🧭 When to suspect

Chlamydia trachomatis is the most commonly diagnosed bacterial sexually transmitted infection in the UK, and prevalence is highest in sexually active people aged under 25. The single most important clinical fact is that it is usually silent – around 70–80% of infections cause no symptoms – so the diagnosis depends on testing the right people rather than waiting for a clinical picture to declare itself.

Keep the threshold for testing low. Consider chlamydia in any sexually active person with a new or multiple partners, condomless sex, a partner with a confirmed STI, or any of the genital or extra-genital symptoms below.

Since 2021 the National Chlamydia Screening Programme (NCSP) has refocused on reducing the reproductive harm of untreated infection, because serious complications occur predominantly in women. In practice this changes who is offered opportunistic (asymptomatic) screening: in general practice and pharmacies it is now proactively offered to young women and other people with a womb or ovaries under 25 – not to men. Everyone else, including men, can still be tested, but on indication (symptoms, being a contact, or on request).

Offer opportunistic (asymptomatic) screening	Test on indication – any gender
• Sexually active women / people with a womb or ovaries aged < 25 – offer after each new partner and at least annually.	• Any genital, rectal, or ocular symptoms suggestive of infection (see History).
• Offered routinely when accessing contraception, SRH, termination-of-pregnancy, GP or pharmacy services.	• Sexual contact of someone with confirmed or suspected chlamydia.
• Re-test at ~3 months after a positive result – the peak window for reinfection.	• Another STI diagnosed, or the person requests a test / reports higher-risk exposure.

Source: UKHSA NCSP · BASHH · NICE NG221

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