positive for p24 antigen and which are positive for antibodies. They can also tell the difference between HIV-1 and HIV-2.
Window Times and Precision
During the window period, right after infection, tests may not always be able to find antibodies or p24 antigen, giving a false negative result.
It is difficult to say exactly how long the time period for a test is because it varies from person to person and is a hard subject to study (recently infected individuals would need to know precisely when they were exposed to HIV and then give multiple blood samples over the following weeks and days).
Still, the median window period for fourth-generation tests is thought to be 18 days, and half of all infections are found between 13-24 days after exposure.
Even though sometimes this period is a little longer, 99% of HIV-infected people can be found within 40 days of being exposed.
Are Fourth-Generation HIV Lab Tests Reliable?
The fourth generation of lab tests is very reliable.
The most important ways to measure accuracy are sensitivity (the number of times a test is positive when HIV is present) and specificity (the percentage of correctly negative results when HIV is not present).
The fourth-generation tests are very accurate and sensitive. They are more likely to find an infection early than quick tests. They should be used when results don’t need to be given right away and when people are likely to come back to get their results.
Quality control programs are easier to set up when a central lab does testing.
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