HIV Antigen Test

When and why did the American Red Cross begin testing for the HIV antigen?

The American Red Cross began testing for the HIV antigen on March 17, 1996. Testing for the antigen reduces the "window period" in which a donor may have been exposed to HIV and have a reactive test result. The "window period" for the HIV antibody is approximately 22-25 days. The "window period" for the HIV antigen is approximately 11-14 days. This means that if a person is exposed to HIV, he/she may test reactive for the antigen in 11 days, but may not test reactive to the antibody for 22 days.

What has been the experience with the HIV antigen test to date?

Based on current data, 100% of HIV antigen confirmed positives developed HIV antibodies and were infected with HIV. Conversely, to date (based on six-month follow-up studies) no HIV antigen repeat reactive individual with a negative confirmatory test has become HIV positive. 

Is it true that the HIV antigen test is only available in the volunteer blood donor setting?

Yes, but the tests used by physicians and medical clinics are highly sensitive and effective. We must remember that even the HIV antigen test is not foolproof. In light of this, we all have a responsibility to ensure the safest blood supply possible by donating solely out of a desire to help others.

How will you dissuade people who wish to donate solely to find out their HIV status?

As always, we will rely on the truthfulness of donors, who have shown time and again their willingness to help others. We also have to strive constantly to educate people about the responsibility  we have to maintain a safe blood supply. No test is 100% foolproof, and we all must do all we can to safeguard the lives of the millions of infants, children, women, and men who depend on donated blood to live. Anyone wishing to find out their HIV status should visit their physician or local health department for confidential testing.

What does a "repeat reactive" test result mean?

"Repeat reactive" refers to the results from duplicate tests that are performed if a sample tests reactive on the initial run. A repeat reactive result does not necessarily mean a donor carries an infectious agent. Fewer than three in every 10,000 donors, or 0.03%, will test repeat reactive.

How does this compare with the repeat reactive rate for the HIV antibody test?

Fewer than five in every 10,000 donors, or 0.05%, test repeat reactive for HIV antibodies.

How many of these repeat reactive antigen tests can be expected to be truly positive after they are tested using a more specific confirmatory test called the neutralization test?

Over 99% of HIV antigen repeat reactive donors will not have a positive confirmatory test result and will be classified as false positive. Fewer than one of every 300 repeat reactive donors is true antigen positive, or antigen neutralizable. Initial estimates were than one in 1 million donors would be true HIV antigen positive, but negative on the other HIV test used by blood banks, HIV antibody. In fact, one to two donors in the United States per year fall into the window period between antigen test positivity and antibody test positivity. Identification of these donors will shorten the period between time of infectiousness and test reactivity, now estimated at 22 days, by 6 to 10 days.

A greater number of HIV antigen true positive (neutralizable) donors will also be HIV antibody positive and will be first-time donors. Currently, for every 100,000 donors nationwide, five to seven are detected who are HIV antibody positive.

What are the rules or reinstatement of donors who test repeat reactive?

The FDA advises retesting in eight weeks, all HIV antigen repeat reactive individuals who do not have a positive confirmatory test. If, upon retesting, the donor is negative for all tests and otherwise qualified to be a blood donor, he or she can be reinstated. Donors whose HIV antigen test results remain repeat reactive after eight weeks are not likely to become test negatives, even with a negative confirmatory test result. They must be indefinitely deferred, not because of risk, but because they have persistent false positive test results.