Long before memory falters or confusion sets in, the body may already be carrying a quiet signal in the blood. A large international study has found that elevated levels of a protein called p-tau217 can predict, years in advance, who among cognitively healthy older adults is most likely to develop Alzheimer's-related impairment — offering a glimpse of a future where a simple blood draw might replace costly scans and invasive procedures. Yet the researchers themselves urge patience: a probability is not a destiny, and a tool without a proven intervention to follow it remains, for now, a compass