U.S. health authorities recommended Tuesday that doctors diagnose the risk of developing Alzheimer’s disease before people develop full-blown dementia.

The National Institutes of Health and the Alzheimer’s Association made the recommendation, which could at least double the number of Americans receiving a diagnosis of Alzheimer’s and its early phases. It is the first revision of the guidelines for diagnosing the brain disease in 27 years.

In addition to updating the criteria for diagnosing full Alz­heimer’s, the health authorities created two new categories of the illness: a “preclinical” phase that occurs before patients show any memory loss or other thinking problems, and “mild cognitive impairment,” in which symptoms are subtle.

The recommendations are based on the growing realization that Alzheimer’s is the result of a gradual destruction of brain cells that control memory and other cognitive abilities, a process that begins years before clear-cut dementia becomes apparent.

“The new guidelines reflect today’s understanding of how key changes in the brain lead to Alz­heimer’s disease,” Creighton Phelps of the NIH’s National Institute on Aging told reporters during a briefing held Monday before the guidelines were released.

The recommendations are aimed at helping patients and their families prepare financially, logistically and emotionally for the disease, which can require years of intensive, expensive care.

“For many people, it is important to them to know to make plans to be able to discuss with their family at a point when they are still able to make decisions for themselves,” said Reisa A. Sperling of Harvard’s medical school, who helped develop the guidelines. “And for many people, it will put their mind at ease, even if it is the case that we will say one day that yes, this looks like Alzheimer’s may be beginning in your brain.”

Officials also hope the guidelines will identify more patients in the early stages of the disease who could volunteer for studies testing experimental drugs that could delay the disease, in addition to blood tests, brain scans and spinal taps that could use “biological markers” to provide more definitive diagnoses.

“Ultimately, it may be that we will try these drugs in people before they have symptoms but have evidence of Alzheimer’s in their brain, just as right now we try to lower cholesterol or treat diabetes before there are symptoms,” Sperling said.

About 5.4 million Americans have Alzheimer’s, which robs people of their memory and other thinking abilities. It often dramatically changes personalities and eventually renders those with the disease confused and unable to care for themselves or to recognize their loved ones.

As the population ages, Alz­heimer’s is becoming more common, raising fears about getting the disease and increasing frustration about the dearth of ways to diagnose and treat it. That has intensified the urgency of developing better ways to identify it, distinguish it from other forms of dementia and find ways to treat it, or at least delay symptoms.

Although several drugs have been approved to treat the symptoms of the disease, none are very effective. Experimental medications are being developed to alleviate symptoms and delay the progression, which many experts think is the most promising way to fight the disease.

Alzheimer’s is diagnosed through interviewing patients and family members. Doctors also conduct tests to exclude other causes, such as brain tumors, strokes and side effects of medications. A conclusive diagnosis comes from an examination of brain tissue after death.

Several experimental tests have shown promise for providing a more definitive diagnosis. They include scans that look for telltale buildups of plaque in the brain that damage cells, and spinal cord fluid and blood tests that can identify substances that play a role in creating those plaques.

Experts stressed that more research is needed to validate those tests before they are used outside carefully designed studies, which can only occur if more people receive diagnoses in the early stages so they can volunteer.

Some experts said they fear the recommendations might be misinterpreted by some patients, family members and doctors to mean that the experimental biomarker tests are ready for widespread use.

“At this point, we don’t know enough to be able to advise patients properly about what their risk for later dementia might be,” said John C. Morris of the Washington University School of Medicine in St. Louis. “Even if we do determine what that risk might be, we don’t have treatment to reduce that risk. It will raise a lot of concern.”

The recommendations are based on two years of study by four groups of experts organized by the NIH and the Alzheimer’s Association, which described their conclusions and recommendations in four papers being published in the journal Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association.

The hope is that Alzheimer’s will become more like heart disease, in which patients at increased risk can be identified early, just as patients at risk of heart attacks and strokes can be identified through cholesterol and blood pressure tests.

“The new criteria are really extending the range of our ability to investigate this disease and eventually find the treatments that will be so necessary to avoid the epidemic of Alzheimer’s disease that we see facing us,” said William Thies of the Alzheimer’s Association.