(640) Baseline Anti-Factor Xa Impact on Thrombotic Risk After Andexanet Alfa

Introduction: Factor Xa inhibitor (FXaI) serum concentrations have been shown to correlate with clinically-available low molecular weight heparin (LMWH)-calibrated anti-Factor Xa (AXA) assays. AXA assays may be used to estimate FXaI presence, but there is currently no consensus on the ideal threshold for FXaI reversal. Furthermore, the relationship between baseline AXA and thromboembolic events (TE) after andexanet alfa (AA) it is unknown. This study aims to evaluate safety outcomes in patients reversed with AA based on their baseline AXA activity.

Methods: This two-center, retrospective cohort study included patients with an AXA drawn prior to AA administration for apixaban or rivaroxaban reversal. TE were defined as one of the following within 30-days after AA administration: acute venous thromboembolism (VTE), pulmonary embolism, acute ischemic stroke (AIS), and myocardial infarction. The primary outcome compared TE incidence between baseline AXA cohorts (AXA >1.8 IU/mL vs < 1.8 IU/mL, AXA >1.0 vs < 1.0, AXA >0.5 vs < 0.5). A secondary outcome quantitatively assessed baseline AXA differences between patients with and without TE events.

Results: A total of 126 patients were included with a mean (SD) baseline AXA of 1.62 (0.34) IU/mL and 30-day TE rate of 20.6%. VTE (10.3%) and AIS (4.8%) were the most common TE. There was a significantly higher TE rate in patients with AXA >1.8 [12/81 (14.8%)] versus AXA < 1.8 [14/47 (29.8%)]; p=0.042. No statistical difference was seen when comparing AXA >1.0 versus AXA < 1.0 [21/112 (18.8%) vs 5/14 (35.7%); p=0.139] and AXA >0.5 versus AXA < 0.5 [24/120 (20%) vs 2/5 (40%); p=0.280]. There was no significant difference between baseline AXA concentration in patients with a TE [1.45 (0.54) IU/mL] compared to without a TE [1.60 (0.40) IU/mL]; p=0.11.

Conclusions: Patients receiving apixaban or rivaroxaban reversal with AA demonstrated a higher incidence of TE with baseline AXA < 1.8 IU/mL compared to >1.8 IU/mL in this study cohort. Current data is limited by an overall low representation of patients with baseline AXA < 1.0 IU/mL and detectability thresholds for the institutional AXA assay. Further research is warranted to establish a baseline AXA threshold that warrants reversal with AA while minimizing TE risk.