The diagnosis of HIV is a two-step process. The initial screening tests, including ELISA and the rapid tests, are highly sensitive and can detect very low concentrations of anti-HIV antibodies. However, these tests are less specific and can lead to false positive results. Thus, a positive screening test should always be validated with a confirmatory test. According to the CDC, ELISA combined with a confirmatory test such as the Western blot or immunoflorocent assay test is 99 percent accurate.
Western Blot
The Western blot is the most common confirmatory test for HIV and it identifies the anti-HIV antibodies based on their molecular weight. The test is usually performed three to five weeks after the initial exposure, which is the time required for the anti-HIV antibodies to appear in the patient's blood. This period is known as the "window period." The most common sample for all the HIV confirmatory tests, including Western blot, is blood--although other body fluids such as oral fluids and urine are used under special circumstances. On average, it takes about one to two weeks to process the results of a HIV confirmatory test. Patients usually have a limited choice, as most tests are not commonly available. Western blot is very specific and rarely gives false-positive results. It can sometimes be inconclusive, especially if the patient does not have sufficient antibody concentration in his blood. In such cases, the test is repeated after a month.
Radioimmunoprecipitation Assay (RIPA)
RIPA involves combining the patient sample with radio labeled antigen (antigen combined with a radioactive substance) to detect the presence of antibodies. When performed after the initial window period of three to five weeks, this method is highly specific and sensitive. But, it is expensive and requires highly skilled personnel. Because of this, it is performed only if the Western blot results are consistently inconclusive despite a strong clinical suspicion.
Immunoflorocent Assay (IFA)
In an IFA, HIV antigen is mixed with a fluorescent compound and then with a sample of the patient's blood. If positive, the mixture radiates under ultraviolet light. Though not commonly available, IFA is as reliable as any other test for HIV if performed three to five weeks after the exposure. This test is generally used to screen blood donor samples and for donor counseling.
HIV Viral Load Measurements
Also known as the PCR test, RNA test or HIV nucleic acid amplification testing (HIV NAAT), this test detects the presence of viral genetic material (RNA) in the test sample as opposed to the antibodies. And although this test can be performed just nine to 11 days after the initial infection, it is used only for testing newborn babies and high-risk individuals. It has not been approved by the FDA for general testing because it is expensive and often difficult to interpret.
p24 Antigen Assay
The p24 antigen test detects the presence of p24 protein, which is present in the HIV virus. Although this test can give results only two weeks after the infection, it is neither very sensitive nor specific for the presence of live virus. It is used for detecting early infections in rape cases and after occupational exposures to infected body fluids. It is also used to resolve inconclusive Western blot results.
Home-Based HIV Tests
The FDA has approved only one home-based test for HIV, and it costs about $40 to $70 depending on the speed of the results. The test kit includes the materials and the instructions for collecting a blood sample. The sample is then sent to the specified laboratory, where the traditional ELISA and Western blot are performed. This test is as reliable as any other test when performed after the window period.
References
- "Practical Medical Microbiology"; J. G. Collee, A. G. Fraser, B. P. Marmion, A. Simmons; 1996
- Center for Disease Control and Prevention: HIV Testing Basics for Consumers
- University of California-San Francisco HIV InSite: Beyond Basics-What Kinds of HIV Screening Tests are Available in the United States?
