Calling it "the answer to the prayers of thousands of Americans" needing blood transfusions, Health and Human Services Secretary Margaret M. Heckler announced approval of a commercial test yesterday that will provide "an additional insurance policy" for preventing transmission of the deadly disease AIDS (acquired immune deficiency syndrome) through the nation's blood supply.

Officials for Abbott Laboratories of North Chicago, one of five companies competing to supply more than 1.5 million tests monthly to the country's 2,300 blood banks and commercial blood-product producers, said they were ready to begin shipments yesterday afternoon.

Officers of the American Red Cross, the nation's largest collector of blood, signed a contract with the company yesterday and said they were prepared to begin phasing-in the test within days. A Red Cross official estimated that the test will cost about $5 per unit of blood to administer, perhaps adding 10 percent to the cost of blood.

Food and Drug Administration Commissioner Frank Young estimated that it will take two to six weeks for the test to be put into use voluntarily in blood-collecting centers nationwide. He said it will later be required by the government.

Blood that is found positive for evidence of AIDS antibody will be destroyed. At some point after the phase-in period, individuals whose blood-donation tests repeatedly show positive results will be notified of this uncertain finding and urged to seek additional medical evaluation.

The Centers for Disease Control report that as of Feb. 25, there had been 8,597 cases of AIDS in the United States and 4,145 deaths. Public health experts estimate, however, that about 400,000 Americans may have been exposed to the AIDS virus. It is thought to be spread through contact with bodily fluids -- sexually, through intravenous needles, from pregnant women to their fetuses and through the blood -- but not through casual contact with carriers.

Blood-related cases account for about 2 percent of cases, including 119 cases in adults and children linked to blood transfusions, and 66 among hemophiliacs who received blood products, said CDC officials.

Although the risk of getting AIDS through a blood transfusion appears to be small, there has been considerable pressure to speed a blood test to the market to reassure the more than 3 million individuals who get transfusions each year. An industry expert estimated yesterday that the annual market for the blood test could be $60 million to $80 million. Abbott officials said that each test kit will sell for $2 to $3.

Despite the test's possible benefits to the blood supply, critics in the public health community and in gay rights groups have expressed concern about the repercussions for those who get a positive test result. Addressing these concerns, Heckler and Young stressed yesterday that the new blood test must be used with caution because of two major limitations:

* It is not a diagnostic test for AIDS. The test detects only the presence of antibodies to the virus, known as Human T-Cell Leukemia Virus III, thought to cause the disease. "The existence of these antibodies means only that the person tested has at some time, in some way, been exposed to the virus or a related virus. It does not -- I repeat does not -- mean that the person whose blood tests 'positive' has AIDS," Heckler said.

Although most people who carry the AIDS antibodies do not seem to get AIDS, many may carry the virus indefinitely and be able to spread it. There is no way to predict which individuals will later fall prey to the disease. Rough government estimates suggest that 25 percent of people may become ill with lymph node problems, weight loss and other symptoms, while 10 percent may come down with the severe form of AIDS, which devastates the body's immune system.

* The test is not error-proof. Although it appears to be highly accurate for screening out blood that carries antibodies to AIDS, "the test is not perfect," Young said. It has the potential for incorrectly indicating that blood contains AIDS antibodies, as well as incorrectly indicating that blood does not.

When a group of AIDS patients was tested, 7 percent were not detected.

But public health authorities agree that individuals in groups at high risk of contracting AIDS -- sexually active homosexual men, intravenous drug abusers, recent Haitian immigrants and the sexual partners of people in these groups -- should not donate blood.

Heckler urged high-risk individuals who wish to get the test to seek out "alternative locations, not blood banks." These would include clinics and private physicians.

Kristine Gebbie, head of Oregon's state health division and chair of the Association of State and Territorial Health Officials, complained yesterday that thus far the federal financial support for testing at alternative sites -- Heckler said about $12 million -- is "just a drop in the bucket." She and Dr. David Sencer, head of the New York City health department, joined many gay-rights activists in urging high-risk individuals not to get the test "until we know more" about what the results tell individuals about their health.

Jeff Levi of the National Gay Task Force, which has been concerned that the test will be used for discriminatory purposes, was pleased that the government-required labeling for the blood-test kit emphasizes that "it is inappropriate to use this test as a screen for AIDS or as a screen for members of groups at increased risk for AIDS in the general population."

In a letter being sent to doctors and health-care providers, FDA's Young noted the need for confidentiality of test results "because disclosure could lead to serious social and employment consequences."

Rep. Henry A. Waxman (D-Calif.), whose House health subcommittee has accused the government of spending too little on AIDS research, said in a statement that "while this blood test will help make a safe blood supply safer, it has an undeserved reputation. This is not the answer to the AIDS epidemic . . . . I'm certain that the administration will take a lot of credit when the license is granted, but they should be deeply ashamed of how little they have done."

Heckler said yesterday that in the coming week, at least one other company will be granted a government blood test license. Sources said they expected it to be Electro-Nucleonics Inc. of Columbia, Md.