http://www.catie.ca/en/pif/fall-2010/detecting-hiv-earlier-advances-hiv-testing
Detecting HIV earlier: Advances in HIV testing
Since the early days of the HIV epidemic, HIV tests have improved considerably. We now have multiple methods to test for HIV infection, and today’s testing technologies are getting better at detecting new infections. This has led some HIV testing experts to suggest that we can detect the vast majority of HIV infections much earlier than many people realize. This article will review the scientific evidence on the early detection of HIV to help guide our messaging about HIV testing.
...Newer testing technologies can now detect HIV antibodies when there are lower concentrations in the blood and hence are able to detect an HIV infection much sooner.
...Older HIV antibody tests were significantly less able than newer tests to detect low levels of HIV antibodies in a person’s blood, which is why in the past the window period was set at 6 months.
Across Canada, all labs use newer, more sensitive antibody tests (this includes rapid (point-of-care) HIV tests). For more on rapid HIV testing, see A rapid approach to community-based HIV testing. Research shows us that with these new tests, as many as 95% of people who test positive will do so within 34 days of exposure to HIV.10 However, for the remaining 5%, the window period for these HIV antibody tests is generally accepted to be three months so as to ensure people who take longer to develop antibodies are not overlooked. This means that if someone tests negative for HIV antibodies during the window period, they should be re-tested three months after possible exposure, to fully rule out HIV infection.
There are some rare exceptions which may require someone to retest up to six months after exposure:
• People who have a severely impaired immune system may take longer to develop HIV antibodies.
• People who have taken post-exposure prophylaxis (PEP).
Tests that directly detect HIV
There are a number of technologies that detect HIV itself. However, at this time, these tests are not offered uniformly throughout Canada or within provinces and territories. It is important to do some research and find out which testing services are available for your clients to help ensure effective messaging for someone who thinks they have recently been exposed to HIV.
The two most commonly used tests that detect HIV directly are the p24 antigen test and the HIV nucleic acid amplification test (NAT).
...The HIV p24 antigen test, the most widely available of the two, is designed to detect a protein (the p24 protein) associated with HIV. The p24 antigen test can detect the p24 protein on average 10 to 14 days after infection with HIV.2,3,11,12 One drawback of this test is that levels of the p24 protein peak at around three to four weeks after exposure to HIV and are usually not detectable after five to six weeks (and sometimes even earlier).
...Currently, the most advanced tests combine a p24 antigen test and an antibody test. While combination tests are available in some regions across Canada, they are not yet available everywhere. These tests are seen as beneficial because they combine the early detection abilities of the p24 antigen test with the accuracy of the newer antibody tests. It should be noted that a rapid (point-of-care) version of these tests is not yet available.
...The HIV NAT test is a very sensitive test designed to detect HIV RNA in blood. RNA is the viral equivalent to human DNA. The NAT test is able to detect HIV RNA as early as seven to 14 days after infection with HIV.2,3,11,12 Unlike the p24 test, the NAT test will always give a positive result as long as there is HIV in someone’s blood.
NAT testing is currently being offered in six clinics in British Columbia as part of a five-year study called the Acute HIV Infection Study (
http://www.acutehivstudy.com). One of the objectives of this study is to investigate the impact of new testing technologies on gay men’s testing practices.
...In places where p24 antigen tests or HIV NAT tests are available, these tests are often used for individuals who have recently had a high-risk exposure and are either (a) in the three-month window period of the antibody test, or (b) experiencing symptoms of a new HIV infection (most often flu-like symptoms, including a fever, diarrhea, rash and/or sore throat). The p24 antigen test is also used when indeterminate results are obtained from an HIV antibody test (the test couldn’t give a clear answer).
...It is also important to talk about the symptoms of seroconversion when we talk about testing. Educating clients about the symptoms of seroconversion may increase the likelihood that they will get tested if they experience symptoms. Seroconversion symptoms can occur from two to four weeks after infection and may include flu-like symptoms, such as fatigue, fever, sore throat, swollen lymph nodes, headache, loss of appetite or skin rash. This illness usually lasts less than two weeks although it can last as long as 10 weeks. If a client has had a recent high-risk encounter and experiences any of these symptoms, they should be encouraged to have an HIV test. Depending on the time since infection, the antibody test may not give an accurate result. However, if available, the p24 antigen test will be able to give an accurate result two to four weeks following infection and the NAT test will be accurate in as little as seven to 14 days after infection.
...The early detection of HIV is important because people who are newly infected are very infectious and may inadvertently transmit HIV to others. There is still a lot of misunderstanding about how soon one should get tested after potential HIV exposure. Many people still believe they have to wait three months. However, new and improved testing technologies are continually decreasing the amount of time it takes for a new HIV infection to be detected.
In the case of clients at high risk of HIV, testing can be done as early as one month after exposure for standard antibody assays and rapid point-of-care tests. Clients who test positive will know for certain they are HIV-positive. Of those who test negative, 95% are in fact negative. It is important to realize that 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 testing negative at one month are advised to return for repeat testing once the three-month window period is covered.
The p24 antigen test and the HIV NAT test could be used to test for HIV in people who think they were recently exposed to HIV. These tests are useful for people who think they have recently seroconverted. As these tests may not be readily available in all cities and towns across Canada, it may be useful to find out when and where these new and improved testing technologies will be offered in your area.
http://www.catie.ca/en/pif/fall-2010/detecting-hiv-earlier-advances-hiv-testing
