Background:This study evaluated whether measuring prothrombin time (PT) using particular reagents of interest predicted apixaban-associated anticoagulant activity in Japanese patients with non-valvular atrial fibrillation (NVAF).Methods and Results:Two reagents, Shinplastin Excel S and Coagpia PT-N, were used to evaluate PT under apixaban therapy. From June 2013 to February 2014, 103 NVAF patients were recruited, and PT was measured at 3 time points: (1) anytime in the outpatient clinic, (2) at peak, and (3) at trough. In spike-in experiments using pooled citrated normal human platelet-poor plasma with these PT reagents, apixaban prolonged PT values in a concentration-dependent manner. PT values significantly correlated between both reagents (r=0.97) in outpatients. PT values in outpatients taking 5-mg apixaban bid were significantly prolonged and had wide inter- and intraindividual variability. Peak values were significantly higher than trough values, with both values higher than normal. The dose change of apixaban from 5 mg bid to 2.5 mg bid in outpatients halved the degree of PT prolongation in each NVAF patient.Conclusions:The PT value measured by these specific reagents can predict apixaban-associated anticoagulant activity, although there is significant interpatient variability. (Circ J 2014; 78: 2651–2656)