A device documented arrhythmia is confirmed in cardiac activity by obtaining a cardiac activity (CA) data set that includes CA signals (151) for a series of cardiac events and includes device documented (DD) markers within the series of cardiac events. The device documented markers are indicative of atrial fibrillation (AF) detected by an ICM (100) utilizing an on-board R-R interval irregularity (ORI) process (136) to analyze the CA signals. A feature enhancement function is applied to the CA signals (151) to form modified CA signals with enhanced sinus features. The enhanced sinus features in the modified CA signals are analyzed and a confirmatory feature detection process (137) is utilized to identify false AF detection by the ORI process (136). A result of the analysis identifying false AF detection by the ORI process (136) is recorded.
