WASHINGTON (Reuters) - U.S. regulators gave the nod to an imaging test from Eli Lilly and Co. that can for the first time help doctors detect brain plaque tied to Alzheimer's disease, the company said.

The U.S. Food and Drug Administration approved the radioactive dye, called Amyvid, to help doctors rule out whether patients have Alzheimer's, the most common form of dementia, Lilly announced late on Friday.

The dye binds to clumps of a toxic protein called beta amyloid that accumulates in the brains of patients with Alzheimer's. Doctors can then see the plaque light up on a positron emission tomography, or PET, scan.

Patients with Alzheimer's always have some brain plaque, so its absence in the test would tell doctors to look for other causes of mental decline, such as depression or medications, Lilly has said.

But Lilly, which plans to sell the drug through its unit Avid Radiopharmaceuticals Inc, said the test should not be used to diagnose Alzheimer's, since brain plaque can also be tied to other neurologic conditions and may occur naturally in older people with normal mental states.

An FDA advisory panel recommended against approving the dye last year, saying doctors might have trouble interpreting scans of the plaque, and the FDA rejected Amyvid last March.

Since then, Eli Lilly said it has worked to identify better ways of training doctors to use the test.

Dr. Daniel Skovronsky, CEO of Avid, said one in five patients who are diagnosed with Alzheimer's turn out not to have the disease after an autopsy.

"The approval of Amyvid offers physicians a tool that, in conjunction with other diagnostic evaluations, can provide information to help physicians evaluate their patients," he said in the company's statement from Friday.

There is currently no cure for Alzheimer's, a mind-robbing disease that affects more than 35 million people worldwide and gets worse with age.

But an early hint that something is wrong might improve the success of drugs meant to prevent or delay disease progression, researchers believe.

Avid has been in the lead in the race for imaging agents for Alzheimer's, which are estimated to have a potential global market anywhere from $1 billion to $5 billion.

General Electric Co and Bayer AG are developing rival products.

Lilly, as well as Pfizer Inc, are the farthest along in developing experimental medicines to treat Alzheimer's. Lilly expects to release final data for its contender, solanezumab, as soon as this summer.

(Reporting by Anna Yukhananov; editing by Todd Eastham)