HIV Testing at 90 Days Post-Exposure
HIV testing performed at 90 days (approximately 12-13 weeks) post-exposure is highly accurate and should definitively rule out HIV infection, even in patients who have taken PEP or PrEP, provided both laboratory-based antigen/antibody testing and nucleic acid testing (NAT) are performed together. 1
Recommended Testing Approach at 90 Days
The CDC 2025 guidelines explicitly recommend final follow-up testing at 12 weeks after PEP initiation (which equals approximately 8 weeks after completing the 28-day PEP course) to definitively rule out HIV infection. 1 This timing accounts for:
- Antiretroviral washout period: ARVs taken as PEP or PrEP can suppress HIV viral load and delay seroconversion, but by 12 weeks post-exposure, these effects have resolved 1, 2
- Test window periods: Fourth-generation antigen/antibody tests can detect HIV at a median of 18 days post-infection, with a 99th percentile window period of 44 days 2, 3, 4
- Combined testing sensitivity: Using both laboratory-based Ag/Ab testing AND diagnostic NAT together maximizes detection capability 1, 5
Why Both Tests Are Critical at This Timepoint
You must perform BOTH tests simultaneously at 90 days:
- Laboratory-based Ag/Ab test: Detects both HIV antigen (p24) and antibodies, with fourth-generation tests having a median window period of 18 days and probability of false-negative results dropping to 0.01 by 42 days post-exposure 3, 2
- Diagnostic NAT (nucleic acid test): Detects HIV RNA approximately 10-14 days post-infection, significantly earlier than antibody tests, and can detect acute infection approximately 1 week before Ag/Ab tests 1, 5
The dual testing approach is specifically recommended because ARVs can suppress viral load and delay antibody formation, making either test alone potentially unreliable in the PEP/PrEP context. 1, 2
Special Considerations for PEP/PrEP Users
Critical caveat: The 12-week (90-day) timepoint is specifically chosen based on data about ARV washout timelines. 1 If you test earlier:
- At 4-6 weeks post-PEP initiation, a negative result does NOT rule out HIV because ARVs may still be suppressing viral detection for more than 2 weeks after stopping medications 1, 2
- Testing can be deferred at the 4-6 week mark ONLY if the patient started PEP within 24 hours of exposure and completed the full 28-day course with good adherence 1
What If Only Ag/Ab Testing Is Available?
If NAT is unavailable (though strongly discouraged), laboratory-based Ag/Ab testing alone at 12 weeks post-exposure should detect the vast majority of infections when accounting for antiretroviral washout. 2 However, this creates a diagnostic gap where highly infectious individuals with acute infection may go undetected, which is problematic since persons with acute HIV are highly infectious. 5, 2
The inability to provide NAT should not prevent PEP provision, but healthcare professionals should use the most sensitive accessible HIV test available. 1
Comparison to Older Guidelines
Older 2001 guidelines recommended HIV antibody testing for at least 6 months post-exposure (at baseline, 6 weeks, 3 months, and 6 months). 1 However, these recommendations predate fourth-generation Ag/Ab tests and routine NAT availability. The current 2025 CDC guidelines have shortened this to 12 weeks based on improved test sensitivity and understanding of ARV effects on detection. 1
Bottom Line for Clinical Practice
At 90 days post-exposure with combined laboratory Ag/Ab and NAT testing, you can confidently rule out HIV infection and reassure the patient, even if they completed a full PEP course. 1 This represents the definitive testing timepoint that accounts for all known variables including antiretroviral suppression effects and test window periods. 1, 2