A Blood Test for Alzheimer's?

New research hints at the possibility of early detection, but experts balk.

ByAUDREY GRAYSON<br>ABC News Medical Unit
September 08, 2008, 3:37 PM

Sept. 8. 2008&#151;, 2008 -- New research released Monday touts a potential blood test to predict which people are at risk for developing Alzheimer's disease. But experts overwhelmingly agree that the test might not be such a reliable screening tool for Alzheimer's -- and is certainly not ready for prime time.

The study looked at a potential biomarker for Alzheimer's disease -- a protein called ABeta42. Researchers at Columbia University performed blood tests on more than 1,100 elderly patients to examine the relationship between the amount of ABeta42 in a person's blood and their likelihood of developing Alzheimer's. They found that those with high levels of this protein were three times as likely to develop Alzheimer's over a five-year period.

"Our results suggest individuals with [higher ABeta42 levels] are at increased risk of [Alzheimer's disease]," the researchers note in the study, published in the Proceedings of the National Academy of Science.

The study authors surmise that the connection may be useful in determining who may be at higher risk of developing the condition -- and might even lead to a diagnostic test to catch it early.

But Alzheimer's disease researchers not affiliated with the study overwhelmingly disagree that this discovery moves scientists any closer to developing a truly reliable blood biomarker screening tool for the disease.

"[This research] doesn't appear to move us closer [to a blood screening test for Alzheimer's]," said Dr. Myron Weiner, clinical professor of psychiatry and neurology at the UT Southwestern Medical Center in Dallas. "We probably need to explore other potential markers [other] than ABeta42."

Currently, experts depend on a series of brain scans and psychological tests to diagnose Alzheimer's disease. However, such a test is only successful in diagnosing patients who already have symptoms. Thus, the current struggle within the Alzheimer's community is to develop a screening tool that can predict who will develop the condition before symptoms start.

But, experts remain skeptical of diagnostic blood tests for the disease. The reason, according to Dr. Sam Gandy, chair of the national medical and scientific advisory council of the Alzheimer's Association, is that the disease is so complicated from a diagnostic point of view, that it is not possible to lump people into two distinct groups -- those who will go on to develop Alzheimer's, and those who won't.

"Think about the challenge," Gandy said. "About two-thirds of dementia is Alzheimer's disease, so, if you diagnose dementia, you can guess Alzheimer's disease and be right nearly 70 percent of the time. Experts can get to 90 percent [accuracy] with neuropsychological testing.

"I'm not sure that a blood test will be over 90 percent accurate."

Dr. Donald Royall, chief of the division of aging and geriatrics at the University of Texas Health Sciences Center at San Antonio, said that, beyond the inability of this test to distinguish from one form of Alzheimer's to another, the blood test would also carry a high risk of false positives -- that is, telling people they have Alzheimer's when they do not.

"Biomarkers risk too sensitively detecting Alzheimer's disease ... even in non-demented persons," Royall explained.

Furthermore, beyond the fact that most experts said they do not believe a blood test will ever surpass the accuracy of the current neuropsychological test, many experts don't believe that ABeta42 is the protein we should be focusing on as a biomarker for the disease.

"ABeta42 is not specific to Alzheimer's disease and not highly correlated with cognition," Royall said.

In fact, most researchers are holding out hope for another diagnostic tool, altogether: amyloid imaging. An amyloid imaging agent would allow doctors to directly examine the clumps and tangles of this protein in the brain -- a protein that many believe is directly linked to Alzheimer's.

But, with so many exciting new tools for diagnosing Alzheimer's disease still in the pipeline, experts note that, as of yet, there are no diagnostic tools for the disease that are ready for clinical application.

"This is pure research," said Dr. John Messmer, associate professor of family and community medicine at the Penn State College of Medicine. "It is not a final application of a diagnostic test.

"It's just one more incremental step toward understanding the disease."

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