HIV Serology Test

HIV is also known as human T-lymphotrophic virus, type III (HTLV-III), or the lymphadenopathy-associated virus (LAV). There are two types of human immunodeficiency viruses, types 1 and 2. Type 1 is most prevalent in the United States and Western Europe. Type 2 is mostly limited to Western African nations. Those at high risk for AIDS include sexually active male homosexuals, bisexual men and women with multiple partners, intravenous (IV) drug abusers, persons receiving blood products containing HIV, and infants exposed to the virus during gestation and delivery.

Due to of the medical and social sensitivity towards a positive rest for HIV antibody, the test results must be accurate with correct interpretation. Therefore the U.S. Public Health Service has emphasized that an individual can be said to have serologic evidence of HIV infection only after an enzyme immunoassay (EIA) screening test is reactive and another test, such as the Western blot or immunofluorescence assay (IFA), validates the results. However, a positive EIA that is not confirmed by the Western blot test or immunofluorescence should not be considered negative. Repeat testing is required in 3 to 6 months. A person with positive HIV test results does not have AIDS until the patient develops the clinical features of diminished immune ability.


EIA and ELISA HIV serologic tests, which test for antibodies to HIV in serum or plasma, are the most widely used serologic tests for AIDS. They are used for making a clinical diagnosis, screening donor blood and blood products, and testing individuals who believe they may be infected with HIV. It is important to reiterate that serologic tests detect antibodies to HIV. Because they do not detect viral antigens, they cannot detect infection in its earliest stage (before antibodies are formed). There is a period of time (usually within 2 to 12 weeks, but possibly as long as 6 months) it takes for a person who has been infected with HIV to seroconvert (test positive for HIV antibodies). This time is called the “window period.”


The specificity (probability that test results will be nonreactive if the specimen is a true negative) of the serologic tests are approximately 99% for repeatedly nonreactive tests or if potential exposure was beyond the window period. False positive tests do occur, but are rare. To increase the specificity of serologic tests further, a supplemental test (most often the Western blot test, immunofluorescent assay, or viral RNA test is performed to validate repeatedly reactive serologic test results.


The diagnostic tests described previously detect HIV infection based on demonstration of antibodies to HIV. Recently, it has become possible to diagnose HIV infection by the direct detection of HIV RNA. The p24 antigen capture assay is an ELISA-type assay that detects the viral protein p24 in the peripheral blood of HIV-infected individuals, in which it exists either as a free antigen or complexed to anti-p24 antibodies. The p24 antigen may be detectable as early as 2 to 6 weeks after infection. The p24 antigen test can be used to assess the antiviral activity of anti-HIV therapies. The p24 antigen test can also be used to diagnose neonatal HIV infection, detect HIV before seroconversion, detect HIV in donor blood, and determine the progression of AIDS.


Recently, the use of oral fluids for the detection of antibodies to HIV has become available as an alternative to serum testing. These new HIV-1 antibody tests use oral mucosal transudate (OMT), a serum-derived fluid that enters saliva from the gingival crevice and across oral mucosal surfaces. These tests compare favorably to serum test reliability because they are based on the same serology—Western blot algorithm. The OMT tests for HIV involve a simple, safe, and noninvasive method of specimen collection, thus providing an effective epidemiologic tool for HIV testing. In all of the oral tests, a pretreated cotton pad is placed between the lower gums and the cheek. This pad is then sent to the central laboratory for antibody testing as described above. It is hoped that these portable, user-friendly diagnostic tests will facilitate identification of greater numbers of infected individuals, with the ultimate goals of early identification, early treatment, and prevention of disease transmission.


Another noninvasive alternative to blood testing is urine testing for HIV. Only a spot urine collection is required. Testing urine for HIV antibodies is valuable, especially when venipuncture is inconvenient, difficult, or unacceptable. Insurance companies also commonly use it. The sensitivity and specificity (accuracy) are somewhat less than that of the blood and oral fluid tests. This is also an EIA antibody test similar to blood EIA tests and requires a follow-up confirmatory Western Blot using the same urine sample. It is important to note that all urine HIV tests are detecting antibodies and not the HIV particles. Urine does not contain the virus and is not a body fluid capable of infecting others.


Because some persons who undergo HIV testing do not return to learn their test results, there has been a strong push toward rapid HIV testing where results can be available in less than 1 hour. Like conventional HIV enzyme immunoassays, rapid HIV tests require confirmation if reactive. There are several different types of rapid test using either pin prick or OMT. All of the four HIV tests can be interpreted visually by the naked eye and require no instrumentation or any sort of measurement. The HIV antigens will be affixed to a test strip. If HIV antibodies do exist in the specimen, those HIV antibodies will bind with their matching antigens on the strip. As a result, the colorimetric reagent on the strip will bind to the resulting immunoglobulins creating a coloring indicator that can be detected visually. Like conventional EIAs, rapid HIV tests are screening tests. If performed correctly, they detect HIV antibodies with sensitivities similar to currently available EIAs.


Home kits are now available that provide anonymous registration and pretest counseling via a toll-free call. Sample collection in the privacy of one’s home, laboratory processing, and posttest counseling are components of this home-testing process. The procedure involves pricking a finger with a special device, placing drops of blood on a specially treated card, and then mailing the card in to be tested at a licensed laboratory. Test results are available to the client (as described previously) through HAHC, within 3 business days for the Express Kit and 7 days for the Standard Kit after shipment of the sample to the laboratory.



Possibility of False Results:


  • False-positive results can occur in patients who have autoimmune diseases such like  leukemia, lymphoma, syphilis, or lymphoproliferative disease. False results also can occur when testing patients who are alcoholics.

        •  False-negative results can occur in the early incubation stage or the end stage of AIDS. Tests for HIV antigen should be performed at this stage.