Amid the cloud of bad news about the AIDS epidemic shines a major advance: a test to protect the nation's blood supply from transmission of the virus that causes the deadly disease.

"The blood test is extremely good for detecting infection by this virus. So the problem of blood-transfusion AIDS, for the most part, is over," says the National Cancer Institute's Dr. Robert Gallo, the American scientist who discovered the AIDS virus, known as HTLV-3, and helped design the new test.

"The blood supply is safer now than it ever has been before," asserts American Red Cross associate vice president Dr. S. Gerald Sandler, who has been monitoring the test since it received federal approval last March. "We believe this test is very, very effective and virtually every unit of infected blood is being eliminated."

Their sentiments are shared by experts who were initially worried that the new test was being implemented prematurely, but have since been impressed not only by its accuracy but also by the speed with which it moved into universal screening of blood donations around the country.

The blood test is not a test for AIDS. Instead, it measures the presence of antibodies to the AIDS virus, an immune system response indicating that an individual has been exposed at some point in the past to the virus and may be currently infectious.

But because no laboratory test is 100 percent foolproof, blood banks and government health officials continue to urge that individuals in groups at highest risk of getting AIDS -- including homosexual and bisexual men, intravenous drug abusers, hemophiliacs, and sexual partners of people in those groups -- continue to refrain from donating blood. Individuals who wish to find out if they are AIDS-antibody positive are urged instead to go to "alternative testing sites" provided by local health departments, clinics and private physicians.

The combination of donor screening and blood testing responds to a growing national concern that contamination of the blood supply by the AIDS virus posed a risk to the more than 3 million people transfused with 12 million units of blood in the United States each year. The uncertainty created both a physical and emotional threat to the well-being of those receiving blood, particularly in areas such as New York and California, where AIDS cases were highest.

Nearly 200 people -- almost 2 percent of the more than 12,000 adult cases of AIDS reported in the United States since 1981 -- have contracted AIDS following tranfusions with blood or blood products. In addition, 80 hemophiliacs, who receive special blood factors to help their blood clot, account for just under 1 percent of the cases.

Among the approximately 180 American children reported to have AIDS, 15 percent have been linked to blood tranfusions and 5 percent to hemophilia.

But despite current blood screening and a new heat treatment for blood products used by hemophiliacs, more AIDS cases from blood transfusions are likely to occur over the next few years in people who received transfusions before the test became widely available. Unlike many viral diseases, AIDS has a long incubation period. Appearance of the disease is delayed an average of more than two years, and sometimes longer than five years, after initial infection.

Experts with the federal Centers for Disease Control believe that within five years after exposure to the virus, from 5 to 10 percent of those infected will come down with the severe form of AIDS, in which the body's immune system is destroyed and vulnerable to a range of life-threatening infections. They estimate that another 25 percent may become ill with lesser symptoms, including fatigue, fever and weight loss, that are associated with a loosely defined illness known as AIDS-related complex, or ARC. Others who are carrying the virus may have no symptoms but may nonetheless be infectious.

This "paradoxical circumstance should not cause the public to misunderstand the success of the blood test," says the Red Cross' Sandler. While the federal government has not made it mandatory, blood bank and government officials believe that the test is now used routinely on all blood products. Major blood banking groups set a July 1 implementation deadline for their members, but most major collection centers appeared to be using it well before that.

Known as ELISA or EIA, short for enzyme-linked immunosorbent assay, the test picks up signals -- in this case antibody readings -- that a person has been exposed to the AIDS virus. Like a radio, the test picks up a range of signals. For blood screening purposes, the AIDS antibody test is tuned to be extremely sensitive, picking up even the weakest signals, so any blood that is potentially tainted can be discarded.

To maximize the test's accuracy, the American Red Cross, the American Association of Blood Banks and the Council of Community Blood Centers have recommended that the ELISA test be repeated if the first result is positive. If the positive reading is confirmed, then the potentially contaminated blood is discarded.

There is still a small chance that contaminated blood could slip through, since studies at Gallo's NCI lab have suggested that some individuals may be slow to produce antibodies after they are infected with the virus.

While the blood test seems to provide a high degree of security for blood recipients, it also has raised difficult new issues regarding donors whose blood tests positive. The major blood banking organizations have agreed that donors should be notified, but because the AIDS antibody test is set to be highly sensitive, there is concern about false alarms.

To minimize this possibility, particularly until more information is gathered about the test, the national blood banking groups have recommended notificiation only of those individuals whose repeat positive tests have also been confirmed by a second testing method, most commonly a test known as the Western blot.

If the additional test method is negative, such individuals would not be notified, but their names would be kept on a confidential deferral list for special testing if they should donate again, according to the national guidelines.

By July 1, the American Red Cross, which collects half of the nation's blood supply and most of the blood in the metropolitan area, required its regional centers, except where prohibited by state law, to notify donors found positive by the two separate testing methods. By registered mail or personal phone call, these individuals are counseled about the meaning of the test and told to seek medical follow-up.

Using this dual system, the Red Cross experience with the first 1 million tests found that about 20 out of each 10,000 units of donated blood were discarded based on repeatedly reactive ELISA testing. But requiring a second confirmatory test reduces the number of donors who would be notified that they were potential virus carriers to about four per 10,000 units. The Centers for Disease Control reports slightly higher figures.

Over the first year, the Red Cross estimates that at least 1,500 of its donors, most of them males, will be notified of positive results. At least as many might be notified by other blood banks, most of which are following the same policy, said the American Association of Blood Banks' executive director Gilbert M. Clark. But some facilities have chosen to notify based only on repeated ELISA testing, he said.

Sandler and others say that, so far, the majority of donors who test positive turn out to have had sexual or intimate contact that put them at risk of getting AIDS, but apparently these donors did not view themselves as belonging to a high-risk group.

While the original guidelines suggested that sexually active homosexuals or bisexuals with multiple partners posed the highest risk, the blood bank groups are now redrafting their informational statement to be more explicit.

"We need to be certain that any male who has had sex with another male within the past five years clearly understands that he has a risk of transmitting the infection," says Sandler. "The key to a safe blood supply is the elimination of persons who have reason to believe that they are harboring HTLV-3 infection or any other potentially transmissible disease."