Was the exposure in the last 72 hours? Testing now can’t tell you anything about it yet. If your partner has HIV and isn’t on effective treatment, or you don’t know their status, ask about PEP (post-exposure prophylaxis) today. It must start within 72 hours and works best within 24. How to get PEP
Why timing matters
Every HIV test looks for something the body or the virus produces after infection. It takes time for that to build up to a level a test can find. Until then, a person can have HIV and still test negative. That gap is the window period, and each kind of test has its own.
So the question isn’t really “how accurate is this test?” Once a test’s window has passed, a negative result is conclusive. The question is “how far through its window was I when I tested?”
Week by week, test by test
“Too early” means the test can’t detect HIV yet, so a negative result tells you nothing about this exposure. “Inside window” means a negative result is reassuring but not final. “Conclusive” means a negative result rules out HIV from that exposure, as long as you haven’t had another exposure since.
| After exposure | NAT | Lab antigen/antibody (vein) | Rapid antigen/antibody (finger-prick) | Antibody, incl. most home tests |
|---|---|---|---|---|
| Week 1 (day 7) | Too early | Too early | Too early | Too early |
| Week 2 (day 14) | Inside window | Too early | Too early | Too early |
| Week 3 (day 21) | Inside window | Inside window | Inside window | Too early |
| Week 4 (day 28) | Inside window | Inside window | Inside window | Inside window |
| Week 6 (day 42) | Conclusive | Inside window | Inside window | Inside window |
| Week 8 (day 56) | Conclusive | Conclusive | Inside window | Inside window |
| Week 12 (day 84) | Conclusive | Conclusive | Inside window | Inside window |
| Day 90 onward | Conclusive | Conclusive | Conclusive | Conclusive |
These are the CDC window periods the calculator uses:
| Test | Detects HIV from | Negative is conclusive from |
|---|---|---|
| Nucleic acid test (NAT / RNA) | Day 10 | Day 33 |
| Antigen/antibody lab test (4th gen, blood from a vein) | Day 18 | Day 45 |
| Rapid antigen/antibody test (finger-prick) | Day 18 | Day 90 |
| Antibody test (most rapid tests and home self-tests) | Day 23 | Day 90 |
How a negative test lowers an estimate
Here is one example, worked through. It uses the highest-risk kind of sex: one act of receptive anal sex without a condom, with a partner who has HIV and isn’t on treatment. Before any test, the estimate is 1.4% (about 1 in 72).
A negative result changes that number, and the later the test, the bigger the change:
- Week 1, any test: unchanged. It’s too early for any test to see HIV.
- Week 2, negative NAT: 0.51% (about 1 in 200)
- Week 3, negative lab antigen/antibody test: 0.7% (about 1 in 140)
- Week 4, negative lab antigen/antibody test: 0.18% (about 1 in 540)
- Week 6, negative lab antigen/antibody test: 0.012% (about 1 in 8,000)
- Day 45, negative lab antigen/antibody test: 0.0014% (about 1 in 71,000), which is conclusive
- Week 8, negative home antibody test: 0.097% (about 1 in 1,000)
- Week 12, negative home antibody test: 0.011% (about 1 in 9,000)
- Day 90, negative home antibody test: 0.0014% (about 1 in 71,000), which is conclusive
The model is deliberately cautious. It assumes most infections can’t be detected yet at the start of each window, so an early negative counts for less than it might in reality. The methodology page explains how this works.
Most exposures start far lower. For a man who had vaginal sex once without a condom with a woman of unknown status in the US, the estimate is <0.0001% (about 1 in 1,500,000). After a negative lab test at week 4, it’s <0.0001% (about 1 in 11,000,000).
Picking a test for an early answer
- Want the earliest answer? A NAT can detect HIV from about day 10, but it’s expensive and not offered everywhere.
- The best all-round choice: a lab antigen/antibody test using blood from a vein. It’s widely available and conclusive from day 45.
- Using a home test? Most are antibody tests. They’re useful, but only conclusive from day 90. See home HIV tests.
Testing early is fine if it helps you feel calmer. Just plan a follow-up test once the window for that test has closed.
If you took PEP
PEP can change the testing plan. Follow your clinician’s schedule. CDC recommends HIV tests about 4 to 6 weeks and 12 weeks after the exposure.
What to do now
- If it’s been less than 72 hours, ask about PEP before thinking about tests.
- Work out your timing. Count the days since the exposure and see where you are in the table above.
- Test when it can tell you something, ideally with a lab antigen/antibody test from about week 3 onward.
- Retest after the window if your first test was inside it. A negative result after the window, with no new exposure, means you don’t have HIV from that exposure.
- Any positive or reactive result needs a follow-up lab test. See what to do after a positive test.