(1208) Impact of Enoxaparin on Anti-Xa Levels When Transitioning to Unfractionated Heparin Infusion

Introduction: Inpatients requiring therapeutic anticoagulation may receive unfractionated heparin (UFH) infusions with monitoring of anti-factor Xa (anti-Xa) or activated partial thromboplastin time levels. Enoxaparin also increases anti-Xa levels, but it is unknown how this impacts heparin monitoring. The aim of this study was to assess the anti-Xa levels of patients who transitioned from therapeutic or prophylactic enoxaparin regimens to UFH infusions.

Methods: This retrospective, cohort study compared patients who received at least one dose of therapeutic or prophylactic enoxaparin and were transitioned to an UFH infusion within 24 hours. Patients with a total body weight ≥ 150 kg, cirrhosis, using direct-acting oral anticoagulants prior to UFH initiation, or with a starting UFH infusion dose of < 12 units/kg/hour were excluded. The primary outcome was the number of patients with supratherapeutic first monitoring anti-Xa levels (> 0.7 IU/mL) drawn 5-7 hours after starting UFH. Secondary outcomes included supratherapeutic baseline anti-Xa levels and time to reach therapeutic anti-Xa range (0.3 to 0.7 IU/mL) for patients assessed in the primary outcome who continued heparin for at least 72 hours. Categorical data was assessed by Chi square or Fishers exact test. Continuous data was assessed using Mann-Whitney U or a T-test.

Results: 104 and 233 patients received therapeutic and prophylactic enoxaparin, respectively. For patients with data to assess the primary outcome, 10/27 and 15/55 had supratherapeutic anti-Xa levels (37% vs. 27.3%; P = 0.367). Baseline anti-Xa levels were available in 221 patients; 16/68 (23.5%) and 10/153 (6.5%) of these were supratherapeutic (OR, 4.4; 95% CI, 1.88 to 10.3; P < 0.001). With 24 patients included, there was a shorter median time to therapeutic anti-Xa levels found for therapeutic enoxaparin patients (14.8 vs 10.2 hours, P = 0.903).

Conclusions: More patients who received therapeutic enoxaparin had supratherapeutic anti-Xa levels after transitioning to UFH, but this did not reach statistical significance. More baseline anti-Xa levels were supratherapeutic in patients on therapeutic compared to prophylactic enoxaparin regimens.