BACKGROUND:The aim of this study was to determine the frequency and nature of errors made by computer electrocardiogram (ECG) analysis of atrial fibrillation (AF), and the clinical consequences. HYPOTHESIS:Computer software for interpreting ECGs has advanced. METHODS:A total of 10279 ECGs were collected, automatically interpreted by the built-in ECG software, and then reread by 2 cardiologists. AF-related ECGs were classified into 3 groups: overinterpreted AF (rhythms other than AF interpreted as AF), misinterpreted AF (AF interpreted as rhythms other than AF), and true AF (AF interpreted as AF by both computer ECG interpretation and cardiologists). RESULTS:There were 1057 AF-related ECGs from 409 patients. Among these, 840 ECGs (79.5%) were true AF. Overinterpretation occurred in 98 (9.3%) cases. Sinus rhythm and sinus tachycardia with premature atrial contraction and/or baseline artifact and sinus arrhythmia were commonly overinterpreted as AF. Heart rate ≤60 bpm and baseline artifact significantly increased the likelihood of overinterpreted AF. Misdiagnosis occurred in 119 (11.3%) cases, in which AF was usually misdiagnosed as sinus or supraventricular tachycardia. The presence of tachycardia and low-amplitude atrial activity significantly increased the likelihood of misdiagnosis of AF. Among the erroneous computer ECG interpretations, 17 cases (7.8%) were not corrected by the ordering physicians and/or repeat computer-ECG interpretation; inappropriate follow-up studies or treatments of the patients were undertaken with no serious sequelae. CONCLUSIONS:Erroneous computer ECG interpretation of AF was not rare. Attention should be concentrated on educating physicians about ECG appearance and confounding factors of AF, along with ongoing quality control of built-in software for automatic ECG interpretation.