WASHINGTON (AP) _ No single test can determine the cause of the mind-robbing dementia that afflicts many older people, and doctors must take the time to do their own detailed evaluations of each patient, an expert panel said Wednesday.

The committee of specialists assembled by the National Institutes of Health said that although the decline of intellectual capacity signified by dementia often is incurable, virtually every patient can be helped with some kind of treatment.

Dementia, and the forgetfulness, restlessness or apathy often associated with it, frequently is caused by diseases such as Alzheimer's and Parkinsonism, or brain tissue injury caused by blood vessel problems.

However, the panel said, depression and other mental disorders, reactions to prescription drugs, infectious diseases, metabolic problems and nutritional deficiencies are some of the things that can produce the signs of dementia.

Because many of these conditions are treatable, a good evaluation is critical before patients are diagnosed as having dementia, it concluded.

The panel said the highest rate of dementia is in people over age 75, the age group that is increasing more than any other as the general population ages. Experts estimate that between 2 million and 5 million persons in this country are demented to the point that it interferes with working or living a normal life.

Dr. Joseph M. Foley of Case Western Reserve University, the committee chairman, said no test substitutes for the detailed evaluation by a doctor who knows the patient.

This evaluation should include a thorough physical examination and taking a detailed medical history of the patient and his or her family, Foley said.

It may take some time to complete this evaluation because dementia primarily is a behavioral condition and it may be necessary to interview family members and friends before the doctor can reach a conclusion, he continued.

Foley said the panel's recommendations are addressed to general physicians and not specialists in aging or dementia. Most elderly people, the principal group affected by dementia, do not see specialists and depend upon family doctors or other health professionals for their care, he said.

The panel said that almost 80 percent of impairing dementia is caused by irreversible conditions such as Alzheimer's disease and brain tissue death from vessel disease. But the remaining 20 percent is due to a variety of causes that often are reversible upon diagnosis, it said.

However, Foley emphasized at a news briefing, even incurable conditions can be treated.

Foley called past practices of telling patients they had an incurable condition and there was nothing doctors could do ''the shame of the American medical profession.''

''There is no such thing as an untreatable patient,'' Foley said. ''There is always something you can do to treat symptoms or to improve the patient's enviroment to make his life easier.''

Sometimes, people with diagnosed dementia have accompanying medical problems that make their condition worse which can be treated, he added.