Doctors have learned how to detect a virus infection in newborns that causes thousands of cases of deafness every year.

Because the test should lead to earlier detection of many hearing defects -- and much earlier and more effective language training and other help for thousands of children -- the National Institutes of Health yesterday called the test "a major medical advancement."

Two NIH agencies, the National Institute of Child Health and Human Development and the Division of Research Resources -- financed the research and helped develop the research unit where the work was done.

The test method was developed at the University of Alabama Medical Center in Birmingham by a team of pediatricians and virologists who are following 300 children born with these infections.

But all the children potentially could develop hearing loss or other medical problems by age 5.

The virus the test seeks -- cytomegalovirus or CMV -- is a common cause not only of hearing loss, but also of mental and physical impairment.

Unsuspected CMV infection of the birth canal is so common in pregnant women that an estimated 30,000 newborns a year, or one baby in every 100, become infected.

Of these, only 1,500 to 3,000 have any immediate recognizable disease or defect. But 2,700 to 7,800 undetected at birth, will soon developed hearing or other problems.

If hearing loss develops, it is often years before parents or elementary school teachers begin to realize that a silent or apparently slow child cannot hear.

"Early hearing loss is very difficult to detect, and in the first nine months. Unless you specifically test for it, it is almost impossible to detect," Dr. Sergio Stagno, head of the University of Alabama team, said in Birmingham yesterday.

"It is critical that these infants be detected as quickly as possible so training of both the parents and child can begin. Otherwise the development of language and learning are slowed at a critical age."

What Stagno and colleagues leanred was that a well-known test for rheumatoid factor, an antibody or protein involved in rheumatic disease, can also spot 35 to 45 percent of CMV-infected babies.

"We're working to develop a more accurate test still," he said.

University Hospital and several other Birmingham hospitals are screening newborns for CMV, Stagno reported.

The test requires taking a small sample of blood by heel-prick from a newborn. The results can be available in two hours. If the test is positive for rheumatois factor, a final diagnosis must be made by studying a sample of the infant's urine.

The urine test is even more accurate. But it must be made in the first week of life. But it would be too expensive to give a urine test to the 3 million babies born in the United States every year, hence the need for the cheaper, easier new screening test that is now available.

The Alabama research also identified CMV infection as the leading cause in children of sensory or neutral hearing loss, contrasted with hearing loss caused by infection or other problems of the inner ear, which leads to bone conduction deafness.

Work is a under way at Birmingham to develop a drug treatment for CMV infections. Scientists elsewhere hope to develop a vaccine that could protect both mothers and babies against the CMV virus -- and infant deafness.