BACKGROUND: Non-vitamin K antagonist oral anticoagulants (NOACs) are the preferred choice of anticoagulants to prevent stroke in most patients with atrial fibrillation (AF). NOAC's dosing algorithms are defined in the respective Summary of Product Characteristics (SmPC) but the European Heart Rhythm Association (EHRA) Practical Guide can also be used as it considers more complex clinical scenarios. Nevertheless, suboptimal dosing of NOACs compromises the efficacy and safety of this commonly prescribed therapy in the AF population. Clearer objectification of inappropriate dosing and its influencing factors is needed to optimise management of AF patients. OBJECTIVES: The primary aim of this study was  to investigate whether there is a difference in the perceived appropriateness of NOAC dosing with respect to the SmPC or the 2018 EHRA Practical Guide in AF patients criteria and influencing factors. The secondary aim was to explore if there were differences in appropriateness of NOAC dosing between primary care and specialist care, and when using different renal function formulas. METHODS: This retrospective study included AF patients treated with a NOAC in primary or in ambulatory specialist care in Antwerp (Belgium). Appropriateness of the NOAC dose was assessed according to the SmPC and 2018 EHRA recommendations. Univariate/multivariate analyses were performed to explore influencing factors for under- and overdosing of NOACs. RESULTS: Of the included 294 AF patients, 19.4% and 15.6% received an inappropriate dose according to the SmPC and the 2018 EHRA Practical Guide respectively (p = 0.003). Perceived frailty and higher weight were associated with underdosing relative to the SmPC, while a higher body mass index and the use of drugs/alcohol were associated with underdosing relative to the EHRA 2018 recommendations. Lower renal function and treatment with other NOACs than apixaban were associated with relative overdosing compared to both standards. CONCLUSIONS: Inappropriate NOAC dosing is present in almost twenty percent of AF patients according to the SmPC and requires further education of health care professionals and frequent reassessment of NOAC dosing. However, a significant lower prevalence of underdosing was present when judged by the 2018 EHRA criteria, likely reflecting decision making in complex AF patients. Perceived frailty, weight, renal function and type of NOAC are the main determinants of deviated dosing.