“We don’t have enough information about what to tell people,” said Dr. Steven DeKosky, dean of the University of Virginia medical school, who participated in one of the working groups. “Until you can tell a clinician, ‘If you do this test you have X amount of reliability and to do that will make a difference in the life of your patient’ — until then, it remains in the lab.”

But the guidelines reflect a sense in the medical community that the moment when science will have more specific knowledge about biomarkers is not that far off. They are intended to encourage more research so that drugs can be developed to attack early brain changes and to identify people who might benefit from such drugs when they become available.

The goal, said William Thies, chief medical and scientific officer for the Alzheimer’s Association, is “extending the range of our ability to investigate this disease and eventually find the treatment that is going to be so necessary to avoid the epidemic of Alzheimer’s disease that we see facing us over the next 40 years.”

In the short term, the biggest impact is likely to be seen with people who fall into the middle phase, those with mild cognitive impairment linked to Alzheimer’s. Experts say there are at least as many people experiencing this phase as the 5.4 million people estimated to have Alzheimer’s dementia. And they expect others to now ask their doctors if they are showing signs of mild impairment, which include experiencing some difficulty or inefficiency with memory, attention or other mental faculties, while still being able to function independently.

Dr. Albert said that if patients with symptoms of mild cognitive impairment wanted to “increase the certainty” of the diagnosis by getting a brain scan or spinal fluid test, they should obtain such tests in a research trial so they have a better chance of getting accurate results.

The guidelines also clarify diagnosis criteria for people with dementia symptoms, distinguishing Alzheimer’s from other dementias, including vascular, fronto-temporal and Lewy body. And they note that the earliest symptom of Alzheimer’s dementia is not always memory loss, but could be mood changes or problems with language, spatial perception or reasoning.

Dr. Pierre Tariot, director of the Banner Alzheimer’s Institute in Phoenix, who was not involved in drafting the guidelines, called them “a step in the right direction” that he hoped would not be “misconstrued” as a sign that biomarker tests are further along than they are. He added, “The notion that Alzheimer’s disease is a continuum that has an extensive pre-symptomatic phase is a very important message to get out.”

Dr. Phelps said it would hardly be the last word from the medical community on Alzheimer’s.

“We’re not drawing a line and saying this is it,” Dr. Phelps said. “What we’re saying is this is the best of our knowledge and we’re not going to wait 27 years to revisit these again.”