Am I Infected? (A Guide to Testing for HIV) en español
What viral tests are available?
There are a number of tests that look specifically for the virus. Generally speaking, these tests yield conclusive results within 48 to 72 hours after infection has occurred. However, in some case, it can take as long as 28 days for results to be considered accurate.
P24 Antigen Test: This test uses ELISA technology to look directly for key pieces of the HIV virus – the p24 protein found on HIV's outer coat. This test can reduce the chance of a false-negative in standard (antibody) ELISA testing is done too early (i.e., less than 13 weeks after exposure). The p24 antigen test may be ordered if there's been a very recent risky exposure to HIV, such as a healthcare work-related incident. Blood banks also use it for screening donations.
Valuable in detecting HIV infection early in the window period after exposure, this test is only useful for a period of approximately three weeks after exposure, before the production of antibodies begins. A p24 test result should be confirmed by antibody testing once the window period has passed.
Quantitative Polymerase Chain Reaction (PCR): The quantitative PCR, also known as the viral load test, is considered to be highly reliable for someone who may have recently been exposed to the virus, particularly in a high-risk situation. If the virus is present, the quantitative PCR will reveal how much virus is in a person's bloodstream (the viral load). In most cases, a quantitative PCR is highly accurate within 48 to 72 hours. However, a small number of people don't have viral loads that are high enough to confirm a diagnosis until 28 days after exposure. The most widely available quantitative PCR test, Amplicor® version 1.5, can detect subtypes A through G, which account for 99.96% of HIV infections in the U.S.
The standard recommendation is that a negative PCR result be confirmed with an ELISA test at 13 weeks.
Qualitative PCR: The qualitative PCR, also known as the PCR-DNA test, looks for DNA in cells that suggest that HIV infection has taken place. It is not a viral load test, meaning that it will only determine if the virus is present, not how much virus is present. This test is frequently used to determine if an infant born to an HIV-positive is infected with the virus, given that it can detect virus before viral load becomes detectable. However, it's not at all clear if the qualitative PCR test has any advantages over the quantitative PCR test, which appears to be just as reliable, more widely available, and cheaper to perform.
Branched DNA Assay: Like the quantitative PCR test, the branched DNA (bDNA) assay can determine how much virus is present in someone who has been infected with the virus. The results of bDNA testing are comparable to those of quantitative PCR testing and there are no known advantages of using bDNA testing over quantitative PCR testing for diagnostic purposes. This test is known as the Quantiplex® HIV-RNA Assay and is available through most commercial laboratories.
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