Scientists have developed a test that detects a protein found in the brains of people with Alzheimer's disease, an advance they hope will lead to the first reliable method for diagnosing the fatal, degenerative illness.

As many as 4 million Americans may suffer from Alzheimer's disease, which progressively robs people of their memory and leads to dementia, but despite enormous efforts, no cause, cure or easy way to diagnose it has yet been discovered.

"This is the first real diagnostic leap," said Garth Bissette, associate professor of psychiatry at Duke University Medical Center and a participant in the study published in today's Journal of the American Medical Association. "What we all have wanted is a definitive, quick, easy and accurate test. And this one fulfills the promise."

Until now the diagnosis of Alzheimer's disease has relied largely on the ability of scientists to count and classify abnormal growths, called senile plaques, that appear in the brains of patients. But that method has been necessarily subjective, difficult to make and only certain after death. Researchers also have been forced to exclude other forms of dementia that can appear similar to that caused by Alzheimer's disease.

Even the most sophisticated experts are correct no more than two-thirds of the time. But in the study released today, a team from Abbott Laboratories reports that it has developed a test for a protein that seems to appear only in the tissue of people with Alzheimer's.

The new test is not yet widely available for evaluating living people because it can still only be used on brain tissue and must be examined by pathologists. But researchers have found this protein in the spinal fluid as well as the brain, and they say within a year it should be possible to test for the presence of the protein, called Alzheimer's Disease Associated Protein (ADAP), without taking tissue from the brain.

An accurate test for Alzheimer's disease would help researchers in a number of ways. If it proves possible to identify those with the protein before they become sick, scientists will be able to test drugs far earlier in the course of the disease. Until now they have been unable to administer any drugs until late in the illness, when the disease is diagnosed. That has made it particularly difficult for Alzheimer's experts to get a sense of what drug might stop the illness's progression.

There also is growing evidence that genetic factors may play a significant role in developing the disease, and an easy and valid test could have a major impact on the families of patients. Many people would want to know whether they were likely to develop the disease, but, as in the cases of other illnesses with a genetic link that have no cure, the knowledge may be difficult to accept.

"I'm excited about these results," said Zaven Khachaturian, director of the office of Alzheimer's Research at the National Institute on Aging. "It's a very important step, but we should remember it's part of a long process. There are still hurdles to overcome before we have a common, simple and completely reliable diagnostic device."

Khachaturian said that more tests would have to be performed on patients to make certain the protein appears only when associated with Alzheimer's disease and not with others.

In the study, researchers examined after death the brain tissue taken from 111 subjects throughout Europe and the United States. Twenty-seven people who died without neurological disease were used for comparison, or controls; twenty-eight subjects had neurologic impairments but not Alzheimer's disease. Tissue from the rest came from 56 Alzheimer's patients.

None of the 55 people who did not have Alzheimer's disease tested positive for the protein, ADAP. But 48 of 56, or 86 percent, of those diagnosed as Alzheimer's patients did. Of the remaining eight, five patients had dementia that indicated they may have had Alzheimer's, but, because of the time it took to get tissue samples after they died, it could not be proven. None of the researchers knew how patients were classified until after the study was completed.

"I think the test will work better as we get to know it better," said Hossein A. Ghonbari, senior research scientist at Abbott, who led the team. "At this point, it is still too hard to be certain in all cases that a patient has the disease."

The researchers tested 18 separate regions of the brain for the protein and found the parts responsible for memory storage and processing had much higher accumulations than other parts of the brain. Understanding how the protein forms, and where, could help scientists draw a much better picture of how the disease progresses and destroys memory.