For the first time in 27 years, the definition of Alzheimer’s disease is being recast in new medical guidelines that reflect fast-mounting evidence that it begins ravaging the brain years before the symptoms of dementia . 
 
 The guidelines, to be issued Tuesday by the National Institute on Aging and the Alzheimer’s Association, divide the disease into three stages: a phase when dementia has developed, a middle phase in which mild problems emerge but daily functions can still be performed, and the most recently discovered phase, in which no symptoms are evident but changes are brewing in the brain. 
 
 “We’re redefining Alzheimer’s disease and looking at this in a different way than had ever been done,” said Creighton Phelps, director of the National Institute on Aging’s Alzheimer’s Disease Centers Program. “I think we’re going to start to identify it earlier and earlier.” 
 
 The drive to diagnose Alzheimer’s before it has progressed into profound dementia is also reflected in a bill introduced in Congress this month, which would create specific Medicare cost codes for Alzheimer’s diagnosis, including steps involving discussions between the patient’s doctor and caregivers, a recognition that keeping family members well-informed can result in better planning and care. 
 
 “Early diagnosis is really the key to this,” said Representative Edward J. Markey, Democrat of Massachusetts and a sponsor of the bill. “Oftentimes family members notice the symptoms in their loved ones, but it’s only years later that they get diagnosed or understand what resources are available.” 
 
 The most striking addition to the guidelines concerns methods that assess brain changes involved in Alzheimer’s, including brain scans and tests of cerebral spinal fluid. Such methods measure what are called biomarkers, physiological indicators that someone is likely to develop dementia eventually, just as cholesterol and blood pressure are biomarkers of impending heart disease. 
 
 For now, the guidelines specify that Alzheimer’s biomarkers — including abnormal levels of the proteins amyloid and tau, and shrinkage of certain brain areas — should not yet be put into widespread use, but used only with patients enrolled in clinical trials. 
 
 That is because scientists cannot yet standardize the results of the tests, or know “what measure is truly abnormal and what measure is not,” said Marilyn Albert, director of the Johns Hopkins Alzheimer’s Disease Research Center, and a leader of one working group that developed the new guidelines. 
 
 As many as a third of people with amyloid plaques in their brains, for example, have not developed Alzheimer’s symptoms by the time they die. The guidelines also urge caution because there is currently no drug known to halt or significantly delay the onset of symptoms, so people told they are likely to get Alzheimer’s have no effective medication to take. 
 
 “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.” ||||| The first update in nearly 30 years to U.S. guidelines for the diagnosis of Alzheimer's disease expands the definition to include patients with earlier stage symptoms and recognizes the condition as being on a continuum rather than just end-stage dementia. 
 
 People diagnosed with Alzheimer's tend to be 65 and older, but evidence suggests that signs of the memory-robbing disease, like amyloid protein plaques and nerve cell death, may start in the brain five to 10 years before they can currently be detected. In the future, doctors hope to rely more on biomarkers, proteins and other molecules in patients' blood and tissue that are indicators of the disease. 
 
 For now, clinical practice won't change much. Diagnosis will continue to rely on the judgment of clinicians, as it has since the diagnostic criteria were first published in 1984, according to experts, including those from the National Institute on Aging and Alzheimer's Association, who led the new guidelines that were published Tuesday. 
 
 5.4 million Number of Americans who have Alzheimer's disease 
 
 Still, the formal recognition that people can have less severe memory problems known as mild cognitive impairment years before full-fledged Alzheimer's could enable doctors "to investigate the ideas and eventually to find the treatments that are so necessary to avoiding the epidemic of Alzheimer's disease that we see facing us," said William Thies, chief medical and scientific officer for the Alzheimer's Association. 
 
 Earlier diagnosis would enable patients to make plans about their lives when they are better equipped to make such decisions. And once medications become available that are proven to slow the progress of the disease, doctors hope that Alzheimer's patients can begin taking them earlier. 
 
 "We felt that our best chance for disease-modifying therapy is to detect disease and intervene much earlier," said Reisa Sperling, a committee member and director of the Center for Alzheimer's Research and Treatment at Brigham and Women's Hospital. 
 
 While some experts advocate taking currently available drugs in the early stages of Alzheimer's, there is no conclusive evidence that they are effective in slowing down the disease. In recent years, several clinical trials of experimental treatments that have reached late-stage testing have been disappointing. 
 
 Alzheimer's is thought to affect some 5.4 million Americans. An equal number of people may have mild cognitive impairment or earlier stages of the disease, though data about these individuals are lacking. 
 
 The new guidelines detail the importance of studying biomarkers, which are considered to be key to earlier diagnosis because they don't rely on subjective judgments of patients' behavior. Such indicators could also be useful in identifying subjects for research studies. 
 
 Though several biomarkers related to Alzheimer's have been identified already, they aren't ready for clinical usage yet owing to a lack of standardization about how the tests are performed and what should be considered the cutoff point for a normal result, said Marilyn Albert, director of the Johns Hopkins Alzheimer's Disease Research Center. 
 
 Write to Shirley S. Wang at shirley.wang@wsj.com
– Alzheimer's disease is being redefined for the first time in 27 years, with new medical guidelines reflecting the fact that the disease is a "continuum." Growing evidence shows that Alzheimer's starts affecting the brain years before dementia symptoms present themselves, and the new guidelines reflect that by dividing the disease into three stages: end-stage dementia, a middle phase involving mild problems, and a symptomless beginning stage where the brain has already begun to change. "I think we’re going to start to identify it earlier and earlier," says one expert. The new guidelines are concerned with measuring biomarkers, or indicators that a person will likely develop dementia, in clinical trial patients, the New York Times reports. The Wall Street Journal notes that earlier diagnosis of the disease will also allow earlier intervention, and could someday allow patients to take Alzheimer's-slowing drugs earlier.