Challenge: Limitations of HIV Tests
The limitations of tests used to detect HIV infection is another major barrier to identifying recently infected individuals.
Several types of HIV tests are available but for each type of test there is a brief period of time after infection during which they are unable to detect infection in a person who is HIV-positive. The time period from when a person becomes infected with HIV to when an HIV test can detect their infection, is known as the "window period." During the "window period," an HIV test may find a recently infected person to be HIV-negative. The length of the "window period" is different for each type of test and varies from person to person.19
The HIV tests most commonly used in Canada look for antibodies to HIV in the blood. These tests cannot detect HIV infection in someone who has acute HIV infection because the body has not yet produced antibodies. The "window period" for antibody tests varies because some people produce antibodies faster than others. For most people (up to 95%), the window period of the antibody test is approximately one month, but for some individuals it may be as long as three months.
This means that for people at high risk of HIV, testing can be done as early as one month after exposure using standard antibody assays and rapid point-of-care tests. People who test positive will know for certain they are HIV-positive. Of those who test negative, 95% are indeed negative. Up to 5% of people who test negative at one month could later test positive at three months. It is important to ensure that people who test negative at one month are advised to return for repeat testing once the three-month window period is over.
Other HIV tests have been developed, which detect the virus itself -- such as the HIV RNA or the p24 antigen tests. These have shorter window periods (from seven to 14 days) than antibody tests and so can potentially detect HIV infection during the acute phase.20 Unfortunately, access to these new testing technologies varies across Canada.
Another limitation of HIV tests is the time it takes for many of them to produce results. Most HIV tests do not produce results immediately and require people to wait one to two weeks before getting the results. During this time, people may continue to engage in high-risk behaviours and some people may never return to get their test results.
Tests known as rapid or point-of-care (POC) tests can provide results on the same day that a test is performed. Most rapid/POC tests can provide results within minutes. This ensures that a person receives their results. Rapid tests that detect antibodies are available in some parts of Canada but rapid tests that can detect HIV RNA or the p24 antigen do not yet exist.
- Frontline service organizations need to increase people's awareness of the different "window periods" for each type of HIV test, and emphasize that a negative result does not necessarily mean that a person is HIV-negative. Messaging should emphasize that a person who has recently tested HIV-negative may be in the "window period" and may be highly infectious. Knowledge of the "window period" and the increased risk of transmission during this time is particularly important for people who base their decisions of whether or not to have unprotected sex on knowledge of their HIV status or their partner's.
- People who test HIV-negative within the "window period" should be encouraged to refrain from high-risk behaviours and return for another HIV test at an appropriate time. A second test is important to rule out the "window period" as the reason for the test being negative. In Canada, we encourage people who test HIV-negative on an antibody test to test again at the end of the three-month "window period," or sooner if appropriate.
- Organizations should learn if, and where, rapid antibody testing or RNA/p24 testing is available in their area. A person who is suspected of being in the acute stage of HIV infection (for example, if they have recently had a high-risk exposure or they have experienced flu-like symptoms after the exposure) should be referred to a site where RNA or p24 testing is available. Service providers may need to advocate for improved access to rapid antibody testing and RNA/p24 tests in their area.
The goal of HIV prevention is to reduce the number of HIV infections in the communities we serve. A large number of HIV transmissions in your communities may be occurring from recently infected individuals, and therefore represent an important priority for HIV prevention efforts. Although several challenges exist in identifying these individuals and engaging them in prevention services, frontline organizations can play a key role in overcoming these challenges and reducing HIV transmissions.
Hottest at the Start -- A campaign by the Health initiative for Men (HiM) in BC to raise awareness on acute infection and transmission and to encourage gay men who have had a recent potential exposure to HIV to get an "early" HIV test.
James Wilton is the Project Coordinator of the Biomedical Science of HIV Prevention Project at CATIE. James has an undergraduate degree in Microbiology and Immunology from the University of British Columbia.
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