What You Can--and Can’t--Learn From Blood Screenings
Since laboratory tests to detect exposure to the AIDS virus became available two years ago, millions of Americans have had their blood checked.
The most widely used tests do not detect the virus particles themselves. Rather, the tests identify proteins called AIDS antibodies, which are produced by the body’s immune system in response to infection with the virus.
Those who test positive for AIDS antibodies are considered to be infected with the AIDS virus and capable of transmitting it to others through the blood or sexual intercourse for as long as they live. These individuals, however, may or may not become ill with AIDS or related diseases.
To check a person for AIDS antibodies, about one-third of an ounce of blood is obtained from a vein in the arm. Then a screening blood test known as the ELISA (enzyme-linked immunosorbent assay) is performed.
If the test result is negative, no further tests are done. But if the result is positive, it should be confirmed by a second method, most often a more expensive and time-consuming but generally more reliable procedure known as the Western blot.
Physicians and commercial laboratories usually charge patients $40 to $50, which includes the ELISA test, a repeat ELISA test if the results are borderline and the Western blot confirmatory test, if necessary.
Together, the ELISA and Western blot tests have proved to be among the most accurate diagnostic tools available. But like the majority of blood tests, the AIDS antibodies measurements are not foolproof. False results can be caused by laboratory error or the imperfections of the methods themselves.
Some test results may be “falsely positive,” meaning that a person has a positive test result even though he or she has not been exposed to the AIDS virus.
False positive readings underscore the importance of confirmatory tests. But even if the confirmatory test is negative, some physicians recommend another test be done six weeks later as a precaution.
Test results also can be “falsely negative,” meaning an erroneous negative result even though a person is infected with the AIDS virus.
The “false negative” problem
most commonly occurs when the ELISA test is performed too soon after a person has been infected. It usually takes a month or two for AIDS antibodies to become detectable, but in some instances this has taken as long as six months. Therefore, physicians sometimes recommend repeat testing several months later for individuals at high risk of infection, such as intravenous drug users and those with multiple sexual partners.
Illness Comes Years Later
Within five years following exposure to the AIDS virus, 20% to 30% will come down with AIDS, public health officials estimate. Most will not become ill until at least three years after the initial exposure. The virus that causes acquired immune deficiency syndrome attacks the body’s immune system, leaving it vulnerable to a variety of life-threatening infections and tumors.
Also within five years, an additional 25% to 40% of infected individuals will develop the AIDS-related Complex, or ARC. ARC is an imprecise diagnosis that includes patients with mild symptoms, such as swollen lymph glands, as well as those who are seriously ill with infections or tumors but do not meet the exact criteria for diagnosing AIDS.
About 1 million to 2 million Americans are believed to be infected with the AIDS virus. As of Monday, 32,825 Americans had contracted AIDS and 19,021 had died from it.
