After HIV exposure, what is the earliest time each type of HIV test (HIV‑RNA polymerase chain reaction, fourth‑generation antigen/antibody assay, third‑generation antibody test) can become positive?

HIV Test Detection Timeline After Exposure

HIV-RNA nucleic acid testing (NAT) can detect infection earliest at 10-14 days post-exposure, fourth-generation antigen/antibody tests become positive at 18-24 days, and third-generation antibody-only tests require 22-25 days for detection. 1

Test-Specific Window Periods

HIV-RNA NAT (Earliest Detection)

• Detects HIV approximately 10-14 days after exposure 1
• Can identify acute infection approximately 1 week before laboratory-based antigen/antibody tests 1
• The eclipse period (time from exposure to any detectable virus) averages 8-10 days 2
• Diagnostic NAT is the most sensitive early detection method and should be prioritized when recent exposure is suspected 1

Fourth-Generation Antigen/Antibody Tests

• Become positive 18-24 days post-exposure 1
• Detect both HIV p24 antigen (appears 14-22 days post-infection) and HIV antibodies 3
• Laboratory-based fourth-generation tests are more sensitive than rapid point-of-care versions 1, 4
• Can reduce the diagnostic window to 11-14 days in optimal conditions 5

Third-Generation Antibody-Only Tests

• Require 22-25 days for detection 1
• Detect IgM and IgG antibodies, which develop approximately 3-4 weeks post-infection 3
• Less sensitive for acute infection compared to fourth-generation or NAT testing 6
• At least 95% of infected individuals will have detectable antibodies within 6 months 4

Critical Clinical Caveats

The "Second Diagnostic Window" Phenomenon

• Fourth-generation tests can initially turn positive due to p24 antigen detection, then become temporarily negative when antigen levels decline before antibodies fully develop 7
• This creates a second window period where neither antigen nor antibody is detectable 7
• Repeat testing is essential if initial fourth-generation test is positive but subsequent tests are negative 7

Antiretroviral Medication Effects

• PrEP or PEP can suppress viral load, delay seroconversion, and decrease test sensitivity 1, 4
• When recent antiretroviral exposure exists (oral PrEP or cabotegravir injection within past year), baseline testing must include NAT in addition to antigen/antibody testing 1, 4
• A negative test does NOT rule out HIV in persons taking antiretrovirals 1

Oral Fluid Rapid Tests

• Oral fluid-based rapid HIV tests are NOT recommended for post-exposure testing because they are significantly less sensitive for acute or recent infection than blood-based tests 1, 3
• Blood-based rapid tests detected only 22-33% of early infections compared to 76-88% for fourth-generation laboratory tests 6

Recommended Testing Algorithm After Known Exposure

For Persons NOT on Post-Exposure Prophylaxis

• Baseline: Use laboratory-based fourth-generation Ag/Ab test; add NAT if high suspicion for acute infection 1
• 6 weeks post-exposure: Repeat testing 4
• 3 months (12 weeks) post-exposure: Confirmatory testing 4
• 6 months post-exposure: Final testing to definitively rule out infection (rare delayed seroconversion can occur) 4

For Persons ON Post-Exposure Prophylaxis

• Baseline: Laboratory-based Ag/Ab test PLUS diagnostic NAT (mandatory due to potential viral suppression) 1, 4
• 4-6 weeks after PEP initiation: Both laboratory-based Ag/Ab test AND diagnostic NAT required 1, 4
• 12 weeks after PEP initiation: Final testing with both laboratory-based Ag/Ab test AND diagnostic NAT to definitively rule out infection 1, 4
• Testing at 4-6 weeks can be deferred only if PEP started within 24 hours of exposure, full course completed, and not starting PrEP 8

If Acute Retroviral Syndrome Suspected

• Test immediately with HIV-RNA NAT regardless of timeline if symptoms develop (fever, rash, lymphadenopathy) 3
• Do not wait for scheduled testing intervals when clinical suspicion is high 1