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I had unprotected sex. a one time thing. i was given PEP within 72 hours. I took a determine HIV antibody test 1 month after completing PEP and it was non-reactive. I did another determine HIV antibody test 7 weeks after completing pep (11 weeks after exposure) and it was still negative.

Can the negative results change after 12 months post PEP since i have read that PEP delays seroconversion?



Thank you for your inquiry. From what we gather from the question, you were asking about the validity of HIV tests after having PEP(Post Exposure Prophylaxis).

PrEP/PEP: Starting PEP as soon as possible after a potential HIV exposure is important. PEP is effective, but not 100%, so you should continue to use condoms with sex partners. Occurrences of delayed seroconversion have happened in some people taking pre-exposure prophylaxis, in this case with Hepatitis C (1). Currently there is not enough research to make a conclusive statement.

HIV Testing: HIV tests are very accurate. Once confirmatory testing has been performed, the chance of a positive result being false is essentially zero. When outside of the window period, the chance of a negative result being false is very low. The time between when a person may have been exposed to HIV and when a test can tell for sure whether they have HIV is called the window period. The window period varies from person to person and depends on the type of test used to detect HIV.

A Nucleic acid Test (NAT): can usually tell you if you have HIV infection 10 to 33 days after an exposure. Concurrent use of PrEP or PEP may reduce accuracy of results. An Antigen/antibody Test : can usually detect HIV infection 18 to 45 days after an exposure. Antibody tests: can usually take 23 to 90 days to reliably detect HIV infection. Most rapid tests and home tests are antibody tests. Fourth-generation: Antigen test- P24 is detectable immediately after infection, & only for the first few weeks. The antibody (ab) test has a window period of 4-12 weeks. Most HIV specialists consider this Ag/Ab test conclusive at 6 weeks. Official HIV testing guidelines still recommend re-testing at 12 weeks for completely conclusive results. Accuracy is 99.9%

The guidelines recommend testing at six weeks, three months and six months from the day of exposure. Since you already tested negative at 1 month and re tested negative again at almost 3 months. It is less likely for you to have a positive test result.

Recommendation: In concordance of official HIV testing guidelines, please seek re-testing at 12 weeks for "completely conclusive results" , but rest assured that your previous tests are reliable. Please also refer to a physician for other health related questions.


AIDS Vancouver Helpline/Online, (Vardah)