So he received brain scans for beta amyloid and took cognitive tests. He knew that in people like him with mild memory problems, the combination can cinch a diagnosis.

The result? He was in the early stages of Alzheimer’s disease.

Now he worries about his future. Alzheimer’s is “an ugly way to die,” he said. He has told his family that if he gets something like pneumonia, they should withhold treatment.

The sort of testing Dr. Gibbs had can be expensive, and diagnostic brain scans usually are not covered by insurance. The tests are not for the worried well whose memories are intact.

But they are available at some medical centers to those with mild memory problems.

Generally, even people without memory problems who have amyloid plaques in their brains are more likely to progress to Alzheimer’s, said Dr. Ronald Petersen, a neurologist at the Mayo Clinic in Rochester, Minn.

But not everyone does progress. Even when they do, it can be years before there are symptoms.

Outside of research studies, Dr. Petersen said, “we do not do amyloid scans on clinically normal people, because we don’t know what to tell them.”

Dr. Rabinovici, of U.C.S.F., sees people who are distressed by their memory problems, knowing something is wrong and unable to get an answer from their doctors.

“Often doctors can’t definitively tell them if their memory loss is related to aging,” he said. “A lot of times, doctors dismiss it and tell patients: ‘You are fine, it’s normal. You are 75 or 89 and depressed. Why not try an antidepressant?’”