WASHINGTON — A key marker for Alzheimer's disease has been detected outside the brain, possibly paving the way for skin tests for the disorder afflicting 2.5 million Americans, researchers said today.

Harvard researchers said their discovery of amyloid B protein deposits in the skin, intestines and other tissues bolsters the theory that Alzheimer's may involve the whole body, not just the brain. Doctors consider the amyloid "plaques" found in brains of all Alzheimer's victims a hallmark of the disease--the most common cause of intellectual decline in older people.

If the study is refined and findings confirmed by larger studies, a "skin punch" test for Alzheimer's could be available in two years, said Dr. Dennis Selkoe, who headed the Harvard research team. Currently, Alzheimer's can only be positively diagnosed by examining a patient's brain after death.