When Arthur Ashe, 48, underwent heart bypass surgery in 1979 and 1983 in New York City -- and received several units of donated blood -- he faced a far greater risk of contracting AIDS than people who receive blood transfusions today.

A blood screening test to detect HIV, the virus that causes AIDS, was not approved by the Food and Drug Administration until April 1985 -- too late for Ashe and thousands of other blood recipients. Although 4,408 AIDS cases -- about 2 percent of the U.S. total -- have been attributed to blood transfusions, only 20 of those cases resulted from screened blood, according to the U.S. Centers for Disease Control.

While the screening tests are not perfect, the American Red Cross puts the odds nowadays for the average transfusion recipient contracting HIV at one in 45,000. Where the tests fail is that they cannot detect HIV in a donor during a window period between the time of infection and when the body produces antibodies, a window that could stretch as long as six months.

Ashe said yesterday at a news conference that he and his current doctors were "one hundred percent sure" that the cause of his initial HIV infection was a blood transfusion -- he believes most likely a transfusion of two units of blood in 1983 after double bypass surgery at St. Luke's Hospital in New York. Ashe had quadruple bypass surgery there in 1979 after a heart attack. Physicians believe Ashe has a familial predisposition to coronary artery disease.

Ashe said he has been aware since 1988 that he had AIDS, but said, "I am not sick, and I can function very well in all that I have been involved in for the past several years."

Although he was at first ambiguous at the news conference about whether he has AIDS, his doctors said he does have the disease by virtue of the fact that in 1988 he suffered inflammation of the brain due to an attack of toxoplasmosis. A parasitic organism that lies dormant in people with normal immune systems, toxoplasmosis can cause a severe infection in people with impaired immune systems, and means the HIV patient has progressed to AIDS itself.

Nevertheless, "Medically he is a rock," said Henry W. Murray, chief of infectious diseases at New York Hospital-Cornell Medical Center, where Ashe is being treated. "Immunologically, he is obviously on thin ice and has been since 1988, but it does not keep him from following a full travel schedule."

Ashe said that several days after the 1983 bypass surgery he was "not feeling too good" and that his doctor had said he could either wait out his discomfort or receive a couple of units of blood for an immediate boost. The question of HIV risk "never came up," he said. He said he did not plan to sue the hospital.

In 1983, when Ashe was presumably infected, the cause of AIDS still was not known and evidence that it was transmitted by blood was "inconclusive," according to a Red Cross statement in January of that year. However, the Red Cross felt obliged at the time to warn the medical community to use donated blood with caution -- balancing the risks of a person's medical situation against the risk of transfusion.

Ashe's experience "is a chilling realization that prior to April 1985 we were in a completely different situation," said Andrea Smith, director of public information for the New York Blood Center, which provides the community blood supply for the city and its metropolitan area and in all likelihood was the source of the blood for Ashe's transfusion.

With the historically high concentration of AIDS victims in New York, she said, the pool of donors in that city always carried a higher risk of contamination than other areas of the country.

Ashe said his AIDS condition came to light in 1988 when "my right hand went dead. I couldn't move it at all." A brain scan and then a biopsy at New York Hospital Cornell Medical Center revealed that he had toxoplasmosis. A blood test found that Ashe was HIV-positive.

Ashe was treated for toxoplasmosis with two anti-parasitic drugs and regained the use of his hand. He will need to take these drugs for the rest of his life, said Murray, to keep the parasite at bay. Ashe also has been taking AZT, a drug that inhibits progression of the disease, since 1988 as well as pentamidine to protect him against developing pneumocystis carinii, an AIDS-associated form of pneumonia.

For people facing elective surgery, today the Red Cross recommends that to avoid HIV risk, patients donate their own blood before surgery if they are healthy enough to do it. About 5 percent of the nation's blood supply is done this way, it said.

Last month, blood centers began implementing a new blood test that would screen out not only the initial HIV virus known as HIV 1 but also HIV 2, a strain that is prevalent in West Africa but is not yet known to be in the blood supply here.

These tests, coupled with more pointed questioning of potential donors, has reduced the number of contaminated donations, according to the Red Cross. It said that in 1985, 38 of every 100,000 blood donations nationwide tested positive for HIV and had to be destroyed. Today just 5 of every 100,000 donations is found to be contaminated.