Abstract Background/Introduction Previous observational studies reported that off-label underdosed apixaban might be associated with a higher risk of stroke. However, the real-world clinical practice data on the off-label underdosed apixaban in patients with atrial fibrillation (AF) who meet single dose reduction criteria remains scarce. Purpose This study aimed to evaluate the apixaban dosing pattern in patients with AF who met a single dose reduction criterion. The difference in clinical characteristics underlying the on-label and off-label underdose prescription of apixaban was also assessed. Methods The efficAcy and Safety of aPixaban In REal-world practice in Korean frail patients with AF (ASPIRE) study is a multicenter, prospective cohort study. This study included patients with AF having a single dose reduction criterion of apixaban. Study subjects were categorized into two groups according to apixaban dose (on-label 5 mg twice daily [on-label], or off-label underdose 2.5 mg twice daily [off-label]). We evaluated the difference in the clinical characteristics were evaluated. Additionally, the ‘marginal zone’ was defined as age between 75 and 80 years, weight between 60 and 65kg, and creatinine level between 1.2-1.5mg/dL. Results A total of 1997 patients were included in this analysis (mean age 74.3±7.8 years, women 55.8%); 591 (29.7%) patients were 80 years or older, 1250 (62.6%) patients had body weight 60 kg or below, and 156 (7.8%) patients had serum creatinine 1.5mg/dL or higher. Of these patients, 1022 (51.2%) patients were prescribed off-label underdose apixaban. The off-label group was older and had higher mean CHA2DS2-VASC score, Charlson Comorbidity Index, and HAS-BLED scores than the on-label group. Hypertension, heart failure, prior bleeding history, chronic kidney disease, anemia, and concomitant antiplatelet agent usage were more prevalent in the off-label group, but prior stroke history was more common in on-label group. In the age ≥ 80 years group, the marginal zone serum creatinine was more prevalent in off-label group (p=0.011). In the body weight ≤ 60 kg group, the prevalence of marginal zone bodyweight and serum creatinine were higher in off-label group (p<0.001 and p=0.02, respectively). In creatinine ≥ 1.5 mg/dL group, the proportion of marginal zone body weight were more common in off-label underdose group (p=0.001) (Figure). Conclusions Among AF patients with a single dose reduction criterion for apixaban, almost half of the patients were prescribed off-label underdose apixaban. Physicians were more likely to prescribe off-label underdose apixaban in patients with more frail characteristics, whereas on-label apixaban was preferred in patients with prior history of stroke. The clinical outcome of off-label underdose apixaban would be compared with the on-label standard dose in a future study.