What is the recommended follow-up HIV testing schedule for an individual who has completed a 28-day course of post-exposure prophylaxis (PEP) following a needle stick injury to confirm whether the prophylaxis was successful in preventing infection?

HIV Testing Schedule After Completing 28-Day Post-Exposure Prophylaxis

After completing a 28-day course of post-exposure prophylaxis following a needle stick injury, HIV testing should be performed at 4-6 weeks and definitively at 12 weeks post-exposure using both laboratory-based antigen/antibody testing and nucleic acid testing (NAT) to confirm the absence of HIV infection. 1, 2

Recommended Testing Timeline

The most recent CDC guidelines (2025) establish a clear testing schedule for individuals who have completed PEP:

Baseline Testing

• Perform laboratory-based HIV antigen/antibody (Ag/Ab) testing immediately before initiating PEP to establish HIV-negative status 1, 2
• If a rapid point-of-care test is used initially, obtain a laboratory-based Ag/Ab test simultaneously to increase diagnostic sensitivity 2

First Follow-Up: 4-6 Weeks Post-Exposure

• Perform laboratory-based Ag/Ab testing PLUS diagnostic NAT at 4-6 weeks after the exposure 1, 2
• This dual testing approach is critical because antiretroviral medications from PEP can suppress viral load and delay antibody formation, potentially masking early infection 2, 3
• A negative test at this timepoint does not definitively rule out HIV infection due to ongoing antiretroviral suppression effects 2

Final Testing: 12 Weeks Post-Exposure

• Perform both laboratory-based Ag/Ab testing AND NAT at 12 weeks post-exposure for definitive confirmation 1, 2
• The CDC considers fourth-generation HIV tests conclusive after 12 weeks of potential exposure when both Ag/Ab and NAT are performed 2
• This 12-week timepoint accounts for antiretroviral medication washout and the window period for HIV tests 2

Critical Considerations

Why Both Ag/Ab and NAT Are Essential

• NAT can detect HIV infection 10-14 days after exposure, significantly earlier than Ag/Ab tests (18-45 days) 3
• Fourth-generation Ag/Ab combination tests alone may miss acute infections, especially when antiretrovirals have been used 3
• Without NAT, there is a diagnostic gap where highly infectious individuals may go undetected 2

Impact of PEP on Testing

• Antiretroviral medications can suppress viral load, delay antibody formation, and reduce the ability to detect HIV infection 2, 3
• This suppression effect can persist for longer than 2 weeks after stopping PEP medications 2
• This is why the 12-week timepoint is critical—it allows sufficient time for medication washout 2

Special Circumstances Requiring Extended Follow-Up

• Individuals with impaired immune response or hepatitis C virus co-infection in the source patient may require extended follow-up beyond 12 weeks, though this is not routinely recommended with modern combined Ag/Ab and NAT testing 2

Common Pitfalls to Avoid

Do Not Rely on Rapid Tests Alone

• Rapid HIV tests based on oral fluids are NOT recommended for post-exposure prophylaxis follow-up because they are less sensitive for detecting acute or recent infection than blood tests 2
• Always use laboratory-based testing for follow-up 1, 2

Do Not Skip the NAT Component

• The CDC strongly recommends including diagnostic NAT at both 4-6 weeks and 12 weeks for optimal sensitivity 2
• If NAT is unavailable, laboratory-based Ag/Ab testing alone at 12 weeks should detect the vast majority of infections when accounting for antiretroviral washout, but this represents suboptimal care 2

Do Not Stop Follow-Up at 4-6 Weeks

• Even if testing at 4-6 weeks is negative, the 12-week testing is mandatory for definitive confirmation 1, 2
• The earlier timepoint helps identify potential infections but cannot rule them out 2

Clinical Follow-Up Structure

Beyond laboratory testing, the 2025 CDC guidelines recommend:

• A visit at 24 hours (remote or in-person) with a medical provider after starting PEP 1
• Clinical follow-up visits at 4-6 weeks and 12 weeks after exposure for laboratory testing 1
• Counseling about ongoing risk reduction and consideration of pre-exposure prophylaxis (PrEP) if the individual has continuing HIV exposure risk 1