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Direct oral anticoagulant (DOAC) use for stroke prevention in atrial fibrillation (AF) has dose reduction criteria including age, weight, serum creatinine, and creatinine clearance. There is a paucity of data for rates of inappropriate inpatient DOAC dosing in Australia. The objective was to determine the rates of inappropriate inpatient DOAC dosing in AF and identifying its associated underlying factors. We conducted a retrospective cross-sectional study from December 2013 to November 2019 across six South Australian public hospitals utilising a centralised electronic health record. Multivariate analysis was used to identify factors associated with underdosing of patients prescribed apixaban. Of 1882 inpatients, 544 (28.9 %) were inappropriately dosed. Underdosing was the most common form of inappropriate dosing with rates of 22.9 % (n = 295), 7.1 % (n = 7), and 25.1 % (n = 124) for apixaban, dabigatran, and rivaroxaban, respectively. Independent factors predictive of apixaban underdosing included higher age (adjusted odds ratio (aOR) 1.63 [95 % Confidence Interval (CI): 1.47–1.81]), higher serum creatinine (aOR 1.13 [95 % CI: 1.08–1.19]), higher total number of drugs on discharge (aOR 1.08 [95 % CI: 1.04–1.11]), and being already prescribed a DOAC on admission (aOR 1.63 [95 % CI: 1.12–2.38]). Nearly one quarter of all apixaban prescribing was inappropriately underdosed. Older patients with multimorbidity, frailty and polypharmacy present a challenge for clinicians in balancing risks of thromboembolism and bleeding. It is likely prescribers are more conservative in their apixaban dosing in this population. Clinicians should consider alternative drug regimens to avoid DOAC use at inappropriate doses at unknown safety and efficacy. [ABSTRACT FROM AUTHOR]