Chlamydia
Chlamydia trachomatis is an often-silent sexually transmitted infection of columnar epithelium. The high-yield priorities are site-appropriate NAAT, prompt treatment, partner notification, pregnancy-aware prescribing and recognition of ascending or extragenital complications.
Definition
Chlamydia is a sexually transmitted infection caused by the obligate intracellular bacterium Chlamydia trachomatis. Infection is often asymptomatic and is diagnosed with nucleic acid amplification testing from the relevant anatomical site.
Epidemiology
Chlamydia is a common curable bacterial STI in the UK, with the highest diagnosis rates in younger sexually active people. England’s NCSP currently focuses opportunistic community screening on young women and other young people with a womb or ovaries because untreated infection causes disproportionate reproductive harm; sexual health services provide testing for people of any age or gender when indicated.
Pathophysiology
C. trachomatis elementary bodies enter columnar epithelial cells and convert to replicating reticulate bodies before releasing new infectious particles. Local inflammation causes urethritis, cervicitis, proctitis or conjunctivitis. Ascending infection can cause PID and tubal damage; immune-mediated responses can contribute to reactive arthritis.
First principles
The organism is intracellular and the infection is often silent
Chlamydia trachomatis has an elementary-body and reticulate-body life cycle within host cells. It primarily infects columnar epithelium at the cervix, urethra, rectum, pharynx and conjunctiva. The inflammatory response may be mild, so absence of symptoms does not exclude infection or transmission.1,2
Screening matters because symptoms are an unreliable trigger
Many infections are detected through proactive testing rather than presentation. In England, the current NCSP emphasis is on reducing harm from untreated infection: opportunistic community screening focuses on sexually active young women and other young people with a womb or ovaries, while sexual health services continue to test anyone with symptoms, exposure or another indication.3,4
Ascending infection causes the important long-term harm
Untreated endocervical infection can ascend into the upper genital tract and contribute to pelvic inflammatory disease, tubal scarring, ectopic pregnancy and subfertility. The absence of a positive chlamydia test does not exclude PID because other organisms can be involved and clinical diagnosis is deliberately sensitive.1,5
Site of exposure determines the sample and the differential
A genital-only sample can miss rectal or pharyngeal infection. Ask about relevant exposure without assumptions about gender, anatomy or sexual practice, and arrange site-specific NAAT through the sexual-health pathway. Severe proctitis, tenesmus or bloody rectal discharge needs consideration of lymphogranuloma venereum and specialist management.1,2
Presentation
Most people have no symptoms. When symptomatic, chlamydia may present as cervicitis, urethritis, rectal infection, pharyngitis or conjunctivitis; lower abdominal pain, fever or testicular pain should trigger assessment for an ascending or complicated infection.1,2
Cardinal features
- Asymptomatic infection found through screening or partner notification
- Abnormal vaginal discharge, post-coital or intermenstrual bleeding, dysuria or lower abdominal pain
- Urethral discharge or dysuria in a person with a penis
- Rectal discomfort, discharge, bleeding or tenesmus after relevant exposure
- Usually mild or asymptomatic pharyngeal infection
- Unilateral or bilateral conjunctival irritation, redness or discharge
Red flags
- Fever, lower abdominal pain, deep dyspareunia, cervical motion or adnexal tenderness suggesting PID
- Pregnancy or possible pregnancy, which changes antibiotic selection and follow-up
- Sudden severe unilateral testicular pain or a high-riding testis: exclude torsion urgently
- Testicular pain and swelling suggesting epididymo-orchitis
- Severe proctitis, tenesmus or bloody rectal discharge suggesting LGV or another invasive cause
- Eye pain, photophobia, reduced vision or marked discharge requiring urgent eye assessment
Investigations
Sexual and reproductive history with site-directed examination
Clarify symptoms, pregnancy possibility, recent partners, condom use, relevant vaginal, anal or oral exposure, prior STIs, recent antibiotics and safeguarding concerns. Examine for cervicitis, PID, urethritis, epididymo-orchitis, proctitis or conjunctivitis when indicated.
Expected finding: The history identifies the correct sampling sites and whether the patient needs empiric treatment, urgent referral or a broader complication pathway.
