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HIV 4th gen test negative at 6 weeks after PEP

Question: 

Hello,

I took a HIV 4th gen test 43 days post PEP. The test was negative. Do I need to test more or Is this conclusive?

Answer: 

Hello and thank you for contacting AIDS Vancouver Helpline.

We gather that you would like to know if a 4th generation HIV Ag/Ab combo test is considered to be conclusive at 43 days after completion of PEP.

When you take PEP, the window period for any specific HIV test begins when the course of PEP is complete. In British Columbia, a 4th generation HIV Ag/Ab test is recommended at 3 weeks and 6 weeks, and 3 months post completion of PEP. (1)

Tests that detect the p24 antigen generally have shorter window periods than tests that only detect antibodies. This is because most people have detectable levels of p24 antigen by 20 days after infection, which is generally earlier than antibodies can be detected. However, levels of p24 antigen in the blood begin to decrease three to four weeks after infection and are no longer detectable five to six weeks after infection. This is why fourth-generation HIV tests were designed to also detect anti-HIV antibodies, as antibodies generally remain detectable for the entire course of HIV infection. (2)

Fourth-generation HIV tests can detect HIV infection in 50% of people by 18 days after infection; 95% of people by 34 days after infection; and 99% of people by one and a half months after infection. Fourth-generation HIV tests and the INSTI rapid test have a sensitivity of about 99.9%. (2)

Most HIV specialists consider the 4th generation test to be conclusive at 6 weeks (42 days), but some guidelines do recommend re-testing at 3 months.

Recommendation: While you can certainly take comfort in your negative result at 6 weeks post PEP, re-testing at 3 months is recommended for firmly conclusive results.

Regards, AIDS Vancouver Helpline Volunteer, Dyson