Miami Herald - Friday, November 15, 1985
Paul Saltzman, Herald Staff Writer

But it won't prove whether someone has acquired immune deficiency syndrome.

Like the test for hepatitis, the main test for the AIDS virus only indicates possible exposure to the virus.

The test is relatively straightforward, can be done in about 4 1/2 hours and, if interpreted properly, is considered as accurate as most similar diagnostic screens, doctors and laboratory officials at the federal Centers for Disease Control say.

But to meet that standard, the enzyme-linked immunoassay (ELISA) test for the AIDS virus must be done twice with positive results. Then the results must be confirmed by a more esoteric -- as well as time-consuming, expensive and much more difficult -- test called a Western blot.

Taken alone, the ELISA test is far from certain as proof of exposure to the human T-lymphotrophic virus believed to cause AIDS.

Charles Shable, chief of the AIDS laboratory at the Centers for Disease Control in Atlanta, said his agency has found that a pair of positive test results on the ELISA test is confirmed as true exposure only about one in five times.

The American Red Cross, whose labs have performed 3.5 million of the five million tests done to date, estimates the result is confirmed in only one in six cases.

And exposure need not mean disease. The CDC estimates that about one in 10 people with confirmed exposure actually will develop AIDS within five years, Shable said. Perhaps another two in 10 will develop AIDS-related complex, a group of symptoms including fever, chills and swelling that may or may not be a precursor to AIDS.

Abbott Laboratories sells about 80 percent of the test kits that government and private labs use. The kits cost $2 to $5, compared with $40 to $120 for the Western blot.

The ELISA test is done this way:

A diluted drop of blood is spread on a plastic dish treated with tiny samples of the AIDS virus.

If the person has been exposed to the virus, his blood would have produced proteins, called HTLV-III antibodies, to attack the virus. Those antibodies would grab onto the virus that was planted in the dish.

A second antibody, anti-human immunoglobulin, is added. It will attach itself to any HTLV-III antibodies that are present.

Add a chemical that makes the second antibody react, and the whole solution will turn orange, or blue-green, depending which company's test is used.

The darker the color, the more certain the reaction.

With only a slight change in color, there's a 95 percent chance of a false positive reading, Shable said. With a sharp change in color, there's a 95 percent chance the positive reading is right.

In any case, the test almost never shows someone has not been exposed if they have. Shable said the CDC has never found such a false negative reading, though theoretically it's possible that as many as three out of 100 negative readings could be wrong.

CAPTION: PHOTO laboratory technician performs test (2-b)

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