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Interpretive Handbook

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Test 23878 :
HIV-1/-2 Antibody Confirmatory Evaluation, Serum

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

AIDS is caused by at least 2 known types of HIV. HIV type 1 (HIV-1) was isolated from patients with AIDS, AIDS-related complex, and asymptomatic infected individuals at high risk for AIDS. The virus is transmitted by sexual contact, exposure to infected blood or blood products, or from an infected mother to her fetus or infant. HIV type 2 (HIV-2) was isolated from patients in West Africa in 1986. It appears to be endemic only in West Africa and it also has been identified in individuals who had sexual relations with individuals from that geographic region. HIV-2 is similar to HIV-1 in viral morphology, overall genomic structure, and its ability to cause AIDS.


Antibodies against HIV-1 and HIV-2 are usually not detectable until 6 to 12 weeks following exposure and are almost always detectable by 12 months. They may fall into undetectable levels in the terminal stage of AIDS. Routine serologic screening of patients at risk for HIV-1 or HIV-2 infection usually begins with a HIV-1/-2 antibody screening test, which may be performed by various FDA-approved assays, including rapid HIV antibody tests, EIA, and chemiluminescent immunoassay (CIA) methods.


In testing algorithms that begin with EIA or CIA methods, confirmatory HIV antibody tests should only be ordered for patients who are found to be reactive by routine HIV-1/-2 antibody screening tests or by rapid HIV antibody tests. However, for testing algorithms that begin with rapid HIV antibody tests, confirmatory antibody testing should be performed regardless of the result of the EIA or CIA tests. Individuals at risk for HIV infection may have negative HIV antibody screening test results by EIA or CIA with indeterminate Western blot results (especially during early HIV infection). Such individuals may be erroneously interpreted as negative for HIV infection, despite having reactive rapid HIV antibody test results.

Useful For Suggests clinical disorders or settings where the test may be helpful

Confirmatory detection of HIV-1 and/or HIV-2 antibodies in patients with reactive antibody screen or rapid HIV antibody results

Interpretation Provides information to assist in interpretation of the test results

An HIV-1 antibody Western blot (WB) result is interpreted as positive when at least 2 of the 3 following bands are present: p24, gp41, and gp120/160. A positive HIV-1 antibody WB result following a reactive HIV-1/-2 antibody screening test result or reactive rapid HIV antibody test result indicates infection with HIV-1, but it does not indicate the stage of disease. In many US states, positive confirmatory HIV antibody test results are required to be reported to the state department of health.


A negative HIV-1 antibody WB result with either a reactive HIV-1/-2 antibody screening test or a reactive rapid HIV antibody test, in the absence of signs, symptoms, or risk factors for HIV infection, probably indicates a false-positive screening test or rapid test. However, a negative HIV-1 WB does not exclude the possibility of early HIV-1 or HIV-2 infection. If the HIV-1 antibody WB test result is negative, the HIV-2 antibody test by EIA will be performed automatically (at an additional charge).


Indeterminate WB patterns can be found in up to 15% of persons without evidence of HIV infection. Individuals at risk for HIV infection undergo nucleic acid testing if acute HIV infection is suspected or repeat HIV serologic testing in 2 to 4 weeks. If no additional WB bands develop during that time, the patient is considered NOT to be infected with HIV-1.


An unreadable HIV-1 antibody WB result indicates the presence of smear or blotches obscuring proper reading of the WB strip. Such findings indicate probable nonspecific binding of antibodies in the patient's serum to the blot. For specimens with such unreadable WB results, confirmatory HIV-1 antibody test by IFA and HIV-2 antibody test by EIA will be performed automatically (at additional charges).


The following algorithms are available in Special Instructions:

-HIV Testing Algorithm (excludes HIV rapid testing)

-HIV Rapid Serologic Testing Follow-up Algorithm

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

The US Association of Public Health Laboratories recommends verification of all first-time positive confirmatory test results prior to the diagnosis of HIV infection. A second serum specimen should be obtained from the patient and submitted for repeat testing to verify all such positive results.


Although a positive HIV-1 antibody Western blot (WB) result indicates HIV-1 infection, a diagnosis of AIDS can only be made based on the case definition of AIDS established by the CDC.


High-risk individuals with reactive rapid HIV antibody tests, but negative routine HIV-1/-2 antibody screening tests and indeterminate or negative HIV-1 antibody WB results should be retested in 2 to 4 weeks. Repeat testing may be done by a combination of HIV-1 antibody WB with either rapid HIV antibody or routine HIV-1/-2 antibody screening test.


Positive HIV antibody WB results in infants of < or =18 months of age and born to HIV-infected mothers may indicate passive transfer of maternal HIV antibodies. Serologic tests (screening or confirmatory) cannot distinguish between active neonatal HIV infection and passive transfer of maternal HIV antibodies in infants during the postnatal period (up to 18 months). Diagnosis of HIV infection in newborns and infants should be made by consistently positive nucleic acid test results, such as the presence of HIV RNA (HIVQU / HIV-1 RNA Quantification, Plasma) or HIV proviral DNA (PHIV / HIV-1 Proviral DNA Qualitative Detection by PCR, Blood).


This test should be ordered only on sera that are reactive by FDA-licensed HIV-1/-2 screening tests or rapid HIV antibody tests.


Although hemolyzed serum specimens are acceptable for testing, this assay is not FDA-approved for testing cadaveric serum specimens.


Assay performance characteristics have not been established for:

-Cadaveric specimens

-Those that contain particulate matter

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.


See HIV Serologic Interpretive Guide in Special Instructions for further interpretive information.


If this test is ordered as a follow-up test on a patient with a reactive rapid HIV antibody test result, Centers for Disease Control and Prevention (CDC) recommends supplemental testing with WB, even if routine antibody screening result is negative.

Clinical References Provides recommendations for further in-depth reading of a clinical nature

1. Constantine N: HIV antibody assays, May 2006. In HIV InSite Knowledge Base (online textbook). Available from URL:

2. Centers for Disease Control and Prevention: Revised recommendations for HIV testing of adults, adolescents, and pregnant women in health-care settings. MMWR Recomm Rep 2006;55:1-17

3. Hariri S, McKenna MT: Epidemiology of human immunodeficiency virus in the United States. Clin Microbiol Rev 2007;20:478-488

4. Owen SM, Yang C, Spira T, et al: Alternative algorithms for human immunodeficiency virus infection diagnosis using tests that are licensed in the United States. J Clin Microbiol 2008;46:1588-1595