What is the window for Human Immunodeficiency Virus (HIV) detection after potential exposure?

HIV Detection Window Period

The window period for HIV detection depends on the test type: diagnostic nucleic acid tests (NAT) can detect HIV approximately 10-14 days after exposure, fourth-generation antigen/antibody (Ag/Ab) tests detect HIV at 18-24 days, and third-generation antibody-only tests require 22-25 days. 1, 2, 3

Test-Specific Window Periods

Nucleic Acid Testing (NAT/RNA Tests)

• Diagnostic NAT can detect acute HIV infection approximately 1 week before laboratory-based Ag/Ab tests, making it the most sensitive early detection method 1
• The eclipse period (exposure to first RNA detection) is approximately 10 days 2, 3
• NAT is particularly important when recent antiretroviral exposure may suppress antibody response 1

Fourth-Generation Ag/Ab Tests

• Median window period is 18 days (range 16-24 days) after exposure 3
• These tests detect both p24 antigen and HIV antibodies, reducing the window to 11-14 days in optimal conditions 4
• The 99th percentile window period is approximately 44 days, meaning virtually all infections are detectable by this time 2

Third-Generation Antibody Tests

• Median window period is 22 days (range 19-25 days) 3
• The 99th percentile window period extends to 80 days post-exposure 3
• These older tests are less sensitive for acute infection 4

Clinical Testing Algorithm After Exposure

Baseline Testing (Immediate)

• Use rapid point-of-care or laboratory-based Ag/Ab test at initial encounter 1
• Add laboratory-based Ag/Ab test if rapid test used initially to increase sensitivity 1
• Consider adding HIV NAT for baseline testing if the person recently took oral antiretrovirals or received cabotegravir injection within the past year 1

Follow-Up Testing Timeline (Without PEP)

For individuals NOT on post-exposure prophylaxis:

• Test at 6 weeks, 3 months, and 6 months post-exposure using traditional antibody tests 1, 5
• Test immediately if acute retroviral syndrome symptoms develop, regardless of timeline 5

Follow-Up Testing Timeline (With PEP)

For individuals receiving post-exposure prophylaxis, the timeline differs because antiretrovirals can suppress viral load and delay seroconversion:

• First follow-up: 4-6 weeks after PEP initiation (within 2 weeks of completing 28-day course) 1

  • Use both laboratory-based Ag/Ab test AND diagnostic NAT 1
  • A negative test does NOT rule out HIV because PEP medications may suppress detection 1

• Final follow-up: 12 weeks after PEP initiation (8 weeks after PEP completion) 1

  • Use both laboratory-based Ag/Ab test AND diagnostic NAT 1
  • This timing accounts for antiretroviral washout and test window periods 1
  • This is the definitive test to rule out HIV infection from the exposure 1

Important Clinical Caveats

Antiretroviral Impact on Testing

• ARVs taken as PEP or PrEP can suppress HIV viral load, delay seroconversion, and decrease ability to detect HIV infection 1
• This is why both Ag/Ab testing AND NAT are required for follow-up in PEP recipients 1

Test Selection Pitfalls

• Oral fluid-based rapid HIV tests are NOT recommended in the PEP context because they are less sensitive for acute or recent infection than blood tests 1
• If recommended tests unavailable, use the most sensitive accessible HIV test 1

Extended Follow-Up Scenarios

• 12-month extended follow-up is recommended for healthcare workers who become infected with HCV following exposure to a source coinfected with HIV and HCV 5

Probability of False-Negative Results

• For fourth-generation tests: probability of false-negative is 0.01 (1%) at 42 days post-exposure 3
• For third-generation tests: probability of false-negative is 0.01 (1%) at 80 days post-exposure 3