The challenges include lack of funding and problems with diagnosing Alzheimer's.

The test would also save patients painful bouts of invasive probing: It often takes several years of observation and brain scans to diagnose Alzheimer's accurately.

The only way to diagnose Alzheimer's conclusively, they said, is to examine the brain in an autopsy.

There is no good way to diagnose Alzheimer's, and there is no treatment.

The current test to help diagnose Alzheimer's is given with paper and pen and requires a trained technician.

"To diagnose Alzheimer's, you keep ruling out other diseases," Ms. Miller said.

Other than performing a brain biopsy, which is rarely done, doctors diagnose Alzheimer's in living people through verbal tests, physical examination and by excluding other known causes of dementia.

However, finding evidence for this is difficult due to the need for an autopsy to correctly diagnose Alzheimer's in a patient.

The government said it changed its policy because doctors and psychologists can now often diagnose Alzheimer's in its early stages, when patients are most likely to derive significant benefits from treatment and therapy.

Clinicians have become more experienced in diagnosing Alzheimer's in recent years.