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HIV Antibodies, HIV-1/2, EIA with Reflexes

Test code(s) 19728X

This is an outdated version of this FAQ. It was effective 08/08/2013 to 08/14/2014.

The current version is available here.

Question 1. Who should be tested for the presence of HIV infection?

The latest (2006) Centers for Disease Control and Prevention (CDC) recommendations for HIV testing include:

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

The U.S. Preventative Services Task Force (USPSTF) recommends that clinicians screen for HIV infection in adolescents and adults aged 15 to 65 years. Younger adolescents and older adults who are at increased risk should also be screened.1

Despite CDC Guidelines and the USPSTF recommendations, which do not constitute the law, the practice of medicine is regulated under various state laws. For example, the CDC recommendation of testing a person for HIV without specific patient authorization or informed consent may be problematic in certain states. Every physician should know the law in the state of their practice and abide by the law; if in doubt, physicians are encouraged to seek legal advice regarding the compatibility of the CDC Guidelines with their state’s laws.

Question 2. Is informed consent necessary for HIV testing?

Some states require written informed consent for all HIV testing (but not necessarily on a separate form). Others specifically exempt health care providers from having to get informed consent. Several explicitly state that no separate consent is necessary for an HIV test as long as a general consent for medical care is in effect. The CDC recommends incorporating informed consent for HIV testing into the general consent for medical treatment. Specifically, separate written consent for HIV testing should not be required; general consent for medical care should be considered sufficient to encompass consent for HIV testing. However, as noted above, each facility must be familiar with laws governing informed consent in its own state and carefully consider their legal meaning in light of the HIV testing objectives. 

Question 3. What is the difference between an HIV antibody screening test and an HIV antigen/antibody screening test?

HIV antigen/antibody screening tests can simultaneously detect both HIV antigen (ie, viral protein) and antibody, whereas an HIV antibody screening test can just detect the antibody. Antibody screening test results are repeatedly reactive only after a person has developed detectable levels of antibody in his/her serum or other body fluid. They cannot detect HIV 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 is estimated to be approximately 20-25 days.

HIV antigen/antibody combination assays can detect HIV during this window period and afterwards, ie, during the acute and chronic phases of the disease. This ability has 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 in that actions can be taken to reduce the transmission of HIV. People with acute infection have much higher viral loads, making them more likely to transmit the virus. However, if these people are identified during the acute phase, they can be counseled re risk reduction practices and can reduce their viral load with medication. Both of these actions reduce the likelihood of transmission to uninfected partners.

HIV antigen/antibody combination assays have recently become FDA-approved as diagnostic screening tests. They have been called 4th generation HIV tests. Unlike 3rd generation tests, the 4th generation tests can detect most HIV infections during the window period.2

Question 4. What causes a false-positive HIV screening test?

The specificity of an HIV screening test is very high (>99%). However, there are occasional false-positive results observed with this screening test, as with all screening tests. The precise reason for a false positive in any individual specimen is not likely to be definitively known. A few of the causes described in the medical literature include recent administration of vaccines, presence of HLA-DR antibodies, presence of rheumatoid factors, reactive RPR, hypergammaglobulinemia, crossreactive immune response to other exogenous and endogenous retroviruses, and autoimmune conditions.

The potential adverse effects of a false-positive test are ameliorated by use of a confirmatory test (eg, HIV Western blot or HIV-1/2 antibody differentiation assay) on all repeatedly reactive test samples. Testing is only considered diagnostic for HIV infection when the screening test results are repeatedly reactive and the confirmatory test is positive.

Question 5. Is pregnancy associated with a higher frequency of false-positive HIV screening test results?

There have been published anecdotal reports of pregnancy being associated with a higher frequency of biologic false-positive HIV screening tests. 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 did not demonstrate a greater propensity towards biologic false positivity in pregnant subjects. In this study of more than 2 million HIV antibody screening tests, the false-positive rate was actually lower for pregnant women than for others (0.14% versus 0.21%).3

Question 6. What follow-up test is recommended when the screening test is repeatedly reactive, but the confirmatory test is either negative or indeterminate?

Since the time for full seroconversion varies from patient to patient, an HIV-1 RNA, Qualitative TMA (test code 16185) test can be considered if acute HIV infection is suspected. This test is approved by the FDA to:

  1. Aid in confirming the diagnosis of acute HIV-1 infection
  2. Resolve indeterminate or inconclusive HIV-1 Western blot results

Question 7. Why does the lab require a blood sample for HIV testing be collected in a separate tube?

HIV diagnostic tests are extremely sensitive and can detect very low levels of circulating antibodies in a patient’s sample. The laboratory requires sample collection in a separate tube to help eliminate false-positive results that may be associated with 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. U.S.Preventative Services Task Force (USPSTF). Screening for HIV: U.S. Preventative Services Task Force Recommendation Statement. Web. April 2013. www.uspreventiveservicestaskforce.org/uspstf13/hiv/hivfinalrs.htm
  2. Figure 16 in CLSI. Criteria for laboratory testing and diagnosis of human immunodeficiency virus infection; Approved Guideline. M53-A ed. Wayne, PA: Clinical and Laboratory Standards Institute, 2011.
  3. 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.
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.

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