How can you be absolutely sure that any blood transfusion or organ transplant you might get is free of AIDS? You can't, experts say, because testing for AIDS isn't perfect.

Even layering test upon test only adds expense and increases the possibility of human error."That doesn't mean we shouldn't do everything that makes rational sense" to protect against spreading the AIDS virus in transfusions or transplants, said Dr. Mervyn Silverman, president of the American Foundation for AIDS Research.

"But if we're not careful, we can increase the costs tremendously and give a false sense of security," he said.

Americans were reminded again last week of the risks posed by the AIDS virus and its potential to be spread beyond the high risk intravenous-drug users, their sexual partners and homosexuals.

Three people who received organs or tissues from a Virginia man who was shot to death during a 1985 robbery have died of AIDS, health officials said Friday. A Colorado woman who received a hip joint from the man has tested positive for the AIDS virus, and officials are trying to track down other patients who may have received his organs and become infected.

In Chicago, a hospital clinic was closed for a review of its procedures after two girls punctured their fingers with a syringe one of them had pulled out of a waste container in an examination room.

In the same clinic last month, a woman undergoing a gynecological exam was treated with a cotton swab that had been used earlier on a patient infected with HIV, the virus that causes AIDS.

"It's appropriate to be paying attention to these kinds of things because we need to keep on trying," said Dr. June Osborn, head of the National Commission on AIDS.

"On the other hand, to get tense and agitated about rare events and try to make public policy related to those can very easily misplace resources when we're desperately trying to take care of people" with AIDS, she said.

The estimated chance of contracting the virus from a blood transfusion is about 1 in 40,000 to 1 in 153,000, and the risk from a transplanted organ or tissue is "only somewhat higher," said Jeff Nesbit, a spokesman for the Food and Drug Administration.

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In the Virginia man's case, his blood was twice tested for HIV before the organs were removed, and each test was negative. Officials believe he became infected with the virus so close to the time of his death that his body did not register any signs of the virus.

Currently, HIV testing looks for antibodies to the virus. But it can take the body as long as six months to develop antibodies after becoming infected. During this window, the virus is invisible, for current technological purposes.

There is a test that detects the virus itself, but it is not available for commercial use and has not been shown to be any more effective than the existing test that picks up antibodies.

Silverman said he expects the transplant deaths to spark renewed calls for more AIDS testing, but he said he hoped "we don't let irrationality take over."