Scientists in the United States have proposed a new way to define Alzheimer's disease. They want the definition based on biological signs, such as changes in the brain, instead of memory loss and other customary signs of the disease.

　　The Associated Press says the move is aimed at improving Alzheimer's research by using more objective, factual measures, like brain imaging tests. The researchers want to choose patients for studies and get them tested sooner, when treatments may have more chance to help.

　　But it is too soon to use these brain scans and other tests in patient care because U.S. health officials have yet to approve them for that purpose, experts say. For now, American doctors continue to depend on the tools they have long used to study thinking skills to identify most cases.

　　The new definition will have an important effect: Many more people will be considered to have Alzheimer's because the biological signs can show up 15 to 20 years before other signs do.

　　"The numbers will increase dramatically," said Clifford R. Jack Junior of the Mayo Clinic in Rochester, Minnesota. He said, "There are a lot more cognitively normal people who have the pathology in the brain who will now be counted as having Alzheimer's disease."

　　Jack is a brain imaging specialist. He led a group of experts who amended medical guidance on the disease. The group worked with the U.S. government's National Institute on Aging and the Alzheimer's Association, a non-profit group.

　　The new guidelines were published last week in Alzheimer's & Dementia: The Journal of the Alzheimer's Association.

　　About Alzheimer's

　　About 50 million people worldwide have some form of dementia. The word dementia is a general term for a loss of memory and other mental abilities.

　　In the United States, about 5.7 million people have Alzheimer's disease under its current definition. But about one-third of people over age 70 who show no thinking problems actually have brain signs that suggest Alzheimer's, Jack noted.

　　There is no cure. Current medicines just temporarily ease signs of the disease. Many hoped-for treatments have failed, and doctors think one reason may be that the studies involved patients who had already suffered too much brain damage.

　　Dr. Eliezer Masliah is director of the Division of Neuroscience at the Institute on Aging. He said that by the time a doctor identifies a patient as having the disease, it is very late.

　　He said, "What we've realized is that you have to go earlier and earlier," just as doctors found with treating cancer.

　　Another problem is that as many as 30 percent of people being tested in Alzheimer's studies based on behavioral signs did not have the disease. They instead had either other forms of dementia or other medical conditions. That makes it unclear if experimental treatments for Alzheimer's might help. The new definition aims to improve identification of medical conditions through brain imaging and other tests.

　　Many other diseases already are defined by biomarkers. Biomarkers are objective health measures. For example, one biomarker for diabetes is blood sugar level.

　　Biomarkers for Alzheimer's disease were not possible until a few years ago, when brain scans and spinal fluid tests were developed for the purpose. They measure two proteins, amyloid and tau, as well as signs of nerve injury, degeneration and brain shrinkage.

　　The new guidelines identify use of these biomarkers to measure a patient's condition as it worsens.

　　What to do?

　　People may be worried and want these tests for themselves or a family member now.

　　Yet the Mayo Clinic's Clifford Jack advises against doing so. "There's no proven treatment yet," he says.

　　You might find a doctor willing to order them. However, spinal fluid tests are somewhat invasive, and brain scans can cost up to $6,000.

　　In the United States, health care plans usually do not pay for such tests because they are considered experimental research.

　　Americans with signs or a family history of dementia, or even healthy people concerned about the risk can consider taking part in one of the many studies being done.

　　I'm Caty Weaver.

　　Marilynn Marchione reported this story for the Associated Press, George Grow adapted her report for VOA Learning English. Cay Weaver was the editor.