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Calculating HIV window periods

If I have HIV, what are the chances that my HIV test will still show negative?

To find out, drag the slider to the number of days since you think you were exposed to HIV.

 
0 days > 100 days
0days

Probability of false negative test result:

Antibody Test*

100%

Antibody/Antigen Test**

100%

* Measures the body's response to HIV through the detection of antibodies (3rd generation test)

** Measures any HIV proteins in the body as well as antibodies (4th generation test)

What is this tool?

The HIV Window Period Table of Probabilities tool was developed to help clinicians doing HIV pre-test discussion with individuals who may have been exposed to HIV.

As technology improves, the window periods associated with HIV tests get shorter and it's important to counsel patients appropriately about the best time to test for HIV.

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Diagnosing HIV as soon as possible after infection means individuals can be advised on how to prevent transmission when their viral load is high. However, some clinicians may choose to wait three months after a possible exposure to HIV, by which time the chance of a false negative result is less than 1%.

Balancing these priorities can be challenging for clinicians and difficult for individuals to understand. The Table of Probabilities tool was designed to give the probability of a false negative test result if a patient has HIV. This information may help clinicians and individuals decide the best time to test for HIV.

The tool should be used when an individual can give an approximate date of when they were potentially exposed to HIV. It is not useful for people who have condomless sex or regularly engage in other activities that have a higher chance of HIV transmission.

More information can be found about the development of the tool here.

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Point of Care (3rd generation) and 4th generation HIV tests

HIV testing in British Columbia usually starts with a 4th generation test that looks for both HIV antibodies and antigen (the p24 antigen) in the blood. If this test is reactive, additional confirmatory testing is done.

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The 4th generation HIV test has a reported average window period of 14 - 21 days.[1] Since both 3rd generation (Point of Care) and 4th generation HIV tests depend on individual immune responses, the amount of time needed for a person to develop enough antibodies for a test to detect HIV can vary. HIV window periods are based on averages; some people develop antibodies quickly while others take longer, and there are always outliers.

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How to use the calculator

Determine if your client will be given a 3rd generation (Point of Care, or POC) or a 4th generation (non-POC) HIV test. Ask your client to estimate the number of days since they think they were exposed to HIV. Slide the bar to the appropriate number of days; the probability of a false negative will be shown for the 3rd and 4th generation tests.

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Example: Harry is a gay man in a relationship with an HIV positive partner. He and his partner usually use condoms during sex, but one night they had unprotected anal sex, with Harry on the bottom. Harry estimates that this happened about two weeks (14 days) before. His doctor looks at the Table of Probabilities and tells Harry that he can test for HIV, but if he's HIV positive, there is a 79-90% chance that the test will have a false negative result (the test will not detect HIV even when HIV is present). They decide to do it anyway, with the plan to retest at 4 weeks if the first test is negative.

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Limitations

It is important to note that there is inherent uncertainty with window periods, as they can vary for different people. While the Table of Probabilities is based on a large study with a high number of samples, it is still a finite sample and there is always the possibility of outliers in the data. The probabilities provided here are a guideline and should not be used with absolute certainty.

Resources

  1. Taylor D, Durigon M, Davis H, Archibald C, Konrad B, Coombs D, Gilbert M, Cook D, Krajden M, Wong T, Ogilvie G. Probability of a false negative HIV antibody test result during the window period: a tool for pre- and post-test counseling. Int J STD AIDS. 2014 Jul 16.
  2. Gilbert M, Krajden M. Don't wait to test for HIV. BCMJ. Aug 2010;52(6).
  3. Public Health Agency of Canada. Human immunodeficiency virus: HIV screening and testing guide. 2012. Ottawa. Available online at: Catie.ca (PDF document)
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HIV, window period, 4th generation, hiv testing
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