Assessment of Anti–Factor Xa Levels of Patients Undergoing Colorectal Surgery Given Once-Daily Enoxaparin Prophylaxis: A Clinical Study Examining Enoxaparin Pharmacokinetics

IMPORTANCE: Between 4% and 12% of patients undergoing colorectal surgery and receiving enoxaparin, 40 mg per day, have a postoperative VTE event. An improved understanding of why “breakthrough” VTE events occur despite guideline-compliant prophylaxis is an important patient safety question. OBJECTIVE: To determine the proportion of patients undergoing colorectal surgery who received adequate anticoagulation based on peak anti–factor Xa (aFXa) levels while receiving enoxaparin at 40 mg per day. DESIGN, SETTING, AND PARTICIPANTS: This prospective, nonrandomized clinical trial was conducted between February 2017 and July 2018 with 90-day follow-up at a quaternary academic medical center in the Intermountain West and included patients undergoing colorectal surgery who had surgery after receiving general anesthesia, were admitted for at least 3 days, and received enoxaparin, 40 mg once daily. INTERVENTIONS: All patients had aFXa levels measured after receiving enoxaparin 40mg per day. Patients whose aFXa level was out of range entered the trial’s interventional arm where real-time enoxaparin dose adjustment and repeated aFXa measurement were performed. MAIN OUTCOMES AND MEASURES: Primary outcome: in-range peak aFXa levels (goal range, 0.3-0.5 IU/mL) with enoxaparin, 40 mg per day. Secondary outcomes: (1) in-range trough aFXa levels (goal range, 0.1-0.2 IU/mL) and (2) the proportion of patients with in-range peak aFXa levels from enoxaparin, 40 mg once daily, vs the real-time enoxaparin dose adjustment protocol. RESULTS: Over 16 months, 116 patients undergoing colorectal surgery (65 women [56.0%]; 99 white individuals [85.3%], 13 Hispanic or Latino individuals [11.2%], and 4 Pacific Islander individuals [3.5%]; mean [range] age, 52.1 [18-85] years) were enrolled. Among 106 patients (91.4%) whose peak aFXa level was appropriately drawn, 72 (67.9%) received inadequate anticoagulation (aFXa