HIV Post-Exposure Prophylaxis (PEP) has been established as a cornerstone of HIV prevention in the last 20 years. With newer, well-tolerated medications and better access to medical care, PEP has shown itself to be a safe and effective weapon in our arsenal against HIV infection.

Read: A Guide to HIV PrEP & HIV PEP (Pills for HIV Prevention)

For those individuals who have been in the unfortunate position of needing PEP, the next question is invariable:

When can I test for HIV after I finish PEP?

There are very few studies that have examined the “earliest time” you can do an HIV test after taking PEP and have conclusive results. There are some theories that taking PEP can increase the window period for testing by delaying the appearance of HIV antigen or antibodies in the blood – these have neither been proven or debunked, although evidence is weighted towards the latter.
Current data would suggest that p24 antigen and antibody response is not significantly affected by these antiviral medications, but these studies have mainly been done on individuals taking Pre-Exposure Prophylaxis (PrEP). As the medications used for PrEP and PEP are essentially the same, we can extrapolate this data to post-PEP testing as well. Confounding factors to these studies include patient adherence to PrEP/PEP regime as well as any instances of ongoing high-risk exposures during the course of medication and/or after.

Immediately After Completing PEP

Both the US CDC 2016 guidelines and joint WHO/ILO 2005 guidelines for PEP recommend HIV testing at baseline before starting medication and immediately after completing PEP (i.e. 4 weeks post-exposure as per regular non-PEP users), as well as 3-6 months post-exposure.
Importantly, the US CDC guidelines also states that patients keen to start on HIV PrEP after their HIV PEP can do so immediately following the completion of 28-day PEP, and provided the patient has a negative 4th generation HIV Antigen/Antibody test: “Because no evidence exists that prophylactic antiretroviral use delays seroconversion and PEP is highly effective when taken as prescribed, a gap is unnecessary between ending PEP and beginning PrEP.” Again, this suggests strongly that testing immediately after completing PEP is sufficiently conclusive to exclude an HIV infection.
Some studies done in animals have shown HIV DNA/RNA PCR testing during HIV PEP can sometimes even show a positive HIV viral load, with follow up testing after completing the medication showing a negative result. This could represent a successfully aborted infection, which is exactly the point of PEP in the first place.
We do not usually recommend HIV DNA/RNA PCR testing during or after completion of HIV PEP, as we know that the medication can and will suppress the viral load the same way it does in chronic HIV infection – this means that even a negative PCR viral load test may be a false negative, with viral load increasing again in a true infection a few months after the medication has been stopped.

Read: HIV Window Period (Timelines for Accurate HIV Test)

Personally, I have not encountered any patients who have taken PEP and tested negative at 28 days to subsequently have their results change to HIV positive in follow-up testing, except in a single case where the patient had ongoing high-risk exposures during and after their PEP medication.

Read: How to Get An Anonymous HIV Test in Singapore

In summary, post-PEP testing can be performed immediately following the completion of medication (i.e. 28 days or more post-exposure) with a 4th generation Antigen/Antibody test with good certainty, although guidelines will still recommend one last test at 3 months or more post-exposure as a precaution.
Current clinical experience indicates that the test at the end of PEP is essentially conclusive and sufficient to ensure that PEP is successful and HIV infection has been prevented, but higher-powered studies are required to confirm this.
It is still best to speak to the doctor who prescribed you the PEP or who will be following up with your care, as they will be most familiar with local testing methods and guidelines.
If you believe you have had a potential high-risk exposure within the last 72 hours, you may consider Post-Exposure Prophylaxis (PEP). Please contact us for a consultation if you think you need PEP.
If you wish to speak to any of our doctors in regards to HIV PEP or HIV Testing, you can visit any of our clinics or drop us an email at hello@dtapclinic.com.sg for an appointment.
Take Care!

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