How soon after sex can an STI test pick something up?

A condom split, or you woke up wishing last night had gone differently, and the question arrives before breakfast: if I get tested today, will it show anything?

For last night, the short answer is no. Every STI test looks for something that takes time to build up in the body, and a few hours isn't long enough. That doesn't make today a wasted day. A doctor can check for anything you picked up earlier, look at symptoms, and give you a date when a test for last night will actually work.

If you'd rather skip the reading, our GP-led sexual health and GUM clinic at 117a Harley Street sees people seven days a week, walk-in or booked, with STI screens from £250. If you'd prefer a specialist testing clinic, our sister clinic, Sexual Health Clinic London, offers that too.

If it's HIV you're worried about, timing matters most

If the contact was within the last 72 hours, NHS advice is to go to a sexual health clinic or A&E for emergency HIV medicine, which may stop infection. It should be started as soon as possible, ideally within 24 hours, and no later than 72 hours. A laboratory HIV test taken this morning can't show an infection from last night, so the medicine comes first and the test comes later.

What is a window period?

It's the gap between catching an infection and a test being able to see it. Think of a test as a reading rather than a photograph of last night. A urine or swab test for chlamydia or gonorrhoea looks for genetic material from the bacteria, and the lab can only report it once there's enough to measure. A blood test for HIV, syphilis or hepatitis looks for virus, antigen or antibodies, and each of those appears on its own clock.

So a negative result can be accurate about the past and still say nothing about a risk that's only hours old. Each infection runs to its own timetable:

When should I test for chlamydia and gonorrhoea?

Around two weeks after the contact. These are nucleic acid amplification tests (NAATs) on first-catch urine, meaning the first part of the stream rather than a mid-stream sample, or on a swab from the area at risk.

BASHH's UK guideline for chlamydia suggests testing when you first come in, then a repeat NAAT two weeks after the exposure if that's the one you're worried about. The chlamydia test you take today covers earlier partners. The one in a fortnight covers last night. If you come in before then, the doctor can still examine you, treat on clinical grounds when that's needed, and book the repeat test that makes the lab result mean something.

How long is the window for HIV, syphilis and hepatitis?

Longer. HIV testing is most accurate after 28 days, though fourth-generation tests can detect infection from 10 days, and the 2020 guidelines from BHIVA, BASHH and the British Infection Association put the window for fourth-generation laboratory serology at 45 days. NHS.uk notes you may be asked to repeat the test a few weeks later after a recent exposure. The doctor will tell you which clock applies to you.

Syphilis is a blood test, and NHS sexual health services commonly quote around twelve weeks for serology. An early negative is less reassuring here than a two-week chlamydia result. Hepatitis runs on a longer clock again. For hepatitis B, UKHSA puts the incubation period at 40 to 160 days, with the surface antigen test most commonly positive by 60 to 90 days. For hepatitis C, BASHH notes the antibody test is positive three months after exposure in 90% of people but can take up to nine months, while an antigen or RNA test picks infection up much sooner. So after a hepatitis risk, the doctor will plan a three-month blood test rather than rely on a single early one. FAST Complex (£450) tests for hepatitis B and C alongside HIV, syphilis, chlamydia and gonorrhoea, with every result in six hours.

Is herpes a swab or a blood test?

Usually a swab, taken while sores or blisters are present. The swab looks for the virus in the sore itself. A blood test looks for antibodies from past exposure, which can't date a contact or tell you who passed it on. If you have ulcers, come while they're there, because a healed patch gives a weak sample. A tingle with nothing on the skin is better seen once something appears.

What if I already have symptoms?

Then come now rather than waiting for the lab clock. Discharge, pain when passing urine, pelvic or testicular pain, sores or a fever need a doctor whatever the timing, and a too-early negative test won't explain or treat them. If symptoms carry on after a clear result, our guide to symptoms after a negative test explains what a doctor checks next.

If you're severely unwell, in severe pain, unable to pass urine or have a high fever, call 999 or go to A&E rather than booking at Harley Street. NHS 111 can advise on urgent symptoms that aren't life-threatening.

Feeling fine tells you very little either way, because many STIs cause no symptoms. UKHSA recorded about 401,800 new STI diagnoses in England in 2023, and chlamydia was still the most common.

How often should I get an STI test?

Window periods tell you when to test after one contact. For regular testing, UKHSA gives a simple rhythm:

• If you have condomless sex with new or casual partners, have a full STI screen, including an HIV test, at least once a year.
• If you are a gay, bisexual or other man who has sex with men, test for HIV and STIs every year, or every three months if you have condomless sex with new or casual partners.
• If you are under 25, sexually active and have a womb or ovaries, have a chlamydia test every year and whenever you change partner.
• If you test every three months for HIV, BASHH advises adding a hepatitis C test at least once a year.

A new relationship is a good moment too, because a screen for both of you gives a clear starting point. Our Bronze screen covers chlamydia, gonorrhoea, syphilis and HIV for £250, with a doctor's consultation included. If you'd like to compare it with a free clinic first, our guide to NHS or private STI testing sets out the differences.

What happens at the clinic?

Care here is confidential and GP-led. You can be assessed, tested and, when it's clinically indicated, treated, with nothing on your NHS record unless you ask us to share it, and you're welcome to use a name of your choice. Walk in or book at 117a Harley Street, Monday to Friday 09:00 to 18:00 and weekends 10:00 to 14:00, subject to capacity.

Every screening package includes a doctor consultation. If you want answers fast, FAST Simple (£350) and FAST Complex (£450) return every result in six hours, urine and swab tests included. Screening packages run from £250 for Bronze to £665. On the other packages, blood results are generally back within 24 hours, most urine and swab results take up to three working days, and a herpes swab takes three to five. Treatment is included with packages when it's clinically indicated. We can't promise every infection will be treated on the day, or that every infection can be managed here.

Our doctors work to BASHH guidance and samples go to UKAS-accredited laboratories. The doctor reads each result alongside your history and examination, because a lab number on its own isn't a diagnosis. Bring your current medicines, when any symptoms started and the timing of recent contacts, as far as you're happy to share, and you'll leave with a plan and a date for any follow-up test.

Which test, and when?

• If last night was the only risk and you feel well, book a chlamydia and gonorrhoea test for around two weeks later, and ask the doctor when to test for HIV and syphilis.
• If you've had other partners in the past month, say so in outline. A test today can still give a useful answer about them.
• If you have discharge, pain, sores or a fever, come in now.
• If a six-hour FAST result is clear but you're still inside the HIV or syphilis window, keep the later blood test in your plans.
• If you've read a day-count on a forum, bring the question. The doctor dates the window from your actual contact, not from the day you started worrying.

Related guides: genital warts treatment choices and adult HPV vaccination with Gardasil 9. Questions before you come in? Contact the clinic on 0207 499 1991.