parental nutrition therapy, and nil per os status. He was able to take some oral (PO) medications; however, he reported finding whole capsules in his collection device. He refused enoxaparin injections for venous thromboembolism treatment. The decision was made to load with apixaban therapy with anti-Xa monitoring. After two 10-mg doses, peak apixaban anti-Xa level was 146 ng/ml which fell within on-therapy levels in the AMPLIFY study, prothrombin time 18 s, partial thromboplastin time 35.5 s, international normalized ration 1.5. Monitoring was an important factor in this patient with questionable absorption and further complicated by his morbid obesity which has not been adequately studied in clinical trials.