The primary purpose of this study is to evaluate the risk of major bleeding and stroke/systemic embolism (SE) among novel oral anti-coagulant (OAC) naïve and warfarin naïve Medicare Advantage patients with non-valvular atrial fibrillation (NVAF) treated with apixaban, dabigatran, rivaroxaban, or warfarin.

Inclusion Criteria:

• Had at least 1 pharmacy claim for warfarin, apixaban, dabigatran, or rivaroxaban during the identification period (01-Jan-2013 through 31-Dec-2015)
• Had continuous health plan enrollment with medical and pharmacy benefits for 6 months pre-index date (baseline period)
• Had continuous health plan enrollment with medical and pharmacy benefits for at least 1 month following index date
• Had at least 1 medical claim for atrial fibrillation any time before or on index date

Exclusion Criteria:

• Had medical claims with a diagnosis code for rheumatic mitral valvular heart disease, mitral valve stenosis or heart valve replacement/transplant during the 6-month baseline period
• Had medical claims with a diagnosis code for dialysis, kidney transplant, or end-stage chronic kidney disease during the 6 month baseline period
• Had medical claims indicating a diagnosis of venous thromboembolism during the 6-month baseline period
• Had claims indicating a diagnosis or procedure code of hip or knee replacement surgery within 6 weeks prior to the index date
• Had claims for a diagnosis or procedure code for reversible atrial fibrillation
• Had medical claims indicating pregnancy during the study period
• Had a pharmacy claim for warfarin, apixaban, dabigatran, edoxaban, or rivaroxaban during the 6-month baseline period
• Had > 1 oral anticoagulant prescription claim on the index date

Patients will be selected from the MORE Registry according to the inclusion/exclusion criteria. Four different cohorts will be evaluated: patients who were prescribed warfarin, apixaban, dabigatran or rivaroxaban. Only users of oral anticoagulant treatment from January 1, 2013 to January 1, 2016 will be included in this study.