Background There are limited data on the association of diabetes mellitus (DM) and levels of glycated hemoglobin (HbA1c) with outcomes in patients with atrial fibrillation (AF). Methods This retrospective cohort study included patients who were recently hospitalized with a primary or secondary diagnosis of AF from December 2015 through June 2018. Kaplan–Meier curves and Cox-regression adjusted hazard ratios (aHR) were calculated for the primary outcome of all-cause mortality and for the secondary outcomes of cardiovascular (CV) mortality and the composite outcome of CV death or hospitalization. Competing-risk regression analyses were performed to calculate the cumulative risk of stroke, major bleeding, AF- or HF-hospitalizations adjusted for the competing risk of all-cause death. Spline curve models were fitted to investigate associations of HbA1c values and mortality among patients with AF and DM. Results In total 1109 AF patients were included, of whom 373 (33.6%) had DM. During a median follow-up of 2.6 years, 414 (37.3%) patients died. The presence of DM was associated with a higher risk of all-cause mortality (aHR = 1.40 95% confidence intervals [CI] 1.11–1.75), CV mortality (aHR = 1.39, 95% CI 1.07–1.81), sudden cardiac death (aHR = 1.73, 95% CI 1.19–2.52), stroke (aHR = 1.87, 95% CI 1.01–3.45) and the composite outcome of hospitalization or CV death (aHR = 1.27, 95% CI 1.06–1.53). In AF patients with comorbid DM, the spline curves showed a positive linear association between HbA1c levels and outcomes, with values 7.6–8.2% being independent predictors of increased all-cause mortality, and values < 6.2% predicting significantly decreased all-cause and CV mortality. Conclusions The presence of DM on top of AF was associated with substantially increased risk for all-cause or CV mortality, sudden cardiac death and excess morbidity. HbA1c levels lower than 6.2% were independently related to better survival rates suggesting that optimal DM control could be associated with better clinical outcomes in AF patients with DM.

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