This page is general health information, not medical advice. It cannot tell you whether you have an infection. A clinician can, and the testing options on this site are a good place to start.

You had a sexual encounter, a condom broke, or a partner just told you they tested positive. Now you want to know when a test will give you a useful answer.

The honest answer is that it depends on the infection and the type of test. This guide walks you through the steps in order, with the CDC's published numbers and the places in Houston where you can test for free or on a sliding scale.

What a window period is

A window period is the time between an exposure and the point when a test can reliably pick up an infection. Testing inside that window can return a negative result even when an infection is present. That is why timing matters as much as the test itself.

Window periods are about the test, not about you or your choices. Many people test more than once for exactly this reason, and clinicians expect it.

What you need before you start

The date of the exposure, or your best estimate of it.
A short list of what kind of contact it was (vaginal, anal, oral, shared needles or other). This helps a clinician choose the right tests and body sites.
Any symptoms you have noticed, with the date they started.
Your phone and a calendar, to book a visit and mark a retest date.
An ID and any insurance card, if you have one. Several Houston options do not require insurance.
A list of current medicines, including any PrEP.

Step 1: Check whether you are within 72 hours

This is the only step with a hard clock, so look at it first. The CDC says post-exposure prophylaxis (PEP) for HIV works best when it is started as soon as possible, and no later than 72 hours after a possible exposure.

PEP is a prescription medicine. A clinician decides whether it fits your situation and writes the prescription. You do not need to wait for a test result to ask about it.

In Houston, the Houston Health Department says its health centers offer nPEP by appointment at 832-395-9800. Legacy Community Health lists PEP among its services as well. If you are close to the 72-hour mark, call right away and say that you are asking about PEP. If you are past 72 hours, you can still talk with a clinician about testing and next steps.

The CDC also says that, for some people, a clinician may prescribe doxycycline as post-exposure prevention (doxy-PEP) after sex, to lower the chance of certain bacterial STIs. The 2024 CDC guidance focuses on gay, bisexual and other men who have sex with men, and transgender women, who had a bacterial STI in the past year. It is a clinician's decision, and it is not a replacement for testing.

Step 2: Look up the window period for each infection

The CDC publishes detailed window periods for HIV tests. For other infections, the CDC STI Treatment Guidelines (2021) describe who to test and when, but they do not give one fixed number of days that fits every test. For those, a clinician can tell you what timing makes sense for the specific test they use.

Infection and test	What the CDC says about timing
HIV: nucleic acid test (NAT)	Can usually detect HIV 10 to 33 days after exposure, according to the CDC.
HIV: antigen/antibody test, blood drawn from a vein (lab)	Usually 18 to 45 days after exposure, according to the CDC.
HIV: antigen/antibody test, fingerstick (rapid)	Usually 18 to 90 days after exposure, according to the CDC.
HIV: antibody test, including most self-tests	Usually 23 to 90 days after exposure, according to the CDC.
Chlamydia, gonorrhea, trichomoniasis	The CDC does not list one fixed window for these. Ask the clinician how soon after exposure their test is meant to be used. The CDC recommends retesting about 3 months after treatment.
Syphilis	Blood tests can be negative early on, so a clinician may repeat the test. The CDC recommends syphilis testing for anyone with a known exposure, and follow-up testing is common.
Herpes and HPV	The CDC does not recommend routine screening for herpes without symptoms, and there is no approved HPV test for screening in men. A clinician can explain what testing applies to you.

The HIV ranges are the CDC's. A test taken at the early end of a range is less likely to detect an infection than one taken at the later end. A test taken after the full range has passed is the most informative, according to the CDC.

Step 3: Test now, then plan a retest

Many people ask how soon they can test for an STD. A clinician can often test the same day, and there are good reasons to go early.

A first test sets a baseline for your sexual health.
Some infections may already be detectable, especially if you have symptoms.
A clinician can talk about PEP, treatment options and preventing spread to partners.

Because a negative result inside the window period may not be final, ask the clinic to help you plan a second test. For HIV, the CDC's ranges point to a follow-up test at the later end of the window for the test you used. For other infections, ask the clinician which date they recommend.

Write both dates on your calendar before you leave the clinic. It is easy to skip the second visit when the first result looks fine. That second visit is what makes the answer more reliable.

Step 4: Choose where to test in Houston

You have several free or low-cost options. For hours, current services and the full list, see the local testing list. For prices and a longer comparison, see our guide to STD testing in Houston and what it costs.

