STI Window Periods: A Practical Timeline for When to Test After an Exposure

Last week we covered why the date on a result matters: a negative result speaks for one day. This is the practical follow-up. If something happened recently, when should you test, and when should you test again?

The short answer is that there is no single number. The gap between an exposure and the point a test can reliably pick it up is called the window period, and it differs by infection and by test type. Here is a timeline you can plan around, built from CDC and HIV.gov guidance.

The first 72 hours

If the exposure involved a possible HIV risk and it was within the last three days, do not wait for a test window. CDC guidance says post-exposure prophylaxis (PEP) should be started as soon as possible, and no later than 72 hours after exposure. An urgent care clinic, emergency department or sexual health clinic can evaluate you the same day.

Around two weeks: chlamydia and gonorrhea

One NHS sexual health service advises waiting about two weeks after sex before testing for chlamydia and gonorrhea. Testing sooner can come back negative simply because there was not yet enough to detect.

Around four weeks and beyond: syphilis and HIV

The same service suggests about four weeks for syphilis and HIV, with a second test at three months in some circumstances. For HIV, the CDC gives ranges by test type: a nucleic acid test can usually detect HIV 10 to 33 days after exposure, a lab antigen/antibody test 18 to 45 days, and an antibody test 23 to 90 days. If you test negative before the window period for your test has passed, the CDC says to get tested again after it.

Syphilis is a good reminder that an early negative is not always the last word. CDC guidelines recommend that people who had sexual contact with someone diagnosed with early syphilis in the previous 90 days be treated presumptively, even if their own test results are negative.

If you took PEP, there is a follow-up schedule

CDC recommendations for people who start PEP include HIV testing at the start, again 4 to 6 weeks after exposure, and a final round 12 weeks after exposure. Your provider will set the exact schedule, and it is worth putting the dates in your calendar before you leave the appointment.

Testing again after treatment

A positive chlamydia or gonorrhea result has its own timeline. CDC guidance says people treated for chlamydia should be retested about three months after treatment, because most repeat infections come from reinfection rather than a treatment that did not work. The CDC also discourages chlamydia testing within four weeks of finishing treatment, since leftover genetic material can cause a false positive.

Telling a partner what the timeline is

A date and a plan is a gentler thing to share than a bare result. Something like: “I tested on [date]. I am testing again on [date] to cover the window period.” It tells the other person what your result covers, and it shows you are taking the gap seriously without making it a big moment.

Sources

• Antiretroviral Postexposure Prophylaxis After Sexual, Injection Drug Use, or Other Nonoccupational Exposure to HIV: CDC Recommendations, United States, 2025, CDC (MMWR Recommendations and Reports, 2025)
• Getting Tested for HIV, CDC
• HIV Testing Overview, HIV.gov (U.S. Department of Health and Human Services)
• How long should I wait before I test?, Sexual Health Oxfordshire (NHS)
• Syphilis, STI Treatment Guidelines, CDC
• Chlamydial Infections, STI Treatment Guidelines, CDC