HIV Antibody Rapid Test Confirmatory Profile, Serum
Follow-up serologic testing of patients with reactive rapid HIV antibody screening test results
Confirmatory testing by Western blot is recommended on serum specimens that are reactive by the rapid HIV antibody tests, even if the chemiluminescence immunoassay results are non-reactive in these patients
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 antibody testing should only be performed on specimens that are repeatedly reactive by EIA or CIA.
In 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. Since individuals at risk for HIV infection may have negative HIV antibody screening test results by EIA or CIA with indeterminate Western blot (WB) results (especially during early stage of infection), such individuals may be interpreted erroneously not to have HIV infection despite having reactive rapid HIV antibody test results. The positive predictive values of rapid HIV antibody tests are low (<50%) in populations with low prevalence rates (<1%) of HIV infection. To rule-out HIV infection in high-risk, rapid test-positive patients, the appropriate follow-up serologic tests should include HIV-1 antibody by WB assay (see WBAR / HIV-1/-2 Antibody Confirmatory Evaluation, Serum) with or without HIV-1/-2 antibody screening by EIA or CIA. HIV-1 antibody by WB assay should be done regardless of the anti-HIV-1 screening test result. The WB testing contains sequential reflexive testing for HIV-2 antibody by EIA (see HIV2 / HIV-2 Antibody Evaluation, Serum), HIV-1 antibody confirmation by IFA (see HIVFA / HIV-1 Antibody Confirmation by Immunofluorescence, Serum), and HIV-2 antibody confirmation by immunoblot assay when necessary.
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.
A reactive HIV-1/-2 antibody screening test result suggests the presence of HIV-1 and/or HIV-2 infection. However, it does not differentiate between HIV-1 and HIV-2 antibody reactivity. Confirmatory testing by HIV-1 antibody-specific Western blot (WB) or immunofluorescence assay (IFA) is necessary to verify the presence of HIV-1 infection. Presence of HIV-2 infection is confirmed by the HIV-2 antibody-specific immunoblot assay.
A negative HIV-1/-2 antibody screening test result indicates the absence of HIV-1 or HIV-2 infection. However, confirmatory HIV-1 antibody testing by WB (WBAR / HIV-1/-2 Antibody Confirmatory Evaluation, Serum) is necessary for specimens that are reactive by the rapid HIV antibody tests; even if the screening test results are negative.
Result of the confirmatory HIV-1 antibody WB assay 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 with a reactive rapid HIV antibody test result indicates presence of HIV-1 infection, regardless of the HIV-1/-2 antibody screening test result, but it does not indicate the stage of the HIV-1 infection. Positive confirmatory HIV antibody test results are required under laws in many states to be reported to the departments of health of the respective states where the patients reside.
For asymptomatic individuals with reactive rapid HIV test results, negative HIV-1 antibody WB results with either reactive or negative HIV-1/-2 antibody screening test results most likely indicate false-reactive rapid HIV antibody test results. However, a negative HIV-1 antibody WB result 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 HIV-1 antibody WB results occur when bands are present but the patterns do not meet the criteria for a positive result. Indeterminate results may indicate a) early, late, or incomplete HIV-1 antibody reactivity, b) presence of HIV-2 antibodies, or c) presence of nonspecific cross-reactive antibodies. For specimens with indeterminate WB results, confirmatory HIV-1 antibody testing by IFA) and HIV-2 antibody testing by EIA will be performed automatically (at additional charges).
Indeterminate WB patterns can be found in up to 15% of persons without evidence of HIV infection. Individuals with persistent, stable, indeterminate HIV-1 antibody WB results but without risk factors should be monitored for 6 months. If no additional WB bands develop during that time, the patient is determined not to be HIV-infected.
An uninterpretable HIV-1 antibody WB result indicates the presence of smear or blotches obscuring proper reading of the WB strip, in the absence of discrete bands. Such findings indicate probable nonspecific binding of antibodies in the patient's serum to the WB strip. For specimens with such uninterpretable WB results, confirmatory HIV-1 antibody test by IFA and HIV-2 antibody testing by EIA will be performed automatically (at additional charges).
The following algorithms are available in Special Instructions:
-HIV Rapid Serologic Testing Follow-up Algorithm-HIV Treatment Monitoring 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 positive confirmatory HIV antibody test results prior to the clinical diagnosis of HIV infection. A second serum specimen should be obtained from the patient and submitted for repeat testing to verify all positive confirmatory test results.
Although repeatedly positive confirmatory HIV antibody test results indicates HIV 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-1 antibody test results but negative HIV-1/-2 antibody screening test results and indeterminate or negative HIV-1 antibody Western blot (WB) results should be retested 1 to 3 months. Repeat testing may be done by a combination of either rapid HIV antibody test with HIV-1 antibody WB or routine HIV-1/-2 antibody screening test with HIV-1 antibody WB.
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 and confirmatory) for HIV-1/-2 antibodies cannot distinguish between active neonatal HIV infection and passive transfer of maternal HIV antibodies in infants during the postnatal period (up to 18 months of age). Diagnosis of HIV infection in newborns and infants should be made by virologic tests such as detection of HIV RNA (HIVQU / HIV-1 RNA Quantification, Plasma) or HIV proviral DNA (PHIV / HIV-1 Proviral DNA Qualitative Detection by PCR, Blood).
Assay performance characteristics have not been established for the following specimen characteristics:
-Grossly hemolyzed (hemoglobin level of >500 mg/dL)
-Grossly lipemic (triolein level of >3,000 mg/dL)
-Grossly icteric (total bilirubin level of >20 mg/dL)
-Containing particulate matter
-Individuals of <2 years of age
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
1. Center for Disease Control and Prevention: Protocols for confirmation of reactive rapid HIV tests. MMWR Morb Mortal Wkly Rep 2004;53:221-222
2. Branson BM, Handsfield HH, Lampe MA: Center 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. Constantine N: HIV antibody assays May 2006. HIV InSite Knowledge Base (online textbook). Available from URL: http://hivinsite.ucsf.edu/InSite?page=kb-00&doc=kb-02-02-01
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