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AIDS is caused by 2 known types of HIV. HIV type 1 (HIV-1) is found in patients with AIDS, AIDS-related complex, and asymptomatic infected individuals at high risk for AIDS. The virus is transmitted by sexual contact, by exposure to infected blood or blood products, or from an infected mother to her fetus or infant. HIV type 2 (HIV-2) infection is endemic only in West Africa, and it 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 to undetectable levels (ie, seroreversion) in the terminal stage of AIDS when the patient's immune system is severely depressed.
Routine serologic screening of patients at risk for HIV-1 or HIV-2 infection usually begins with a HIV-1/-2 antigen and/or antibody screening test, which may be performed by various FDA-approved assay methods, including rapid HIV antibody tests, enzyme immunoassays, and chemiluminescent immunoassays. In testing algorithms that begin with these methods, supplemental or confirmatory testing should be requested only for specimens that are repeatedly reactive by these methods according to assay manufacturers' instructions for use.
Screening for HIV-1 and/or HIV-2 infection in asymptomatic patients
Diagnosis of HIV-1 and/or HIV-2 infection in symptomatic patients
Follow-up testing of individuals with reactive results from rapid HIV tests
Negative HIV-1/-2 antigen and antibody screening test results usually indicate absence of HIV-1 and HIV-2 infection. However, such negative results do not rule out acute HIV infection. If acute HIV-1 infection is suspected, detection of HIV RNA (HIVDQ / HIV-1 RNA Detection and Quantification, Plasma) is recommended.
Reactive HIV-1/-2 antigen and antibody screening test results suggest the presence of HIV-1 and/or HIV-2 infection, but it is not diagnostic for HIV infection and should be considered preliminary. Reactive result of this assay does not differentiate among reactivity with HIV-1 p24 antigen, HIV-1 antibody, and HIV-2 antibody. Diagnosis of HIV infection must be based on results of supplemental tests, such as HIV-1/-2 antibody differentiation test (automatically reflexed on all samples with reactive screen test results at an additional charge), HIV-1 antibody confirmation by Western blot, and HIV-2 antibody confirmation. See these individual tests for interpretation of their results.
All initially positive supplemental or confirmatory HIV test results (by serologic or molecular test methods) should be verified by submitting a second serum specimen for repeat testing. Such positive HIV test results are required under laws in many states in the United States to be reported to the departments of health of the respective states where the patients reside.
See HIV Testing Algorithm (Fourth Generation Screening Assay) Including Follow-up of Reactive HIV Rapid Serologic Test Results in Special Instructions.
This test is not offered as a screening or confirmatory test for blood donor specimens.
Reactive result of this assay does not differentiate among reactivity with HIV-1 p24 antigen, HIV-1 antibody, and HIV-2 antibody.
A reactive screening test result is not diagnostic for HIV infection and should be considered preliminary.
The positive predictive value of a reactive screening test result is highly dependent on the prevalence of HIV infection in the population tested. The lower the prevalence of HIV infection, the lower the positive predictive value and higher the false-positive rate of the test. Diagnosis of HIV infection must be based on positive results of the supplemental or confirmatory serologic or molecular tests.
Recipients of experimental HIV-1 vaccines may have false-reactive HIV antibody test results without infection due to the presence of HIV-1 antibodies.
Negative serologic or molecular HIV screening test results should be evaluated with caution in patients with clinical symptoms and/or a history of high-risk behavior for HIV infection. Repeat testing in 1 to 2 months is recommended in these at-risk individuals.
Screening, supplemental or confirmatory serologic tests for HIV-1 or HIV-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). Diagnosis of HIV infection in newborns and infants up to 18 months should be made by virologic tests, such as detection of HIV RNA (HIVDQ / HIV-1 RNA Detection and Quantification, Plasma).
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 >1250 mg/dL)
-Grossly icteric (total bilirubin level of >20 mg/dL)
-Presence of particulate matter
1. Bennett B, Branson B, Delaney K, et al: HIV testing algorithms: a status report. A publication from The Association of Public Health Laboratories. April 2009. Available from http://www.aphl.org/aphlprograms/infectious/hiv/Documents/ID_2009April_HIV-Testing-Algorithms-Status-Report.pdf
2. Chavez P, Wesolowski L, Patel P, et al: Evaluation of the performance of the Abbott ARCHITECT HIV Ag/Ab Combo assay. J Clin Virol 2011;52(Suppl 1):S51-S55
3. Centers for Disease Control and Prevention and Association of Public Health Laboratories. Laboratory testing for the diagnosis of HIV infection: Updated recommendations. June 27, 2014. Available from http://stacks.cdc.gov/view/cdc/23447