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HIV-1/2 Antigen and Antibodies, Fourth Generation, with Reflexes

Test code(s) 91431(X)

This is an outdated version of this FAQ. It was effective 01/13/2016 to 07/10/2017.

The current version is available here.

HIV-1/2 Antigen and Antibodies, Fourth Generation, with Reflexes

Question 1. Are these particular tests appropriate for use in infants below the age of 2 years?

No. The HIV antigen/antibody assay listed above detects HIV antibodies that could be passively transferred from infected mothers to their infants. The appropriate tests to aid in the diagnosis of infant HIV infection include1:

  • HIV-1 RNA, Qualitative TMA (test code16185[X])
  • HIV-1 DNA, Qualitative PCR (test code 8401[X])
  • HIV-2 DNA-RNA, Qualitative RT-PCR (test code 34977[X])

Question 2. Who should be screened for the presence of HIV infection?

The latest (2006) Centers for Disease Control and Prevention (CDC) recommendations for HIV testing of individuals encountered in healthcare settings include2:

  • Routine, rather than risk-based, HIV screening for all patients aged 13-64 years unless the prevalence of infection is extremely low (<1 in 1000 persons)
  • Opt-out testing, ie, HIV test performed unless patient declines
  • Written patient informed consent not required
  • Patient pre-test counseling not required

The U.S. Preventive Services Task Force (USPSTF) recommends clinicians screen for HIV infection in adolescents and adults aged 15 to 65. Younger adolescents and older adults who are at increased risk should also be screened.3

Question 3. What is the difference between an HIV antigen/antibody (“fourth generation”) screening test [code 91431(X)] and an HIV antibody (“third generation”) screening test [formerly available as code 19728(X)]?

HIV antigen/antibody fourth generation screening tests can simultaneously detect both HIV-1 antigen (ie, viral protein) and HIV-1/2 antibodies, whereas HIV-1/2 antibody third generation screening tests can only detect antibodies. Third generation test results are repeatedly reactive only after a person has developed detectable levels of antibody in his/her serum or other body fluid. Third generation tests cannot detect HIV infection during the “window” period, ie, the time interval between virus acquisition and antibody production. The length of this window period varies among individuals, but on average, third generation tests cannot detect the presence of HIV antibodies until 20 to 30 days after virus acquisition.4

In contrast, fourth generation screening tests can detect HIV during a portion of this window period and afterwards, ie, during a portion of the acute phase and all of the chronic phase of the disease. This ability has substantial individual and societal benefits. The patient benefits in that he/she can be more quickly evaluated for administration of appropriate antiviral medications, which may reduce the adverse early impacts of infection. Society benefits because actions can be taken to reduce HIV transmission. Individuals with acute HIV infection have much higher viral loads, making them more likely to transmit the virus. However, if individuals are diagnosed during the acute phase of the illness, they can be counseled regarding risk-reduction practices and can reduce their viral load with antiviral medication. Both of these actions reduce the likelihood of transmission to uninfected partners.

The CDC recommends using a fourth generation HIV antigen/antibody test for screening and diagnosis because it can detect HIV during both the acute and chronic phases of infection.5

Question 4. What is the CDC-recommended testing algorithm for the diagnosis of HIV infection? 

The key concepts associated with the 2014 CDC recommendations are illustrated here.5

New CDC Recommendations for HIV Testing in Laboratories

Question 5. Why does the CDC no longer recommend the HIV-1 Western blot test as the supplemental antibody test?

The CDC no longer recommends using the Western blot technique to confirm the presence of HIV-1 antibodies in repeatedly reactive specimens because:

  • The HIV-1/HIV-2 antibody differentiation test is more sensitive than the Western blot early in infection.6
  • The HIV-1/HIV-2 antibody differentiation test can be performed more rapidly than can the Western blot      method, so the turnaround time for confirmation of HIV infection is shorter.
  • Studies have shown that the HIV-1/HIV-2 antibody differentiation test more accurately differentiates HIV-1 and HIV-2 infections.7 HIV-2 infection, while rare in the United States, is most commonly seen in persons from West Africa. It's important to differentiate HIV-1 from HIV-2, as HIV-2 may not respond to some HIV medications.9

Question 6. How is the following combination of HIV test results interpreted?

A patient with these test results…


…is unlikely to have an HIV infection, since 2 confirmatory tests (HIV-1/2 antibody differentiation and HIV-1 RNA) are negative. In this particular situation, the repeatedly reactive fourth generation test result in a patient who was exposed to HIV ≥2 weeks before specimen collection would be presumed to be a biological false-positive screening result.5 If recent exposure to HIV-2 is of concern, then follow-up testing with HIV-2 DNA/RNA, Qualitative, Real-Time PCR (TC 34977) should be considered to diagnose acute HIV-2 infection.

A patient with these test results…


…has acute HIV-1 infection, confirmed by detection of HIV-1 RNA prior to the development of HIV-1 antibodies.

Question 7. What causes a biological false-positive HIV screening test?

