AIDS Blood Screens: Chapters 2 and 3
Silberner, Joanne, Science News
AIDS Blood Screens: Chapters 2 and 3
In March 1985, the U.S. Food and Drug Administration approved the marketing of several blood screens that detect antibodies to the AIDS virus. The nation's blood banks immediately began screening donated blood and pulling positive units from their shelves. The screens have been credited with halting new infections from blood or blood products.
But while the tests are very sensitive--they identify just about all contaminated blood--they also have their problems:
* They give positive results for some blood that would not transmit AIDS. In fact, says Max Essex, an AIDS researcher at Harvard University, "Ninety to 95 percent of the people who test positive don't really have the virus." Inclded in this group are people who test positive because they have other cross-reactive antibodies, unrelated to the AIDS virus.
* Of the apparently healthy "true positives" it does identify, the type of screen used today does not pinpoint which people will go on to develop aIDS within five years--a fate that will befall an estimated 20 to 30 percent of them, according to the Centers for Disease Control in Atlanta.
* A small number of people whose tests come up negative actually have AIDS virus in their blood, and at least one person has contracted the virus from an "antibody-negative" donor.
In the wake of such difficulties, researchers and industry have been searching for a second-generation blood screen, and several are expected to be approved by the Food and Drug Administration (FDA) in the next few months. While the second-generation tests, like the first generation, detect antibodies rather than the virus itself, they are more specific and less prone to false positives.
Unfortunately, they won't be any better than the first-generation tests at picking out people who harbor the virus but have not raised antibodies against it. To detect these antibody-negative, virus-positive people will take third-generation tests that hunt for the virus itself. Such tests are already being developed by several companies.
The blood screens are the first practical application of AIDS laboratory research. Since there is as yet no cure for AIDS, screening blood, sticking to safe sexual practices and avoiding intravenous drug abuse are the only steps that can be taken against the syndrome.
The American Red Cross, while following the development of the second-and third-generation screens, is expressing confidence in the sensitivity of the current antibody screen despite its problems. "Since the initiation of the screen there has not been a report of any transfusion-associated AIDS," says Joseph O'Malley, a medical specialist at the American Red Cross in Washington, D.C., "although recently there has been one case of seroconversion [development of antibodies]."
The seroconversion, detailed in the June 20 MORBIDITY AND MORTALITY WEEKLY REPORT, occrred in a 60-year-old man who had received blood during surgery in August 1985. The blood, which had tested negative, came from a donor who had had homosexual relations with one partner; a blood sample taken several months after the donation that caused the seroconversion tested positive. "The blood was [initially] non-reactive due to the fact that the donor was in a 'window' period between infection and seroconversion," says O'Malley. "As a homosexual, the individual should not have donated blood. The only way around a case like this is to develop a test for the virus itself."
Both the first-and second-generation screens are ELISAs, enzyme-linked immunosorbent assays. They are augmenting the crude screening--simply requesting that members of high-risk groups not donate blood--on which blood banks had been depending.
In an ELISA, the blood is added to small wells containing bits of the AIDS virus, which was previously known as HTLV-III or LAV-I, but has been recently designated HIV (human immunodeficiency virus) by an international committee of virologists. …