What is the HIV window period?

What the window period means

When HIV enters the body, nothing a test can find shows up straight away. First the virus multiplies. Then the immune system starts making antibodies. Each kind of HIV test looks for one or more of these signs, so each has its own window.

During the window, a person can have HIV and still get a negative result. That’s why a negative test taken soon after an exposure isn’t the final answer. Once the window has passed, a negative result is conclusive: with no new exposure since, it means you don’t have HIV from that exposure.

The window for each test

CDC gives each test a range. The first number is the earliest the test can usually detect HIV. The second is when a negative result becomes conclusive.

NAT (nucleic acid test)

A NAT looks for the virus’s genetic material in blood. It has the shortest window, 10 to 33 days, but it costs more and not every clinic offers it. It’s mostly used after a recent high-risk exposure or when someone has symptoms of early infection.

Lab antigen/antibody test

This is the standard test at most clinics and labs. It looks for HIV antibodies and for p24, a protein from the virus that appears before antibodies do. With blood drawn from a vein, the window is 18 to 45 days.

Rapid antigen/antibody test

The same kind of test, run on a drop of blood from a finger-prick, with results in minutes. Its window is longer: 18 to 90 days.

Antibody tests

These look only for antibodies, which take the longest to appear. Most rapid tests and home self-tests, including oral-swab tests, are antibody tests. The window is 23 to 90 days. More on these in home HIV tests.

When to retest

CDC’s advice is simple: if you tested after a possible exposure and the result was negative, test again after the window period for the test you used.

• Tested inside the window? Retest once it has closed. For a lab antigen/antibody test that’s day 45. For a home antibody test, day 90.
• Tested after the window, with no new exposure? You don’t need to retest for that exposure.
• Had another exposure since? The window starts again from the newest exposure.
• Took PEP (post-exposure prophylaxis)? Follow your clinician’s plan. CDC recommends HIV tests about 4 to 6 weeks and 12 weeks after the exposure.

A negative result inside the window still counts for something. After receptive anal sex with a partner who has HIV and isn’t on treatment, the estimate is 1.4% (about 1 in 72). A negative lab antigen/antibody test at four weeks lowers it to 0.18% (about 1 in 540). See HIV test accuracy by week for every week.

Can HIV be passed on during the window?

Yes. Someone who was infected recently may not test positive yet, but the amount of virus in their blood is often at its highest in the first weeks. Transmission is several times more likely during this early phase than later. If you might have been exposed, it makes sense to use condoms or avoid sex until you have a conclusive result.

The window period and PEP

No test can say anything about an exposure in the first few days. But that’s exactly when PEP can stop an infection. Don’t wait for a test result before asking about PEP. It has to start within 72 hours.

What to do now

1. Note the date of the exposure and count the days since.
2. If it’s under 72 hours, ask about PEP first.
3. Choose your test. A lab antigen/antibody test from about day 18 gives an early read and is conclusive at day 45.
4. Book your retest for the end of that test’s window if your first test was inside it.
5. In the US, you can find free testing at gettested.cdc.gov. CDC recommends everyone aged 13 to 64 get tested at least once.