Despite the increasingly sophisticated methods clinicians have for diagnosing disease, Alzheimer's remains out of reach for even the most advanced imaging and molecular probes. The degenerative illness can be definitively diagnosed only at autopsy, when pathologists can confirm the presence of hallmark plaques and tangles in the brain. But a promising new blood test may help confirm a diagnosis early in the disease's progression, which opens the possibility for prevention of dementia and mental decline even before the earliest onset of symptoms. The new test analyzes more than two dozen proteins in the blood, and is 80% accurate in identifying patients with the disease. It is only the latest in a series of new methods, including tests of spinal fluid, aimed at detecting and confirming Alzheimer's earlier in patients' lives. Quicker diagnoses could help patients take advantage of behavioral interventions — such as keeping the mind active by maintaining social contacts and learning new things — that may slow the mental deterioration of Alzheimer's.