Using Combination HIV Tests for Faster, More Accurate Results

4th Generation Tests Cut Window Period by U to Four Weeks

Alere Determine HIV-1/2 Ag/Ab Combo. Courtesy Alere, Inc.

On June 27, 2014, the U.S. Centers for Disease Control and Prevention (CDC) updated their HIV testing recommendations by calling for the use of 4th generation combination assays able to detect HIV infection up to four weeks earlier than antibody-based tests.

There are two such tests currently approved for use: the Alere Determine HIV 1/2 Ag/Ab Combo approved by the U.S. Food and Drug Administration (FDA) on August 8, 2013, and the Abbott ARCHITECT HIV Ag/Ab Combo, which received FDA approval on June 18, 2010.

Rather than relying solely on the detection of HIV antibodies (the defensive proteins produced in response to HIV), these combination assays are able to detect both antibodies and antigens.

Why Is This Significant?

HIV antibodies generally take weeks, and sometimes up three months, to reach detectable levels in a person's blood or saliva. Because of this, false negatives can often occur if a person is tested too early, during what is known as the window period.

By contrast, HIV antigens are the very proteins that elicit the immune response in the first place, signalling the body to create antibodies. These proteins—called  p24 antigens—are found on the surface of the virus and can be detected in as little as 12 to 26 days.

Earlier confirmation of HIV infection is considered key to preventing HIV transmission during the window period. Studies are also suggesting that treatment during the early stages of HIV infection may prevent the development of latent reservoirs, where HIV is known to persist even in the face of successful antiretroviral therapy

CDC Recommendations for Combination HIV Testing

Unlike the rapid OraQuick In-Home HIV-1 Test, the new Ag/Ab combos were neither designed for home use nor for oral (saliva) testing. Rather they are administered either as a fingerstick or venous blood test in a lab or testing center, and (like the OraQuick) can produce results in as little as 20 minutes.

The CDC recommends that testing be conducted in three steps:

  1. Conduct initial testing with Ag/Ab combo assays. If the result is negative, then no further testing is needed.
  2. If the first result is positive, conduct a second test using one of several FDA-approved assays able to differentiate HIV-1 and HIV-2 antibodies. (The Alere Determine can perform both of these functions at once.) This step is considered important as it can help inform the course of therapy for the HIV-infected individual.
  3. If the second result is negative, a third test would be performed with a so-called HIV-1 nucleic acid test (NAT), which is able to detect HIV RNA in as little as 1-2 weeks after infection. The NAT assay is able differentiate acute HIV-1 infection from an initial false positive result with a high degree of reliability. (There are currently no such assays available for HIV-2 infection.)

The three-step strategy is known to increase the sensitivity of HIV testing, while making the Western Blot test—previously considered vital to HIV confirmation—obsolete. Based on the current CDC recommendations, no such confirmation is needed.


Branson, B.; Owen, S.; Wesolowski, M.; et al. "Laboratory Testing for the Diagnosis of HIV Infection: Updated Recommendations." U.S. Centers for Disease Control and Prevention (CDC). Atlanta, Georgia; released June 27, 2014.

U.S. Food and Drug Administration (FDA). "June 18, 2010 Approval Letter  - ARCHITECT HIV Ag/Ab Combo." Silver Spring, Maryland; issued December 22, 2009.

U.S. Food and Drug Administration (FDA). "FDA approve first rapid diagnostic test to detect both HIV-1 antigen and HIV-1/2 antibodies." Silver Spring, Maryland; press release issued August 8, 2013.

Heinrich, T. and Gandhi, R. "Early Treatment and HIV Reservoirs: A Stitch in Time?" Journal of Infectious Diseases. July 2013; doi: 10.1093/infdis/jit307.

Pilcher, D.; Louie, B; Facente, S.; et al. "Performance of Rapid Point-of-Care and Laboratory Tests for Acute and Established HIV Infection in San Francisco." PLOS|One. December 12, 2013; DOI: 10.1371/journal.pone.0080629.

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