Houston Health Department health centers at Northside (8504 Schuller Rd., 77093), Sharpstown (6201 Bonhomme Rd., 3rd Floor, South Tower, 77036) and Sunnyside (4410 Reed Rd., 77051). The department lists a sliding-scale cost. The phone number is 832-393-4220. The department says walk-ins are accepted as capacity permits, and appointments have been encouraged since September 1, 2026, so call first.
Harris County Public Health Antoine Clinic, 5815 Antoine Dr., 77091, phone 832-927-7350. The county lists HIV, syphilis, gonorrhea and chlamydia testing, PrEP and STI treatment at no cost, and walk-ins are welcome. Its page lists Monday to Thursday 8am to 7pm, closed Friday, and every other Saturday 8am to 2pm. Confirm before you go.
Legacy Community Health, Montrose, 1415 California St., 77006, phone (832) 548-5100. Legacy lists free walk-in HIV, syphilis and hepatitis C testing. A full STI panel is by appointment, and the page does not list a price for it.
Avenue 360 Health & Wellness at Heights (2150 West 18th Street, Suite 300) and Midtown (2920 Fannin St.), phone (713) 426-0027. Avenue 360 lists free walk-in rapid HIV, hepatitis C and syphilis testing Monday to Friday, 8:30am to 4:30pm.
The Montrose Center, 401 Branard Street, 77006, phone 713-529-0037. It lists free walk-in rapid HIV, hepatitis C and syphilis testing Monday to Thursday, 9am to 2pm. It does not list a full STI panel.

If you would rather not go in person, the Texas Department of State Health Services lists Test for Texas, a free mail-order self-collection kit for HIV, syphilis, gonorrhea and chlamydia for anyone in Texas aged 13 and older, with no insurance needed. You order it at testfortexas.org. Remember that a mailed kit follows the same window periods as any other test. The CDC's GetTested tool is another way to find testing sites.

Step 5: At the visit, say what you want tested

Clinicians do not always test for everything by default. Tell them about the exposure and the type of contact, and ask what they will test for and which body sites (genital, throat, rectal) they will sample. The CDC recommends testing at the sites where exposure happened, because infections there can be missed by a urine test alone.

You do not have to explain your personal life beyond what helps with testing. Clinicians see this every day and are there to help, not to judge.

If you are under 18, the Texas Family Code (Sec. 32.003) allows a minor of any age to consent to diagnosis and treatment of a reportable infectious disease, which covers reportable STIs and HIV. Parental consent is not needed. The same statute allows a physician to tell a parent or guardian, so a provider cannot promise that a parent will never be told. The Houston Health Department also notes that a Texas law that took effect September 1, 2025 gives parents access to a minor's electronic health record. Ask the clinic about its privacy practices before you share details.

Step 6: Get your results and take the next step

Ask how and when you will hear back, and who to call if you do not. Results are usually provided by the clinic, not by the health department. In Texas, chlamydia, gonorrhea, syphilis and HIV are reportable conditions, according to the Texas DSHS notifiable conditions list. That means a positive result is reported to public health authorities, which is a routine step in care.

If a result is positive, a clinician explains the options. According to the CDC, chlamydia, gonorrhea, syphilis and trichomoniasis are curable with treatment from a clinician. HIV, herpes and HPV are not curable, but they are treatable or manageable. Treatment is always prescribed by a clinician.

If a partner may need care too, ask about expedited partner therapy. The CDC lists Texas rules that allow clinicians to prescribe for the partners of an established patient. Rules differ, so ask what applies to you.

Tips that help

Call ahead. The Houston Health Department says walk-ins are seen only as capacity permits.
Test at more than one point. One early test and one later test give a fuller picture than a single visit.
Ask about symptoms. If you notice something new, see a clinician sooner rather than waiting for a retest date. Our guide to symptoms and when to test explains common signs, though many infections cause none.
Use protection while you wait. Condoms and other barriers lower the chance of passing on an infection while you are waiting for results. If you are on PrEP, keep taking it as your clinician directed.
Tell recent partners when you are ready. A clinician or the health department can help you plan how.

Common mistakes to avoid

Treating one early negative as the final answer. Inside the window, a negative result may not be final.
Waiting past 72 hours when PEP is a question. If you think you might want PEP, call right away.
Skipping the retest after treatment. The CDC recommends retesting about 3 months after treatment for chlamydia, gonorrhea and trichomoniasis, because reinfection is common.
Testing only one body site. Ask which sites should be sampled based on the contact you had.
Letting worry or embarrassment decide. Testing is routine health care. It is not a judgment.

Expected results

A negative result after the full window period for the test you used suggests that an infection from that exposure is unlikely, according to the CDC's HIV guidance. A negative result before the window has passed may change with a later test. A positive result can be confirmed with additional testing, and a clinician can walk you through what it means and what care is available.

No test can predict your future exposures, so regular testing is part of routine care if you are sexually active. The USPSTF and the CDC recommend screening based on age, pregnancy status and risk, and a clinician can tell you what schedule fits you.

Why Houston testing matters

Houston sits in the second largest county in Texas, and STI numbers there are higher than the state average. The Texas DSHS 2022 STD Surveillance Report lists 31,406 chlamydia cases in Harris County, a rate of 656.9 per 100,000 people. The statewide rate that year was 519.9 per 100,000. For HIV, the Houston Health Department reports 1,343 new diagnoses in Houston/Harris County in 2023, a rate of 27.5 per 100,000.

These numbers describe a community, not any one person. They are one reason local health agencies make testing easy to reach.

When to Get Tested After Exposure: STD Window Periods Explained