The method and system includes detecting atrial fibrillation in a patient by monitoring the blood oxygen saturation level over a period of time. The method and system produces a plethysmographic waveform from the monitored blood oxygen saturation level and analyzes the plethysmographic waveform and detected intervals and determines whether the patient is in atrial fibrillation. The method and system is preferably implemented in a software application and may be configured to report to the user on the current state of atrial fibrillation (AFIB) and a current trend.