BackgroundAtrial fibrillation (AF) is the most common cardiac arrhythmia in patients with chronic kidney disease (CKD) and acute coronary syndrome (ACS). This study aimed to explore the frequency and impact of AF on clinical outcomes in CKD patients with ACS.MethodsCKD inpatients with ACS between November 2014 and December 2018 were included based on the improving care for cardiovascular disease in China-ACS (CCC-ACS) project. Included patients were divided into an AF group and a non-AF group according to the discharge diagnosis. Multivariable logistic regression was used to adjust for potential confounders.ResultsA total of 16,533 CKD patients with ACS were included. A total of 1418 (8.6%) patients had clinically recognized AF during hospitalization, 654 of whom had an eGFR of 45 to < 60 ml/min/1.73 m2, and 764 had an estimated glomerular filtration rate (eGFR) < 45 ml/min/1.73 m2. Compared with the non-AF group, the AF group had a higher risk of in-hospital mortality [OR 1.250; 95% CI (1.001–1.560), P = 0.049] and major adverse cardiovascular events (MACEs) [OR 1.361; 95% CI (1.197–1.547), P < 0.001]. We also found that compared with patients with eGFR 45 to < 60 ml/min/1.73 m2, patients with eGFR < 45 ml/min/1.73 m2 had a 1.512-fold increased risk of mortality and a 1.435-fold increased risk of MACEs.ConclusionsAF was a risk factor affecting the short-term prognosis of ACS patients in the CKD population. Furthermore, the lower the eGFR, the higher the risk of in-hospital mortality and MACEs in CKD patients with ACS.Trial registry: Clinicaltrial.gov, NCT02306616. Registered 29 November 2014, https://clinicaltrials.gov/ct2/show/NCT02306616?term=NCT02306616&draw=2&rank=1