STI Testing: When to Test and What For

A new partner, a split condom, or simply a long time since your last check. No symptoms does not mean no infection: tests are chosen by age, risk and how long ago the contact was. After a possible exposure to HIV, the clock runs in hours, not weeks — preventive treatment has to start within 72 hours.

When to see a doctor

Right away: after sexual assault. If you are in danger, call 999 in the UK, 911 in the US, 112 in the EU. Care is more than tests weeks later: within hours — emergency contraception and HIV prevention; at the first visit — an examination, baseline tests and, where indicated, preventive treatment for other infections and vaccination (CDC, US). In the UK, sexual assault referral centres (SARCs), A&E and sexual health clinics help whether or not you decide to go to the police (NHS); Rape Crisis (England and Wales): 0808 500 2222. In the US, the National Sexual Assault Hotline: 800-656-4673. Also call an ambulance for severe or worsening pain low in the abdomen, pain with heavy bleeding, sudden sharp pain, fainting, high temperature or severe vomiting (NHS, pelvic inflammatory disease), and for sudden severe pain in a testicle — possible torsion (NHS).

The same day: possible exposure to HIV — post-exposure prophylaxis (PEP) must start within 72 hours and works better within the first 24. In the UK, a sexual health clinic, or A&E when it is closed (NHS); in the US, your doctor, an emergency room or urgent care (CDC). Pelvic pain, pain during sex, bleeding between periods or fever — possible pelvic inflammatory disease — needs a doctor the same day, especially if you could be pregnant (NHS). New discharge, pain when urinating, sores, rash or lumps on the genitals, or a partner telling you they have an infection: book a clinic now — with symptoms you are tested at once, without waiting for any window.

Routinely: screening by age and risk, below.

What is happening

Many infections are silent. Most people with chlamydia have no symptoms; gonorrhoea does not always cause them either (NHS). Untreated chlamydia can spread to the womb and tubes and lead to pelvic inflammatory disease, infertility and ectopic pregnancy (NHS).

Why a test can be too early. A test finds the germ itself or the body's response to it, and both take time to reach a detectable level. That time is the window period:

Chlamydia and gonorrhoea: about 2 weeks after contact (Sexual Health Oxfordshire, an NHS service).
Syphilis: from about 4 weeks, and an early negative does not rule it out — the clinic may ask you to repeat it. The NHS says some infections take up to 7 weeks to show on a test.
HIV, UK (BHIVA): a laboratory fourth-generation test is reliable 45 days after exposure; rapid clinic tests and self-tests — 90 days.
HIV, US (CDC): a nucleic acid test detects it 10–33 days after exposure; a laboratory antigen/antibody test 18–45 days; a finger-prick rapid antigen/antibody test 18–90 days; antibody tests, including self-tests, 23–90 days.

A negative result inside the window does not exclude infection, and a new contact restarts the count (BHIVA). PEP and PrEP change HIV test timing: the medicines can delay detection, so testing follows the clinic's schedule, ending around 12 weeks after the exposure in CDC's 2025 PEP guidance. A self-test taken while on PEP or PrEP can mislead.

What to test for

US screening without symptoms (CDC):

Chlamydia and gonorrhoea: yearly for sexually active women under 25; from 25, with a new partner or several. For heterosexual men at low risk, evidence is insufficient for routine screening.
Men who have sex with men: at least yearly chlamydia, gonorrhoea (at every site of contact, including throat and rectum), syphilis and HIV; every 3–6 months at higher risk.
HIV: everyone aged 13–64 at least once; at least yearly with ongoing risk.
Hepatitis B and C: every adult 18+ at least once (hepatitis B screening since CDC's 2023 recommendation).
Pregnancy: HIV, syphilis and hepatitis B at the first visit.
Herpes: blood tests without symptoms are not routine.

In England, chlamydia screening is offered to women under 25 (national programme since 2021).

Where. UK sexual health clinics see you without a GP referral; the visit is not shared with your GP without consent, and the clinic can tell partners without naming you (NHS). In the US, CDC's gettested.cdc.gov lists testing sites.

What to do

Say when the contact was, what kind and any symptoms: the clinic picks the tests and sites.
If a result is positive, treatment covers partners too, or you can be reinfected; a repeat test follows the doctor's plan. Talking about it: how to discuss your sex life with a partner.
An unexpected result is sometimes repeated: an abnormal test result: what happens next.
With ongoing risk, ask about pre-exposure prophylaxis (PrEP); PEP is an emergency measure (CDC).
Condoms are the only contraception that also protects against STIs — how effective each contraceptive method is.

When this is not the case

With symptoms the window does not apply: test now.
Pregnancy or possible pregnancy: tell the clinic; some infections matter for the pregnancy, and pain with a late period is an emergency, see above.
After unprotected sex you may also need emergency contraception.
Preparing for the visit: how to talk to a doctor so they listen.

Sources

Checked: 24 September 2026