AIMS To investigate plasma apixaban concentrations and thrombin generation assay (TGA) parameters across different apixaban doses in atrial fibrillation patients who had dose-reduction criteria for apixaban.

METHODS This observational study included 374 patients (mean age 75.6 ± 7.7 years, 54.8% female) with dose-reduction criteria for apixaban. The patients were divided into 3 groups: (i) on-label standard dose (5 mg twice daily, n = 166); (ii) on-label reduced dose (2.5 mg twice daily, n = 55); and (iii) off-label underdose (2.5 mg twice daily, n = 153). Apixaban concentrations determined via the anti-Xa assay and TGA parameters were compared at trough levels.

RESULTS The off-label underdose group exhibited significantly lower apixaban trough concentrations than the on-label reduced-dose and standard-dose groups (56.7 ± 42.9 vs. 83.7 ± 70.4 vs. 129.9 ± 101.8 ng/mL, all P < .001). Less than 70% of all patients fell within the expected range of apixaban concentrations. Proportions exceeding the upper limit of the expected range were significantly lower in the off-label underdose group (1.3%) than in the on-label reduced-dose (9.1%, P = .005) and standard-dose (12.7%, P < .001) groups. The TGA parameters showed the on-label standard-dose group displaying the lowest thrombogenic profiles. Lower creatinine clearance was the most significant predictor of higher apixaban concentrations.

CONCLUSION Off-label underdosed apixaban resulted in lower apixaban concentrations than both on-label standard and reduced-dose regimens. A considerable proportion of the patients exhibited apixaban concentrations outside the expected range, suggesting the potential benefits of plasma concentration monitoring. Further studies are needed to compare dosages directly, investigate the impact of plasma apixaban concentration monitoring and validate the current dose-reduction criteria.