When should lab tests be conducted after a potential HIV exposure?

HIV Testing Timeline After Potential Exposure

For HIV exposures, perform baseline testing immediately, then follow-up HIV antibody testing at 6 weeks, 3 months, and 6 months post-exposure, with the most recent CDC guidelines (2025) now recommending both antigen/antibody combination testing plus diagnostic nucleic acid testing (NAT) at 4-6 weeks and 12 weeks for optimal detection. 1

Immediate Baseline Testing (Day 0)

• Test the exposed person immediately using a rapid (point-of-care) HIV test AND/OR laboratory-based antigen/antibody (Ag/Ab) combination test to exclude pre-existing HIV infection before initiating post-exposure prophylaxis (PEP). 1

• Test the source patient for HIV antibody (consider rapid testing to expedite results within 24-48 hours) to guide PEP decisions. 1

• Add diagnostic HIV NAT to baseline testing if the exposed person has received long-acting injectable PrEP (cabotegravir) within the past 12 months, as antiretrovirals can suppress viral load and delay seroconversion. 1

• Obtain baseline renal and liver function tests before starting PEP to ensure safe medication selection. 1

Interim Testing (4-6 Weeks Post-Exposure)

• Perform both laboratory-based HIV Ag/Ab test PLUS diagnostic HIV NAT at 4-6 weeks after exposure (within 2 weeks of completing the 28-day PEP course). 1

• This dual testing approach is critical because ARVs taken as PEP can suppress HIV viral load, delay seroconversion, and decrease the ability to detect HIV infection with antibody tests alone. 1

• This testing may be deferred only for persons who: (1) started PEP within 24 hours of exposure, (2) completed the full 28-day course without missing doses, and (3) are not considering starting PrEP. 1

• The 4-6 week visit is also the optimal time to assess for PrEP indications and initiate PrEP if ongoing risk exists. 1

Final Testing (12 Weeks Post-Exposure)

• Perform final HIV testing using both laboratory-based HIV Ag/Ab combination immunoassay AND diagnostic HIV NAT at 12 weeks (3 months) after exposure. 1

• This represents updated guidance from the 2025 CDC recommendations, which now emphasizes dual testing at this timepoint rather than relying solely on antibody testing. 1

Extended Follow-Up (6 Months Post-Exposure)

• The older 2001 guidelines recommended HIV antibody testing for at least 6 months post-exposure (baseline, 6 weeks, 3 months, and 6 months). 1

• Current practice has shifted toward the 12-week endpoint with more sensitive dual testing (Ag/Ab plus NAT), though some experts may extend follow-up to 6 months in specific circumstances. 1

Additional Testing Considerations

For Acute Illness

• Perform immediate HIV testing if any illness compatible with acute retroviral syndrome occurs during the follow-up period (fever, rash, lymphadenopathy, pharyngitis). 1

For HBV Co-Exposure

• Test for anti-HBs 1-2 months after the last dose of hepatitis B vaccine if vaccination was given as part of post-exposure management. 1
• Note that anti-HBs response cannot be accurately assessed if HBIG was received in the previous 3-4 months. 1

For HCV Co-Exposure

• Perform baseline and follow-up testing for anti-HCV and ALT at 4-6 months after exposure. 1
• Consider HCV RNA testing at 4-6 weeks if earlier diagnosis is desired. 1

Critical Testing Pitfalls to Avoid

• Do not use oral fluid-based rapid HIV tests in the PEP context—they are less sensitive for detecting acute or recent infection than blood tests. 1

• Do not delay PEP initiation while waiting for source patient testing results; PEP should be started immediately and can be discontinued if the source is later confirmed HIV-negative. 1

• Do not test discarded needles or syringes for virus contamination—this is not recommended for exposure assessment. 1

• Do not rely on antibody testing alone at follow-up visits, as ARV exposure can suppress antibody development and create false-negative results. 1

• Evaluate exposed persons taking PEP within 72 hours after exposure and monitor for drug toxicity for at least 2 weeks. 1

Testing Algorithm Summary

The testing schedule prioritizes early detection while accounting for the window period and the impact of antiretroviral medications on test sensitivity. The shift from the older 6-month antibody-only approach to the current dual-testing strategy at 4-6 weeks and 12 weeks reflects improved understanding of how PEP affects HIV detection and the availability of more sensitive testing modalities. 1