Researchers have developed a blood test to distinguish sufferers of chronic fatigue syndrome - or "yuppie flu" - from those feeling run-down for other reasons, a scientist says.

The test checks levels of white blood cells, AIDS researcher Jay Levy said Sunday at a national conference on the chronic fatigue syndrome. He led the group that developed the test at the University of California at San Francisco.Levy said most of the 120 chronic fatigue patients tested at the university had abnormal levels of three types of white blood cells produced when the body fights a virus.

No one virus was present in all the patients, but Levy said levels of those three cells were either consistently high or consistently low in most of the patients.

Therefore, he said, a check of those white blood cells, plus a review of the patient's medical history, could determine whether a person has chronic fatigue syndrome. None of the 80 healthy people studied had abnormal levels, Levy said.

Chronic fatigue syndrome is characterized by exhaustion, joint and muscle aches, fever and other problems that persist for more than six months and often for years. Its cause is unknown.

Tens of thousands of Americans believe they have chronic fatigue syndrome, although the extent of it is unclear. It was dubbed "yuppie flu" because it has been widely reported among well-educated women in their 30s and 40s.

The weekend conference, titled "Unraveling the Mystery," drew 400 people, including researchers and people afflicted with the ailment.

Most researchers now agree the illness stems from immune system abnormalities, not depression or other psychological factors, Levy said.

He called it "the disease of the '90s."

"This is going to be the 10 years that we see the greatest advancement in understanding this disease," he said.

He suggested the name be changed to "chronic immune activation syndrome" because research indicates that patients' immune systems are chronically "activated," as if fighting a continuing viral infection.

With most viral infections, such as the flu, the immune system fights vigorously, causing fatigue and muscle aches. But it quiets down when the virus is killed. In chronic fatigue syndrome, the immune system stays on the attack, causing the symptoms to persist, he said.

Besides trying to determine the cause, researchers are struggling to learn how the illness is transmitted and whether it is contagious.

Dr. Walter Gunn, a researcher with the Centers for Disease Control in Atlanta, said an inter-agency committee had been formed to determine the extent of the illness.