TUESDAY, April 5 (HealthDay News) -- New research shows that levels of a particular protein can indicate the severity of Alzheimer's disease, but it doesn't serve as an early warning sign of the illness.

Still, the findings are helpful because they reveal that signs of Alzheimer's lurk in the blood, said one of the co-authors of the study.

"What's promising is that we can get such clear signals" through an indicator known as a biomarker, said Monique Breteler, an epidemiologist with University Medical Center Rotterdam in the Netherlands. "It will be a matter of time, and resources, to find the markers or panels of markers in blood that can be used to help predict or diagnose the disease."

Alzheimer's disease, which robs its victims of their memories, affects an estimated 5.2 million people in the United States. That number is expected to grow to as many as 7.7 million Americans by 2030 as the population ages.

At the moment, physicians can't diagnose Alzheimer's with certainty in living patients; it can only be diagnosed with certainty after death.

Scientists have known for some time that there are higher levels of a protein called clusterin in the blood of people with Alzheimer's, Breteler said, and recent research has linked variations in the protein to the disease. Those findings, in turn, raise the prospect that the protein could actually be a factor in causing Alzheimer's, she said. If so, physicians might be able to diagnose the disease early on and maybe even try to stop it in its tracks.

Unfortunately, the new study revealed the limitations of clusterin testing.

From 1997-99, researchers tested levels of the protein in 60 people with Alzheimer's and a randomly chosen group of 926 people, and additionally, in 156 people diagnosed with Alzheimer's before 2007. They found that those with the highest levels of clusterin were also most likely to have Alzheimer's even after they adjusted statistics so they wouldn't be thrown off by factors such as age, education level and gender. Alzheimer's patients with the highest levels had the worst disease.

At the moment, one potential use for a clusterin blood test would be to predict rapid decline in Alzheimer's patients, said Greg Cole, associate director of the Alzheimer's Center at the University of California Los Angeles. "But," he said, "I don't think it is ready for that use."

Dr. James R. Burke, director of the Memory Disorders Clinic at Duke University Medical Center, identified a potential problem with such a test: clusterin levels don't specify whether a person has Alzheimer's disease or vascular dementia, a form of senility related to impaired blood flow, Burke said. "So, measurements of clusterin do not appear to be useful clinically," he noted.

Even so, research like this latest study is "important to prevent widespread adoption of tests that are not useful to answer clinical questions, such as, 'Am I going to get Alzheimer's disease?'" he added.

The study appears in the April 6 issue of the Journal of the American Medical Association.

More information

For more about Alzheimer's disease, try the U.S. National Library of Medicine.