What is the HIV window period for accurate testing after exposure?

HIV Window Period for Accurate Testing After Exposure

The HIV window period has been significantly shortened with modern testing: laboratory-based antigen/antibody (Ag/Ab) combination tests can detect HIV as early as 18-45 days post-exposure, while nucleic acid tests (NAT) can detect infection approximately 10-14 days after exposure. 1

Modern Testing Timeline and Window Periods

Baseline Testing (Immediately After Exposure)

• Rapid or laboratory-based HIV Ag/Ab combination test should be performed at initial evaluation to establish baseline HIV status before initiating post-exposure prophylaxis (PEP). 1
• Do not delay PEP initiation while awaiting test results—start treatment immediately based on exposure history. 1, 2
• For persons with long-acting injectable PrEP exposure in the past 12 months, add an HIV NAT at baseline because antiretrovirals can suppress viral load and delay antibody development. 1

Follow-Up Testing Schedule

At 4-6 weeks post-exposure:

• Perform both laboratory-based HIV Ag/Ab test PLUS diagnostic HIV NAT. 1
• This interim testing may be deferred ONLY if PEP was started within 24 hours of exposure, the full 28-day course was completed without missed doses, and the person is not transitioning to PrEP. 1
• Testing at this timepoint can identify HIV infection in persons who did not adhere to PEP or who had subsequent exposures. 1

At 12 weeks (3 months) post-exposure:

• Final HIV testing using both laboratory-based Ag/Ab combination immunoassay AND diagnostic HIV NAT is required to definitively rule out HIV infection. 1
• This 12-week timepoint accounts for antiretroviral washout periods and the window period of HIV tests. 1
• Most laboratory-based Ag/Ab tests should detect HIV acquisition from the original exposure by this time. 1

Older Guidelines vs. Current Recommendations

The 2001 occupational exposure guidelines recommended HIV antibody testing for at least 6 months post-exposure (at baseline, 6 weeks, 3 months, and 6 months). 1 However, the 2025 CDC guidelines have shortened this to 12 weeks when using modern Ag/Ab tests combined with NAT, reflecting improved test sensitivity. 1

Test Sensitivity and Detection Windows

Diagnostic Test Hierarchy (Most to Least Sensitive)

1. HIV NAT (nucleic acid test): Detects HIV RNA approximately 10-14 days post-infection, about 1 week before Ag/Ab tests. 1
2. Fourth-generation Ag/Ab combination assays: Detect HIV p24 antigen and antibodies, typically positive 18-45 days post-infection. 1
3. Third-generation antibody-only tests: Longer window period of 3-4 weeks. 3
4. Rapid antibody tests: May miss early infections; oral fluid tests are less sensitive than blood-based tests. 1, 3

Critical Caveat: Antiretroviral Interference

Antiretrovirals taken as PEP or PrEP can suppress HIV viral load, delay seroconversion, and decrease the ability to detect HIV infection. 1, 4

• PEP/PrEP can result in false-negative HIV NAT results during treatment. 4
• Starting antiretrovirals during acute HIV infection may result in delayed or complete absence of HIV-specific antibody development. 4
• This is why both Ag/Ab testing AND NAT are required at 12 weeks—to account for potential suppression effects. 1

Common Pitfalls to Avoid

Never rely on rapid antibody tests alone in high-risk populations or during the window period—they miss 20% of HIV infections compared to combined antibody/NAT testing in men who have sex with men. 3

Never use only antibody testing for persons who recently took antiretrovirals—always include NAT because ARVs can delay or prevent antibody development. 1, 4

Never stop follow-up testing before 12 weeks post-exposure when modern Ag/Ab and NAT testing is used, as this is the definitive timepoint to rule out infection. 1

Perform immediate HIV testing if any illness compatible with acute retroviral syndrome occurs during follow-up (fever, rash, lymphadenopathy, pharyngitis)—this may indicate acute HIV infection. 1

Testing Strategy Summary

For accurate HIV diagnosis after exposure:

• Use laboratory-based Ag/Ab combination tests, not rapid antibody-only tests, whenever possible. 1
• Always include diagnostic HIV NAT in follow-up testing at 4-6 weeks and 12 weeks. 1
• The 12-week timepoint with combined Ag/Ab and NAT testing is the definitive window to rule out HIV infection in the modern testing era. 1
• Diagnostic NATs are preferred over viral load assays because they detect lower levels of HIV. 1