Anti–factor Xa activity assays of direct‐acting oral anticoagulants during clinical care: An observational study

BACKGROUND: Direct‐acting oral anticoagulants (DOACs) are increasingly used to prevent and treat thromboembolism. Although measurement of DOAC concentrations is not currently recommended as part of routine patient care, measurement of DOAC concentrations with anti–factor Xa activity assays have recently become clinically available. OBJECTIVES: Our goal was to determine the clinical conditions under which DOAC concentration measurements are requested. MATERIALS AND METHODS: Retrospective electronic medical record analysis of indications for DOAC concentration measurements by anti–factor Xa activity assay at a single academic medical center from July 2015 through April 2020. RESULTS AND CONCLUSIONS: Ninety‐one DOAC concentration measurements were made in 69 patients: 28 received apixaban and 41 received rivaroxaban. The most frequent indication for concentration measurement was drug exposure assessment (38/69; 55%) in patients with potentially altered pharmacokinetics (altered absorption or clearance), recurrent thromboembolic events, or possible medication nonadherence. Fourteen of 69 patients had repeated measurements during preoperative evaluation before emergent surgery; one‐third of those with detectable levels upon presentation had repeated measurements until concentrations were undetectable. Levels were undetectable in 4 of 4 patients scheduled for elective surgery. Eleven of 69 patients had DOAC measurements in the setting of major bleeding; 5 of these 11 received a specific DOAC reversal agent. While most of the observed indications appear in clinical guidelines, altered absorption does not. Overall, clinicians are requesting DOAC concentration measurements to evaluate drug exposure in patients with conditions that might alter the absorption or clearance of the DOAC, to evaluate surgical bleeding risk, and in the setting of major bleeding.