Health experts warned yesterday that a new test to protect the nation's blood supply against AIDS could backfire, and some urged federal officials to delay its implementation.

Representatives of several health organizations said that it could make the blood supply more hazardous unless precautions are taken.

The Department of Health and Human Services rushed out a statement last night, saying it will stick by its planned mid-February approval of the test. It pledged to "divert manpower and funding" in unspecified amounts to help local officials make sure it is widely available outside of blood banks to those who cannot afford to pay.

Countering criticism that had been quietly growing in recent weeks, HHS Secretary Margaret M. Heckler said the "new test will enhance the safety of our nation's blood supply."

Of concern to the health-organization representatives is the prospect that individuals at high risk of developing AIDS (acquired immune deficiency syndrome) -- including sexually active homosexual men who have stopped donating blood -- will be attracted by the test to give blood in order to find out whether they have been exposed to the AIDS virus.

Because the test will not be foolproof in screening out all AIDS virus carriers, some infected blood could slip into the blood supply, health officials said.

Preliminary estimates suggest that as many as 5 to 30 percent of those carrying the virus will not be picked up by the test, according to Dr. Mervyn F. Silverman, president of the United States Conference of Local Health Officers.

"It would be a tragic twist if the very test that has been designed to assure the safety of the nation's blood supply actually results in its contamination," he said before the HHS statement.

Silverman and other health officials called for increased efforts, particularly by the federal government, to develop alternative test sites or methods that do not require blood donations. He urged in an afternoon interview that approval of the new test be postponed until potential problems are resolved.

He said he plans to air his organization's concerns and those of the American Public Health Association at a news conference in San Francisco today and had told HHS of his concerns.

Belief that the test could backfire also was voiced by participants at a two-day meeting here under the sponsorship of the American Blood Commission, an umbrella group of blood-related groups, and New York's Hastings Center, which specializes in medical ethics.

In addition to the test's possible failure to detect some AIDS virus carriers, there is also worry that a small fraction of tests will be falsely positive, incorrectly showing that a person has been exposed to the virus, Human T-Cell Leukemia Virus III, that is thought to cause the disease.

Experts stress that the test does not suggest that a person has AIDS. Instead it measures antibodies to the virus that indicate whether a person has been exposed to it. The test also does not determine whether a person is currently infectious or predict who will get the disease. Some studies suggest that perhaps 5 to 10 percent of those infected may later contract AIDS.

Because of uncertainty about the test and misuse of it, groups such as the National Gay Task Force have urged members not to take it and have continued to warn against giving blood. But the warning's effectiveness is uncertain.

"Implementation of the test may in fact make the blood supply less safe if there are not alternative test sites to go to," said Dr. William V. Miller, president of the American Blood Commission and head of St. Louis' American Red Cross regional blood services. "This is by far the greatest unresolved issue."

Preventing increased blood donations by high-risk persons could "ironically create increased risk to the blood supply rather than reduced risk," said Ronald Bayer, a medical ethicist who chaired the Blood Commission conference here.

As of Jan. 28, AIDS had struck 8,057 Americans, killing 3,863 or 48 percent. About 2 percent of the cases have been related to blood products.