WASHINGTON - Doctors still don't know the best way to treat or even diagnose attention deficit disorder in children - even though more than a million children take powerful drugs to control their hyperactive behavior, according to the National Institutes of Health.

In a report issued Wednesday, a panel of experts selected by the NIH called attention deficit disorder a "profound problem" that may affect 3 percent to 5 percent of all American schoolchildren and costs schools more than $3 billion.

But a consistent, proven method of diagnosing and treating the disorder remains elusive, the expert said.

"There is no current validated diagnostic test for" the disorder, said Dr. David J. Kupfer, a University of Pittsburgh psychiatry professor and chairman of the panel.

Some treatments are effective in the short term - principally drugs such as Ritalin - but no studies have examined their effect on children who take them for more than 14 months, Kupfer said.

Attention deficit disorder, or ADD, also known as attention deficit hyperactivity disorder, usually is diagnosed in school-age children, often as the result of a child's disruptive behavior.

Symptoms include the inability to sit still for reading, study or even to watch television. Often a child cannot play in group games and will act on inappropriate impulses.

Yet defining precisely what constitutes ADD is difficult, said Dr. Mark Vonnegut, a pediatrician and panel member from Quincy, Mass.

"The diagnosis is a mess," Vonnegut said, "but we all believe we are dealing with a serious core problem."

Nonetheless, Janis Ferre of the Utah Governor's Council for People with Disabilities, also on the panel, said there's a wide inconsistency in how a diagnosis is made. "This results in overdiagnosis and underdiagnosis," she said.

Panel member Donald A. Berry of Duke University Medical Center said he thought Ritalin and other powerful, mind-altering drugs are prescribed too often for children, and he criticized the lack of studies on the drugs' long-term effects.

"There is no gold standard for therapy, so it is difficult to look at the prescribing practice and say what is appropriate or not appropriate," said panel member Dr. Robert S. Baltimore of the Yale University School of Medicine.

Among the panel's conclusions:

• Although Ritalin and other therapies may correct classroom behavior problems, there's no evidence that such correction improves a child's academic performance.
• Short-term trials of Ritalin and other drugs show beneficial effects on. some behaviors and are superior to behavior modification training. Combining the two resulted in improved social skills, and parents and teachers judged this combination "more favorably."
• Many other treatments have been tried including vitamins, herbs, and biofeedback and eliminating some foods such as sugar. None has proved effective.
• Doctors and schools usually do a poor job of communicating and coordinating when treating children with ADD, and follow-up often is poor. Teachers and parents play a key role in successfully treating ADD and fine-tuning medication.