What is the window period for the Insti (HIV home test) to detect HIV antibodies and p24 antigens after potential exposure?

INSTI HIV Home Test Window Period

The INSTI HIV test is an antibody-only rapid test that cannot reliably detect HIV infection until approximately 3-4 weeks after exposure, and a negative result does not exclude HIV infection within at least 5 weeks of potential exposure. 1

Test Classification and Detection Capabilities

The INSTI test is a third-generation antibody-only rapid test that detects HIV IgM and IgG antibodies but does not detect p24 antigen or HIV RNA. 2 This is a critical limitation compared to fourth-generation laboratory tests.

• Third-generation antibody tests become positive 7-14 days after detectable virus by nucleic acid testing (NAT), which translates to approximately 3-4 weeks post-infection 2
• The CDC explicitly states that oral fluid-based rapid HIV tests are not recommended for HIV screening in the context of post-exposure prophylaxis (PEP) services because they are less sensitive for detection of acute or recent infection than blood tests 2

Window Period Timeline

Critical Detection Gaps:

• Days 0-10 to 14: Absolute window period where no markers are detectable by any test 2
• Days 10-14 to 21-28: HIV RNA becomes detectable first, followed by p24 antigen (14-22 days post-infection), but antibody tests like INSTI remain negative 2
• Days 21-28 onwards: Antibodies begin to appear, with third-generation tests (like INSTI) becoming positive 7-14 days after virus detection 2
• 4-6 weeks: The CDC notes that the body typically produces detectable HIV antibodies within 4-6 weeks after infection 3

Real-World Performance Data:

A study specifically evaluating the INSTI test during acute HIV infection found that:

• Only 62.5% of patients with acute HIV infection had reactive INSTI tests 1
• The probability of HIV detection was only 85% at 5 weeks from HIV transmission 1
• Patients with non-reactive tests had significantly shorter time since infection and higher HIV-RNA levels 1

Clinical Implications and Recommendations

For Post-Exposure Testing:

Do not rely on INSTI or any antibody-only rapid test for early post-exposure HIV screening. Instead:

• Use a laboratory-based fourth-generation antigen/antibody combination test at baseline, which can detect infection 4-7 days after detectable virus (approximately 2-3 weeks post-exposure) 2
• If using a rapid point-of-care test initially, also obtain a laboratory-based Ag/Ab test to increase sensitivity 2
• Consider adding HIV NAT (nucleic acid testing) for baseline testing, especially if recent antiretroviral exposure 2

Follow-Up Testing Schedule:

For individuals with potential HIV exposure, the CDC 2025 guidelines recommend:

• 4-6 weeks post-exposure: Laboratory-based Ag/Ab test plus diagnostic NAT 2
• 12 weeks post-exposure: Final testing with laboratory-based Ag/Ab test plus diagnostic NAT to definitively rule out infection 2
• Traditional antibody-only tests require testing at 6 months for definitive exclusion 3, 4

Common Pitfalls to Avoid

Critical Errors:

• Never use INSTI or similar antibody-only rapid tests to rule out HIV infection within 5 weeks of exposure 1
• Do not assume a negative INSTI test excludes HIV infection during the acute phase - over 37% of acute infections were missed in one study 1
• Avoid oral fluid-based rapid tests entirely in post-exposure settings due to inferior sensitivity 2

Additional Considerations:

• Recent antiretroviral use (PEP or PrEP) can suppress viral load and delay seroconversion, making antibody tests even less reliable 2
• Fourth-generation tests can have a "second diagnostic window" when p24 antigen declines before antibodies appear, though this is rare 5
• If acute HIV infection is suspected clinically (fever, rash, lymphadenopathy), proceed directly to HIV RNA testing regardless of antibody test results 2

Bottom Line for Clinical Practice

The INSTI test has a window period of at least 3-4 weeks, with only 85% sensitivity even at 5 weeks post-exposure. For any patient with potential HIV exposure in the preceding 5 weeks, a negative INSTI result is insufficient to exclude infection and requires follow-up with fourth-generation laboratory testing or NAT. 1, 2