Wednesday, 1 August 2007

I've had an HIV test that was negative. Can I still be infected with HIV?

HIV tests tend to be very sensitive - that is, they tend to pick up virtually every person who is HIV positive. (The figure is greater than 99.5%.) Thus, if your test comes back negative, you can virtually be assured that you are actually negative. (If you really want reassurance, you could always ask for another test if it would put your mind at rest.)

However, there's a small catch. The above often only becomes true a few months after the initial infection. The most common tests that are used detect antibodies to the HIV antigens. (Most of the antibody tests are ELISA tests - see my post on this topic if you're interested.) In plain English, this means that it tests whether your body is mounting an immune response to HIV, which it obviously wouldn't do unless you were infected. The problem is that the immune response takes several weeks to develop, and so taking a test within this 'window period' could result in a false negative (i.e. you actually do have HIV, but the test is still negative). To get around this, you just need to take the test after this window period.

How long is the window period? That depends on the specific test and method used, and you'd have to speak to your doctor about it for your individual case. Where I practice, it's around 6 weeks, but as I said, it varies. (Most papers cite an average of 22 days.) So if an antibody test is used, you should take the test outside of this time frame.

There are two other types of tests available. They are both much more expensive than the ELISA test, but they have smaller window periods. They are:

  • The p24 antigen test. This tests for a particular protein that the virus produces tonnes of in the early stages of infection. Since the test looks for part of the virus itself, it doesn't have to wait for the body to mount an immune response, so the window period is shortened. There is still a window period though, since the virus has to make enough p24 to be detected by the tests... which takes it time. The window period averages 16 days (down from 22).
  • Nucleic acid tests. It is possible to test for viral RNA, or DNA derived from it. Again the window period is shorter, but still present for much the same reason. The window period is reduced to an average of about 12 days.

Note: most labs do a separate confirmatory test if you get a positive result. They'll usually start with an antibody test (usually ELISA), since it is cheap and has an extremely high sensitivity. If this is positive, a second test is usually performed by the lab, but using a different method (e.g. RNA-based).

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