Thank you for your inquiry. From what we gather from the question, you were asking about the validity of HIV testing while taking post-exposure prophylaxis (PEP).
PEP, if accessed within 72 hours of an exposure and used consistently and correctly for the 28-day prescription, is an effective way to prevent an HIV transmission (1). In regards to testing, 3rd generation HIV tests that only detect antibodies (Ab) to the HIV virus, like the HIV 1/2 test, have a window period that is 3 months post-exposure. The window period for a 4th generation test, like the HIV combo test, that detects antibodies (Ab) to the HIV virus and the HIV virus itself (Ab) is 6 weeks post-exposure, although some guidelines and physicians recommend re-testing 3 months post-exposure for completely conclusive results.
Testing during a PEP prescription or immediately after PEP completion can be useful if the PEP was not successful early on at preventing the HIV virus from establishing an infection. With this in mind, an HIV test cannot be considered conclusive as these times because the drugs that PEP consists of may have suppressed the replication of the virus. Thus, even if the HIV virus infected a few cells before you started PEP, the drugs would have kept the amount of viral particles, as well as the production of antibodies, at such a low level that it cannot be detected by HIV tests.
The window period of an HIV test "resets" if you access PEP. The new window period after accessing PEP becomes 3 months post-PEP completion for 3rd generation tests and 6 weeks post-PEP completion for 4th generation tests. Completely conclusive results are obtained 3 months post-PEP completion when using either a 3rd or 4th generation HIV test.
Recommendation: Refer to a health care provider for HIV testing outside of the window period for completely conclusive results.
AIDS Vancouver Helpline/Online, Marie
If you are satisfied with the Helpline service, please consider Donating.
1) Post-Exposure Prophylaxis