So the new results also give rise to a difficult question: Should doctors offer, or patients accept, commercially available spinal tap tests to find a disease that is yet untreatable? In the research studies, patients are often not told they may have the disease, but in practice in the real world, many may be told.

Some medical experts say it should be up to doctors and their patients. Others say doctors should refrain from using the spinal fluid test in their practices. They note that it is not reliable enough — results can vary by lab — and has been studied only in research settings where patients are carefully selected to have no other conditions, like strokes or depression, that could affect their memories.

“This is literally on the cutting edge of where the field is,” Dr. DeKosky said. “The field is moving fast. You can get a test that is approved by the F.D.A., and cutting edge doctors will use it.”

But, said Dr. John Trojanowski, a University of Pennsylvania researcher and senior author of the paper, given that people can get the test now, “How early do you want to label people?”

Some, like Dr. John Growdon, a neurology professor at Massachusetts General Hospital who wrote an editorial accompanying the paper, said that decision was up to doctors and their patients.

Sometimes patients with severe memory loss do not have the disease. Doctors might want to use the test in cases where they want to be sure of the diagnosis. And they might want to offer the test to people with milder symptoms who want to know whether they are developing the devastating brain disease.

One drawback, though, is that spinal fluid is obtained with a spinal tap, and that procedure, with its reputation for pain and headaches, makes most doctors and many patients nervous. The procedure involves putting a needle in the spinal space and withdrawing a small amount of fluid.