HONOLULU (Reuters) – Proposed new guidelines for diagnosing Alzheimer's released this weekwould look at the disease at its earliest stages, when clumps of a protein called amyloid are just beginning to form in the brains of people who are otherwise healthy.

This pre-clinical stage about ten years before dementia sets in is seen as the best place to intervene in the disease, and it is why new imaging agents for PET scans, spinal fluid tests and other so-called biomarkers that predict Alzheimer's are becoming so important to researchers and drug companies.

The changes, presented at the Alzheimer's Association meeting in Honolulu, are the first update of the criteria used to diagnose Alzheimer's disease in more than 25 years. They are the work of three expert panels put together by the National Institute on Aging and the Alzheimer's Association.

They suggest Alzheimer's should be diagnosed in three stages – advanced disease, mild disease and the new category called pre-clinical disease.

"I think we're realising that the process of Alzheimer's disease begins many years before dementia," Dr. Reisa Sperling of Brigham and Women's Hospital in Boston, who led the group on pre-clinical Alzheimer's disease, said in an interview.

Many researchers believe most Alzheimer's drugs have failed because they were tried in people whose disease was too advanced to do any good.

Currently, only an autopsy can confirm that a person has Alzheimer's disease, a fatal and incurable deterioration of the brain that affects more than 26 million people globally.

Doctors diagnose Alzheimer's by excluding other potential causes of memory loss, such as stroke, tumours and heavy drinking.

They can also administer simple paper-and-pencil tests.

Bill Thies, chief medical officer of the Alzheimer's Association, said the changes reflect an increased scientific understanding of Alzheimer's, including the use of biomarkers tracked by brain scans, blood and spinal fluid tests.

Many of these new tests will still need to be validated before the recommendations are fully adopted, Thies said.

"I do think it is useful to stage Alzheimer's disease," said Sperling, who runs clinical trials on Alzheimer's drugs.