1,5,6NAAT from the appropriate anatomical site
NAAT is the standard test for chlamydia. Use a self-taken or clinician-taken vulvovaginal swab where appropriate, first-catch urine in men, and rectal or pharyngeal swabs when symptoms or exposure make those sites relevant.
Expected finding: A positive NAAT confirms chlamydia at the sampled site. A negative genital result does not exclude infection at an unsampled extragenital site.
1,7,2Full sexual health screen
Offer testing for gonorrhoea and blood-borne or other STIs according to the sexual history, symptoms and local sexual-health pathway. Chlamydia and gonorrhoea can coexist, and partner notification should include access to a full screen.
Expected finding: Co-infection may alter treatment, follow-up and public-health advice; a negative chlamydia result does not exclude another STI.
1,2Pregnancy test and pregnancy-specific assessment when relevant
Pregnancy changes antibiotic choice, the need for test of cure and the risk-benefit discussion. Do not rely on a patient’s contraceptive history alone when pregnancy is possible.
Expected finding: A positive or uncertain pregnancy status triggers current BASHH, BNF and local obstetric or sexual-health prescribing guidance rather than the routine non-pregnant regimen.
8,9,10Targeted tests for complications
If PID is suspected, diagnosis is clinical and treatment should not wait for a positive NAAT. If epididymo-orchitis is suspected, assess urine and STI tests while excluding torsion. Severe proctitis may need LGV genotyping or specialist testing; eye disease may need urgent ophthalmology.
Expected finding: A positive chlamydia test supports the diagnosis but a negative test does not safely rule out PID, epididymo-orchitis or another cause of the presentation.
5,6,1Management
| Step | Detail | Source |
|---|---|---|
| Treat uncomplicated infection promptly with doxycycline | For uncomplicated urogenital, pharyngeal or rectal chlamydia, BASHH recommends doxycycline 100 mg twice daily for 7 days as first line. Check the current BNF and local sexual-health protocol for exact prescribing, contraindications, interactions, adherence support and any patient-specific adjustment.1,7,9 | BASHH Chlamydia 2015 page, 26 September 2018 treatment update |
| Use pregnancy- and breastfeeding-aware prescribing | Do not apply a blanket legacy rule without checking current guidance. The BASHH 2024 position statement should be read with the current BNF: in selected early pregnancies a short doxycycline course may be considered only when the full course can be completed before 15 weeks, while later pregnancy requires a pregnancy-appropriate alternative. Use the current BASHH, BNF and local maternity or sexual-health pathway for the exact regimen; arrange test of cure in pregnancy.8,1,9,10 | BASHH position statement on doxycycline use in pregnancy, current BNF and BASHH chlamydia guidance |
| Offer partner notification and a full screen | Discuss partner notification at diagnosis and use trained sexual-health staff or the local service to trace, test and treat relevant recent partners. Partners should be offered a full STI screen, including HIV testing where appropriate, and epidemiological treatment may be needed before their results return. Document the outcome.1,3 | BASHH Chlamydia 2015 contact-tracing guidance |
| Prevent reinfection while treatment is completed | Advise no vaginal, anal or oral sex until the patient and relevant partner or partners have completed treatment and the sexual-health service’s advice confirms it is safe to resume. Explain that reinfection is common if partners are untreated and that condoms reduce future STI risk.2,1 | NHS chlamydia information and BASHH partner-management guidance |
| Use test of cure selectively and offer retesting for reinfection | Routine test of cure is not required after successful treatment of uncomplicated genital infection, because residual non-viable DNA can persist for several weeks. Arrange test of cure in pregnancy, persistent symptoms, suspected poor adherence and diagnosed rectal infection according to current BASHH or local guidance; do not test too soon. Offer retesting around 3 months after treatment to detect reinfection, following the current NCSP or sexual-health pathway.1,11,3,2 | BASHH chlamydia guidance, UKHSA NCSP retesting policy and NHS information |
| Treat PID or epididymo-orchitis as a complication, not as simple chlamydia | If PID is suspected, start broad-spectrum empirical treatment promptly and arrange review; do not wait for swab results. Admit or obtain urgent specialist input for severe illness, tubo-ovarian abscess, peritonitis, inability to take oral treatment, diagnostic uncertainty or pregnancy. For epididymo-orchitis, exclude testicular torsion urgently when the pain is sudden or severe and use the current BASHH regimen.5,6 | BASHH PID 2019 and epididymo-orchitis 2020 guidelines |
| Refer unusual or extragenital disease | Refer severe proctitis or suspected LGV, persistent infection after documented adherence, recurrent treatment failure, conjunctivitis with significant eye symptoms, neonatal infection or suspected safeguarding concerns to the appropriate specialist service. Do not repeatedly prescribe empiric single-dose azithromycin without reassessing the diagnosis, site, adherence, reinfection and Mycoplasma genitalium or gonorrhoea possibilities.1,2 | BASHH chlamydia guidance and NHS chlamydia information |
Illustrations
Differentials
Gonorrhoea
Often more purulent discharge; may coexist, so gonorrhoea NAAT and the local culture or resistance pathway are important.
