CHICAGO, SEPT. 3 -- A new test to detect the AIDS virus even before the body's defenses begin to respond may enable doctors to diagnose the disease earlier, track its course with greater certainty and better monitor therapy, researchers report.

Screening tests for acquired immune deficiency syndrome now rely on detecting antibodies produced by the body's immune system as it battles AIDS virus infection. An alternative test requires culturing the virus in a laboratory, a time-consuming and expensive procedure.

The new test detects the presence in blood of an AIDS virus antigen, a protein component of the virus. And it does so before antibodies develop. Several manufacturers are developing AIDS antigen tests for marketing.

The antigen test is not likely to replace antibody testing, however, because it is not as sensitive, according to Dr. Harold Kessler of Rush-Presbyterian-St. Luke's Medical Center in Chicago.

Kessler reported in Friday's edition of the Journal of the American Medical Association that the antigen test correctly diagnosed AIDS virus infections in six patients in the early stages of infection.

The six patients were suffering from what is known as acute HIV infection, a short-term viral syndrome with symptoms similar to flu, rubella and infectious mononucleosis, such as fever, rash, muscle aches and sore throat.

Early diagnosis of acute HIV infection can be difficult because patients may test negative for HIV antibodies when they first seek treatment, Kessler said. It can take as long as six months after exposure to the AIDS virus for antibodies to develop, authorities say.

Because antigen levels in the blood change as the disease progresses, the test may be useful in keeping track of the advancement of the disease, Kessler said. The test may also prove useful in monitoring the effectiveness of anti-AIDS drugs such as AZT, or azidothymidine, Kessler said, by recording changes in antigen levels after administration of the drug.

But researchers concluded more study would be necessary to determine the procedure's full diagnostic potential.

The AIDS antigen test is not likely to replace antibody testing, however, because it is not as sensitive, Kessler said. It would not be as effective in screening donated blood for the presence of AIDS contamination, he said.

AIDS cripples the body's defenses against disease, leaving a person prey to life-threatening infections and certain cancers.