By Julie Steenhuysen

CHICAGO (Reuters) - A new test can accurately detect Alzheimer's disease in its earliest stages, before dementia symptoms surface and widespread damage occurs, U.S. researchers said on Monday.

The test, which measures proteins in spinal fluid that can point to Alzheimer's, was 87 percent accurate at predicting which patients with early memory problems and other symptoms of cognitive impairment would eventually be diagnosed with Alzheimer's, they said.

"With this test, we can reliably detect and track the progression of Alzheimer's disease," said Leslie Shaw of the University of Pennsylvania School of Medicine, whose study appears in the Annals of Neurology.

Such tests, which look for so-called biomarkers of disease, can help researchers better focus trials of Alzheimer's treatments, Shaw said.

The test may also lead to better strategies to keep mild memory impairments from progressing into full-blown Alzheimer's disease, a fatal, mind-robbing ailment that is the most common form of dementia in the elderly, he added.

Many teams have been seeking ways to diagnose Alzheimer's in its early stages, which would allow doctors to give people drugs aimed at slowing the disease.

Shaw and colleagues set out to create a standardized test that focuses on levels of two classic hallmarks of Alzheimer's in the brain: amyloid beta protein, which forms sticky brain plaques, and abnormal levels of the protein tau, which forms fibrous tangles in the brain.

"What we are measuring is the amount of the tau protein and the concentration of the amyloid beta42 polypeptide," Shaw said in a telephone interview.

TEST ALSO RULES OUT DISEASE

The team evaluated spinal fluid taken from 410 patients who were part of a large Alzheimer's study.

They found people with low concentrations of amyloid beta42 were more likely to develop Alzheimer's disease, presumably because the protein was accumulating in plaques in the brain.

They also found people with high levels of tau in their spinal fluid were more likely to develop the disease. "The release of tau into the fluid compartment is thought to be the result of the dying of the nerve cells. They release their contents," Shaw said.

He said these two measures accurately predicted which patients with memory problems would develop Alzheimer's disease in 87 percent of the cases. The test also ruled out the disease in 95.2 percent of the volunteers.

Shaw said the findings are encouraging both for patients and for drug companies, which may finally have a way diagnose the disease early on.

"The general consensus is you are going to have the best chance to improve Alzheimer's disease if you can catch it early, when there is more brain function there to preserve," he said.

An estimated 26 million people have Alzheimer's globally and experts predict this number will grow to 106 million by 2050.

(Editing by Maggie Fox and Eric Walsh)

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