WASHINGTON, United States (AP) — Does it really take an expensive brain scan to diagnose Alzheimer’s? Not everybody needs one but new research suggests that for a surprising number of patients whose memory problems are hard to pin down, positron emission tomography (PET) scans may lead to changes in treatment.

The findings, reported Wednesday, mark a first peek at a huge study under way to help determine if Medicare should start paying for specialised PET scans that find a hallmark of Alzheimer’s — a sticky plaque called amyloid.

Alzheimer’s is the most common form of dementia, and classic symptoms plus memory tests often are enough for a reliable diagnosis. But unusual symptoms could mark another form of dementia that, while there are no cures, could require different symptom care. And on the other end of the spectrum, it’s hard to tell if mild memory loss might be an early Alzheimer’s signal, a more treatable condition such as depression, or even age-related decline.

“We’re not accurate enough,” said Dr Gil Rabinovici of the University of California, San Francisco, who is leading the new research.

“Patients know there’s something wrong. Often they can sense in their gut that it’s not normal aging,” he added. Without a clear-cut test, “doctors are very reluctant to make the diagnosis in many cases.”

Until a few years ago, amyloid build-up could only be seen during autopsies. Older types of PET scans show what region of the brain appears most affected, of limited help.

Yet it’s not clear how best to use the new amyloid-detecting scans, which can cost up to US$6,000. They can rule out Alzheimer’s if there’s little amyloid. But cognitively healthy seniors can harbour amyloid, too, and Medicare won’t pay for the new scans outside of a few research studies.

One of those is the IDEAS study, which is testing the impact of amyloid-detecting PET scans in more than 18,000 Medicare beneficiaries. To enrol, patients either must have atypical dementia with an unclear cause — or have particularly puzzling “mild cognitive impairment,” or MCI, early memory problems that raise the risk of later developing dementia. Researchers check if doctors’ initially recorded treatment plans — medications, counselling or additional testing — were altered by patients’ PET results.