An implantable medical device and associated method for classifying a patient's risk for arrhythmias by sensing a cardiac electrogram (EGM) signal and selecting a first pair of T-wave signals and a second pair of T-wave signals. A first difference between the two T-wave signals of the first pair is compared to a second difference between the two T-wave signals of the second pair. A T-wave alternans phase reversal is detected in response to comparing the first difference and the second difference, and the patient's arrhythmia risk is classified in response to detecting the phase reversal.