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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 pre-test counseling with patients who may have been exposed to HIV.

Because the types of HIV tests available are changing, and the window periods associated with these tests are getting shorter, counseling patients about the best time to test for HIV can be challenging.

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  • On one hand, diagnosing HIV as soon as possible after infection means patients can be advised on how to prevent transmission when their viral load is high.
  • On the other hand, many clinicians 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 patients 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 patients decide the best time to test for HIV.

The tool should be used when a patient can give an approximate date of when they were potentially exposed to HIV. It is not useful for patients who have unprotected sex or other higher risk sexual activities on a regular basis.

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

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3rd and 4th generation HIV tests

HIV testing in BC usually starts with a 4th generation test that looks for antibodies and the p24 antigen in the blood. If this test is reactive, additional confirmatory testing is done, including Western Blot or Nucleic Acid Amplification Testing (NAAT).

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The 4th generation HIV test has a reported average window period of 14 - 21 days.[1] Since both 3rd generation 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 can vary.

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How to use the calculator

Make sure you know if your patient will be tested with a 3rd generation or 4th generation HIV test. In BC, under most circumstances, this will be a 4th generation test. Ask your patient 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. 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 window period can vary for different people. The probabilities provided 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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