HIV Test Window Period Interpreter
HIV Test Window Period Interpreter
Work out what a negative HIV test does and does not exclude, from the days since exposure, the assay generation, and whether PEP or PrEP was taken.
HIV window period
Exposure + assay → what a negative excludes21 days since the exposure, fourth-generation laboratory antigen/antibody test, no PEP or PrEP taken
Window periods
- Fourth generation
- antigen/antibody laboratory immunoassay — detects p24 antigen and antibody; CDC 18–45 days, BHIVA window 45 days
- Third generation
- antibody-only laboratory immunoassay; BHIVA window 60 days
- Point-of-care
- any rapid or self-test; BHIVA window 90 days regardless of generation
- Nucleic acid test
- CDC 10–33 days; not recommended by BHIVA for routine diagnosis
- PEP or PrEP
- test a minimum of 45 days after completing the course — 73 days after exposure for a completed 28-day PEP course
Worked example
21 days since the exposure, fourth-generation laboratory antigen/antibody test, no PEP or PrEP taken
No PEP or PrEP, so the assay's own window applies
Fourth-generation window is 18 to 45 days (CDC) / 45 days (BHIVA)
21 days is past 18, so the test can detect infection — but is short of 45
Cumulative false-negative probability at 21 days is above the 5% seen at 34 days → repeat at 45 days or later
Window periods by assay
| Assay | CDC: usually detects | BHIVA: window to apply clinically |
|---|---|---|
| Nucleic acid test (RNA) | 10 – 33 days | Not recommended for routine diagnosis |
| Fourth-generation laboratory antigen/antibody | 18 – 45 days | 45 days |
| Third-generation laboratory antibody-only | 23 – 90 days (antibody tests) | 60 days |
| Rapid antigen/antibody point-of-care (finger-prick) | 18 – 90 days | 90 days |
| Antibody point-of-care or self-test | 23 – 90 days | 90 days |
Residual false-negative risk behind the recommended windows
| Assay | 5% false-negative by | 1% by | 0% by | Recommended window |
|---|---|---|---|---|
| Fourth generation | 34 days | 42 days | 50 days | 45 days |
| Third generation | 40 days | 85 days | 99 days | 60 days |
A negative test inside the window is not a negative result
Every HIV assay has a period after infection during which it reads negative in an infected person. That period has shortened considerably — a fourth-generation antigen/antibody immunoassay detects the p24 antigen before any antibody appears, which is why its window is 45 days rather than the three months that older guidance quoted — and stale figures are the commonest error on this subject. The numbers worth holding are BHIVA’s: 45 days for a fourth-generation laboratory test, 60 days for a third-generation antibody-only test, and 90 days for any point-of-care or self-test, whichever generation the device claims.
The sentence that matters more than any of them is this: a negative test taken inside the window is not a negative result. It is an uninformative result. The underlying data make the gradient visible — for fourth-generation tests the cumulative probability of a false negative was 5% at 34 days, 1% at 42 days and 0% at 50 days — so a test at three weeks is meaningfully reassuring and meaningfully inconclusive at the same time, and the correct action is to repeat it rather than to file it. The recommendation, not the calculator, is what settles the question: repeat at the window period for the assay used, and involve a sexual health or infectious diseases service if the exposure was high-risk or the history is unclear.
Antiretrovirals change everything on this page. BHIVA’s testing guideline states plainly that “post-exposure prophylaxis, PrEP and early ART initiation in acute infection can blunt the HIV antibody response yielding non-reactive, atypical or non-progressive HIV serology”. Someone who has taken PEP or is on PrEP can therefore be infected and serologically negative well past the ordinary window, and the window has to be counted from the end of the drug rather than from the exposure. The PEP guideline does the arithmetic: test “at a minimum of 45 days after completion of the PEP course”, which for a completed 28-day course is 73 days after the exposure. For sexual exposures, testing at twelve weeks conveniently aligns with syphilis serology. Anyone with atypical serology while on or shortly after PrEP should be retested four and eight weeks after stopping it and discussed with a specialist — this is exactly the situation in which HIV RNA testing earns its place, and BHIVA names it as such while advising against molecular assays for routine diagnosis.
Two final cautions. The window is counted from the exposure being asked about, so repeated exposures reset the clock and a negative test only ever excludes infection acquired before the window it covers. And a reactive result from a point-of-care device or a self-test is a screening result: it needs laboratory confirmation on a venous sample before anything is said to the patient about a diagnosis.
Frequently asked questions
How long is the window period for a fourth-generation HIV test?
BHIVA applies a 45-day window; CDC says an antigen/antibody laboratory test “can usually detect HIV 18 to 45 days after exposure”. A negative fourth-generation test excludes infection acquired more than 45 days before the test.
Does a negative test inside the window mean I do not have HIV?
No. A negative test inside the window period is uninformative rather than negative. For fourth-generation tests the cumulative false-negative probability was still 5% at 34 days and 1% at 42 days, so the test must be repeated at the window period for the assay used.
Do PEP and PrEP change the window period?
Yes, and substantially. Antiretrovirals “can blunt the HIV antibody response yielding non-reactive, atypical or non-progressive HIV serology”. BHIVA’s PEP guideline advises testing a minimum of 45 days after completing the course — 73 days after the exposure for a completed 28-day course. Atypical serology on PrEP needs repeat testing 4 and 8 weeks after stopping and specialist advice.
Is an HIV RNA test the fastest way to get an answer?
It detects infection earliest — CDC gives 10 to 33 days — but BHIVA does not recommend molecular assays for routine diagnosis, reserving them for diagnostic uncertainty such as suspected primary HIV or indeterminate serology on PrEP. A negative RNA test does not replace a fourth-generation test at 45 days.
Why do self-tests have a longer window than laboratory tests?
BHIVA applies a 90-day window to all point-of-care tests regardless of generation, because real-world device performance is poorer than a laboratory immunoassay’s. A reactive self-test result always needs laboratory confirmation on a venous sample.
Related calculators
References
- Palfreeman A, Sullivan A, Peto T, et al. British HIV Association/British Association for Sexual Health and HIV/British Infection Association adult HIV testing guidelines 2020. HIV Med. 2020. doi:10.1111/hiv.13015.
- Cresswell FV, Asanati K, Bhagani S, et al. UK guideline for the use of HIV post-exposure prophylaxis 2021. HIV Med. 2022. doi:10.1111/hiv.13208.
- Centers for Disease Control and Prevention. Getting Tested for HIV. cdc.gov/hiv/testing.
- Branson BM, Owen SM, Wesolowski LG, et al. Laboratory testing for the diagnosis of HIV infection: updated recommendations. Centers for Disease Control and Prevention and Association of Public Health Laboratories, 2014.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.