The precise reason for a biological false-positive result in any individual specimen is not likely to be definitively known. A few of the causes hypothesized in the medical literature include recent administration of selected vaccines, presence of HLA-DR antibodies, presence of rheumatoid factors, reactive RPR, hypergammaglobulinemia, cross-reactive immune response to other exogenous and endogenous retroviruses, and autoimmune conditions.

Although there are occasional biological false-positive results observed with HIV screening tests, as with all screening tests, the specificities of the FDA-approved HIV screening tests are very high (>99%). The potential adverse effects of a biological false-positive screening test result are ameliorated by use of this CDC-recommended multi-step testing algorithm. Test results are only considered to support the diagnosis of HIV infection when the screening test results are repeatedly reactive and either supplemental test (ie, HIV-1/2 antibody differentiation or HIV-1 RNA) is positive.5

Question 8. Which follow-up test is recommended when the screening test is repeatedly reactive, but the confirmatory tests are both negative?

Follow-up testing is generally not recommended. Additional testing is only indicated if the patient is thought to be very recently infected and/or the patient is at continued risk for HIV exposure. Refer to cdc.gov for the latest CDC recommendations on this subject.

Question 9. Is pregnancy associated with a higher frequency of biological false-positive HIV screening test results?

There have been anecdotal reports of pregnancy being associated with a higher frequency of biological false-positive HIV screening test results. This purported anecdotal association may have been due to an initially greater frequency of testing of pregnant women prior to issuance of the 2006 CDC HIV testing recommendations. A more recent large epidemiologic study using a third generation HIV screening method did not demonstrate a greater propensity towards biological false positivity in pregnant subjects. In this study of more than 2 million third generation HIV antibody screening tests, the false-positive rate was actually lower for pregnant women than for others (0.14% versus 0.21%).8

Question 10. Why does the lab require that a blood sample for HIV testing be collected in a separate tube?

HIV screening tests are extremely sensitive and can detect very low levels of circulating viral antibodies and/or antigens (fourth generation test) in a patient’s sample. The laboratory requires a separate sample collection tube for HIV screening testing to help eliminate false-positive results that may be associated with pre-analytic sample handling during the performance of other requested tests. Minimizing pre-analytic sample handling issues is critical, given the clinical significance of a positive HIV test result.



  1. Panel on Antiretroviral Therapy and Medical Management of HIV-Infected Children—A Working Group of the Office of AIDS Research Advisory Council (OARAC). Guidelines for the use of antiretroviral agents in pediatric HIV infection [pages C-1 to C-6]. http://aidsinfo.nih.gov/contentfiles/lvguidelines/pediatricguidelines.pdf. Accessed July 28, 2014.
  2. Branson BM, Handsfield HH, Lampe MA, et al. Revised recommendations for HIV testing of adults, adolescents, and pregnant women in health-care settings. MMWR Recomm Rep. 2006;55(RR-14):1-17. http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5514a1.htm.
  3. Screening for HIV: U.S. Preventive Services Task Force recommendation statement. Published April 2013. www.uspreventiveservicestaskforce.org/uspstf13/hiv/hivfinalrs.htm.Accessed July 28, 2014.
  4. Stekler JD, Branson BM. Detection of acute HIV infection: we can’t close the window. J Infect Dis. 2012;205:521-524.
  5. Branson BM, Owen SM, Wesolowski LG, et al for Centers for Disease Control and Prevention and Association of Public Health Laboratories. Laboratory Testing for the Diagnosis of HIV Infection: Updated Recommendations. http://stacks.cdc.gov/view/cdc/23447. Published June 27, 2014. Accessed June 27, 2014.
  6. Masciotra S, Luo W, Youngpairoj AS, et al. Performance of the Alere Determine™ HIV-1/2 Ag/Ab Combo Rapid Test with specimens from HIV-1 seroconverters from the US and HIV-2 infected individuals from the Ivory Coast. J Clin Virol. 2013;58(suppl 1): e54-58. doi:10.1016/j.jcv.2013.07.002.
  7. Nasrullah M, Ethridge SF, Delaney KP, et al. Comparison of alternative interpretive criteria for the HIV-1 Western blot and results of the Multispot HIV-1/HIV-2 Rapid Test for classifying HIV-1 and HIV-2 infections.   J Clin Virol. 2011;52(suppl 1):S23-27. doi:10.1016/j.jcv.2011.09.020.
  8. Wesolowski LG, Delaney KP, Lampe MA, et al. False-positive human immunodeficiency virus enzyme immunoassay results in pregnant women. PLoS ONE. 2011;6(1):e16538. doi:10.1371/journal.pone.0016538.
  9. HIV-2 infection surveillance – United States, 1987-2009. MMWR. 2011;60(29):985-988. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6029a3.htm.
This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
Document FAQS.106 Version: 2
Version 2 effective 01/13//2016 to present
Version 1 effective 08/15/2014 to 01/12/2016
Version 0 effective 08/08/2013 to 08/14/2014