Mycoplasma genitalium
Persistent or recurrent urethritis or cervicitis, especially after appropriate chlamydia treatment; requires specific testing and resistance-aware treatment.
Pelvic inflammatory disease from mixed organisms
Lower abdominal pain, cervical motion or adnexal tenderness and fever; a negative chlamydia test does not exclude PID.
Bacterial vaginosis or candidiasis
Vaginal discharge or irritation without the cervicitis, urethritis or sexual exposure pattern of chlamydia.
Urinary tract infection
Dysuria with urinary symptoms or culture findings, but STI and UTI can coexist.
Testicular torsion
Sudden severe unilateral testicular pain, often with a high-riding or abnormal testis; an emergency requiring immediate assessment.
Complications
- Pelvic inflammatory disease, tubal-factor infertility and ectopic pregnancy
- Chronic pelvic pain and perihepatitis
- Epididymo-orchitis and possible fertility impairment
- Severe proctitis or lymphogranuloma venereum
- Conjunctivitis and neonatal conjunctivitis or pneumonia
- Reactive arthritis
- Reinfection from untreated or new partners
Prognosis
Uncomplicated infection is usually curable when the correct regimen is completed. Untreated or recurrent infection can cause PID, tubal-factor infertility, ectopic pregnancy, epididymo-orchitis, chronic pelvic pain or neonatal infection. Reinfection is reduced by partner notification, safer sex advice and retesting.
Guidelines
- Chlamydia 2015, including September 2018 treatment update (BASHH, 2018)
- National Chlamydia Screening Programme (UKHSA, 2025)
- Position statement on doxycycline use in pregnancy (BASHH, 2024)
References
- BASHH: Chlamydia 2015, including the 26 September 2018 treatment update (BASHH Chlamydia guideline, updated 26 September 2018)Updated 26 Sept 2018
- NHS: Chlamydia (NHS symptoms, testing, treatment and partner advice)
- UKHSA: National Chlamydia Screening Programme (Current NCSP programme overview and policy)Published 1 Jan 2003 | Updated 6 Jan 2025
- UKHSA: NCSP standards (National Chlamydia Screening Programme standards, eighth edition)Published 1 May 2014 | Updated 7 Mar 2022
- BASHH: PID 2019 (UK national guideline for the management of pelvic inflammatory disease, with 2019 interim update)Updated 26 Jan 2019
- BASHH: Epididymo-orchitis 2020 (UK national guideline for management of epididymo-orchitis)Updated 11 Sept 2020
- NICE CKS: Chlamydia – uncomplicated genital (NICE CKS topic)
- BASHH position statement on doxycycline use in pregnancy (BASHH Clinical Effectiveness Group position statement)Updated 13 May 2024
- BNF: Doxycycline (BNF doxycycline monograph)
- BNF: Azithromycin (BNF azithromycin monograph)
- UKHSA: Chlamydia re-testing following a positive diagnosis (NCSP routine offer of retesting around 3 months after treatment)Published 23 Jun 2015 | Updated 31 Jul 2019
Evidence checked: 2026-08-03
This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.