By DR. CHRISTOPHER TOKIN, ABC News Medical Unit

A new drug may help doctors give patients suffering from Alzheimer's-like symptoms some clarity they desperately want: a conclusive diagnosis.

On Friday, the FDA approved Eli Lily's Amyvid to help doctors determine whether patients have Alzheimer's disease.

Until now, a definitive Alzheimer's diagnosis has only been possible  after a patient has already died since the only known method of detection involves cutting into the brain and taking samples to look for the presence of a protein called amyloid. Doctors are left to base their diagnoses on symptoms of Alzheimer's alone.

The new drug acts by binding to amyloid plaques in the brain, allowing them to be captured in a type of imaging test called a PET scan.

"In my opinion this is very big news," Dr. Michael Weiner,  director for the Center of Imaging of Neurodegenerative Diseases at the University of California, San Francisco. "Now for the first time, using this agent, we can identify the amyloid plaques in the brain of living people.

"This allows us to determine who has Alzheimer's disease in their brain and who does not," he said.

The number of people diagnosed with Alzheimer's dementia has doubled in the last few decades; today an estimated 5.4 million Americans live with the disease. Alzheimer's is currently the sixth leading cause of death in the United States, and mortality rates are on the rise.

While this new drug may help doctors identify patients who may have Alzheimer's disease, there is some debate as to how useful the information revealed by the use of the drugwill be.

Part of the problem, says Dr. Clifford Saper, Chairman of Neurology and Harvard's Beth Israel Deaconess Medical Center, is that most people older than 80 have some level of amyloid in their brains, and the amount of amyloid present doesn't necessarily indicate whether the patient has the disease or not.

Plus, he added, practical applications are still a pipe dream, at least for now.

"There is no change in the care of most patients based upon knowing this information, as we have no specific treatment for Alzheimer's disease," he says.

The prevalence of amyloid in the brains of many elderly people may also lead to a new flood of positive scans - which in turn may lead to over-diagnosis of Alzheimer's.

"[Amyvid will] raise false hope and increase costs," says Dr. Peter Whitehouse, a professor of Neurology at Case Western University.

With spending on Alzheimer's disease already topping $130 billion annually by Medicare and Medicaid alone, the question remains whether government or private insurers will pay for the Amyvid diagnostic test since it comes with a significant price tag.

Nevertheless, while there are currently no treatments shown to slow the progression of Alzheimer's, there is clinical testing underway now for several potential treatments. Once one becomes available, Amyloid imaging could help doctors better select patients for treatment and to monitor effectiveness. In fact, Amyvid is already being used for these purposes in ongoing clinical research.

But until new treatment options become available, Amyvid's use will likely be limited to helping doctors rule out Alzheimer's disease in patients with dementia.