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HIV window periods
Window periods are based on averages or statistics from studies. Different HIV tests have different window periods. As HIV testing technology improves, window periods may become more specific or even get shorter.
HIV tests used in British Columbia
These tests measure antibodies to HIV. Antibodies are proteins made by the immune system when the body is exposed to an infection. It takes time for the body to make antibodies after it is exposed to HIV, and different people make antibodies at different rates.
The window period for antibody tests is between 3 weeks and 3 months. Up to 95% of people will have antibodies after 6 weeks, and 99% of people will have antibodies after 3 months.
The Enzyme Immunoassay Test (EIA) and the point-of-care (or rapid) HIV test are common antibody tests in BC. Immunoblot tests and Western Blot tests are other antibody tests which are used to confirm a preliminary positive result.
The antigen test measures a viral protein called p24 antigen, which shows up in the blood soon after a person gets HIV. Ninety-five percent of these tests will be positive 6 weeks after a person gets HIV.
The 4th generation EIA test is a combined antigen / antibody test. It is now the standard HIV laboratory screening test used in BC.
NAAT (Nucleic Acid Amplification Test)
This test looks for the genetic material of HIV in the blood. It is also known as the "early HIV test" or "RNA test". Ninety percent of NAATs are positive 10 to 12 days after a person gets HIV, and over 99% are positive after 6 weeks.
The RNA NAAT can be specially ordered by doctors or nurses if someone has had a recent high-risk exposure to HIV. As this test is expensive, it is sometimes used on “pools” of samples from specific clinics where HIV is more commonly diagnosed in BC. If the “pooled” samples test positive, then each of the samples are tested individually to find out which one is positive. The pooled NAAT test is only available at certain clinics in BC.
The below graphs show the window periods for the different HIV tests available in BC. The shading shows the earliest time when a test could have a positive result. *Symptoms refers to symptoms of serconversion illness.
What happens at the laboratory?
When blood is sent to the BC Public Health Laboratory, the first test done is the 4th generation combined antibody/antigen test. Depending on results, the lab may continue with further antibody tests or a pooled NAAT.
What does this mean for me?
For most people, the best time to test is 6 weeks after having unprotected (condomless) sex with a new partner. If the test is negative, there is a 95% chance you do not have HIV from that sexual contact.
However, if you have recently had high-risk unprotected sex, and at least 10 days have passed since you had sex, an HIV NAAT is a could be done if it’s available in your area. If this test is negative, there is about a 90% chance that you do not have HIV from that sexual contact.
With any HIV test, you should test again at 3 months to be sure.
Because HIV and other sexually transmitted infection (STI) rates are high in gay, bisexual and other men who have sex with men, routine screening every 3 months is recommended.