Lewy body dementia (DLB), the most common form of neurodegenerative dementia after Alzheimer disease (AD), is frequently preceded years earlier by idiopathic REM sleep behavior disorder (iRBD), a predictor also of other synucleinopathies, specifically Parkinson disease (PD) and multiple systems atrophy. Research criteria for prodromal DLB consider Lewy body–associated mild cognitive impairment (MCI-LB)1 and recognize that patients with iRBD who exhibit MCI, especially multidomain MCI, are more likely to develop DLB than PD.2 With clinical and pathologic overlaps between DLB and AD, more sensitive and specific biomarkers for DLB are now being sought, including blood-based biomarkers, an effort supported by the American Brain Foundation in its “Cure One, Cure Many” initiative.