Validation of apixaban anti-factor Xa assay and impact of body weight.

BACKGROUND: Direct oral anticoagulants (DOACs) are widely used as therapies for venous thromboembolism and other cardiovascular diseases. However, routine coagulation monitoring is not required, but may be clinically indicated in high risk populations including obese patients. OBJECTIVES: The aims of this study were two fold; to validate a chromogenic assay for anti-factor Xa measurement in patients taking apixaban, and correlate it with PT/INR and PTT, and to measure anti-factor Xa levels in patients who weighed >120 kg. PATIENTS/METHODS: Patients who were taking apixaban had 3 blood samples drawn over a 4 h period. Apixaban levels were determined using an anti-factor Xa activity assay (STA-Liquid Anti-Xa) using STA-Apixaban Calibrator and STA-Apixaban Controls. The PT/INR was determined using standard methodology. Apix MS, using manufacturer provided apixaban standard, was performed on plasma. RESULTS AND CONCLUSIONS: 18 normal weight patients, 39 obese patients and 14 controls were enrolled. There was a strong correlation between apixaban anti-factor Xa activity compared to plasma Apix MS (r = 0.95). In patients >120 kg, there was a statistically significant decreased rate of change in anti-factor Xa levels after ingestion. Further, the area under the curve for apixaban anti-factor Xa levels was significantly lower in patients over 120 kg. While INR correlated with apixaban MS and apixaban anti-factor Xa activity in both normal weight and obese patients, the association was not sufficiently strong to clinically manage patients, normal weight or obese. Given these findings, research is necessary to investigate the clinical utility of apixaban anti-factor Xa activity measurement in selected populations.