Most common bacterial STI in the US — often asymptomatic; serotypes D-K cause urogenital infection. Cervicitis/urethritis treated with doxycycline.
Also known as: chlamydia, Chlamydia trachomatis, urethritis, cervicitis, PID
Overview
Infection of genitourinary epithelium by Chlamydia trachomatis, an obligate intracellular gram-negative bacterium. Serovars D-K cause urogenital infection; A-C cause trachoma (ocular); L1-L3 cause lymphogranuloma venereum. Vertical transmission causes neonatal conjunctivitis and pneumonia.
Epidemiology
Most commonly reported notifiable infection in the US — 1.6 million cases in 2022 (CDC), with true incidence likely much higher due to asymptomatic infection. Peaks in ages 15-24; women disproportionately affected. Frequently coinfects with gonorrhea.
Try two board-style Chlamydia trachomatis Genital Infection questions
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Question 1Infectious DiseaseMedium
A 22-year-old woman has mucopurulent cervicitis after unprotected sex. NAAT is positive for Chlamydia trachomatis and negative for gonorrhea. Which of the following is the most appropriate treatment?
ASingle-dose intramuscular ceftriaxone
BAcyclovir three times daily for ten days
CSingle-dose oral metronidazole therapy
DDoxycycline twice daily for seven days
Reveal answer & full explanation
Correct answer: D — Doxycycline twice daily for seven days
ASingle-dose intramuscular ceftriaxone
BAcyclovir three times daily for ten days
CSingle-dose oral metronidazole therapy
DDoxycycline twice daily for seven days✓
Why Doxycycline twice daily for seven days is correct
Current CDC guidance treats uncomplicated genital chlamydia with doxycycline 100 mg twice daily for 7 days in nonpregnant adults.
NAAT confirmed C. trachomatis and excluded gonorrhea, so dual coverage is unnecessary.
Partners should be treated and intercourse avoided until therapy is complete to prevent reinfection.
Why the others are wrong
Single-dose intramuscular ceftriaxone — Gonorrhea trap: ceftriaxone is the gonococcal drug, but the GC NAAT is negative and it does not reliably clear chlamydia.
Single-dose oral metronidazole therapy — Trichomoniasis/BV trap: metronidazole has no activity against the intracellular chlamydia organism.
Acyclovir three times daily for ten days — HSV trap: cervicitis here is bacterial; acyclovir treats herpetic ulcers, not chlamydial infection.
Question 2Infectious DiseaseMedium
A 26-year-old sexually active woman has mucopurulent cervicitis. NAAT is positive for Chlamydia trachomatis. Which of the following is the most appropriate treatment?
AOral metronidazole for seven days
BIntramuscular ceftriaxone single dose
COral azithromycin single dose
DOral doxycycline for seven days
Reveal answer & full explanation
Correct answer: D — Oral doxycycline for seven days
AOral metronidazole for seven days
BIntramuscular ceftriaxone single dose
COral azithromycin single dose
DOral doxycycline for seven days✓
Why Oral doxycycline for seven days is correct
Doxycycline 100 mg twice daily for 7 days is the CDC-preferred regimen for uncomplicated chlamydia in nonpregnant adults.
It clears urogenital infection more reliably than single-dose alternatives, especially for rectal coinfection.
Partners require evaluation and treatment to prevent reinfection.
Why the others are wrong
Oral azithromycin single dose — Azithromycin is now only an alternative for chlamydia and is preferred in pregnancy; in a nonpregnant patient doxycycline is favored for higher cure rates.
Intramuscular ceftriaxone single dose — Ceftriaxone treats gonorrhea; it is added when gonorrhea is present but does not treat chlamydia, which the NAAT confirmed.
Oral metronidazole for seven days — Metronidazole targets trichomoniasis and bacterial vaginosis, not the chlamydial cervicitis identified here.
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New partner within 60 days, inconsistent condom use
Prior STI history
Sex work, MSM (rectal/pharyngeal infection)
Mother with untreated chlamydia (neonatal infection)
Pathophysiology
Elementary bodies (infectious form) attach to and invade columnar epithelial cells of cervix, urethra, rectum, conjunctiva. Inside, they convert to reticulate bodies and replicate within inclusions, then revert to elementary bodies and are released. Persistent infection drives chronic inflammation that scars the upper genital tract — causing tubal infertility and chronic pelvic pain.
Clinical presentation
Symptoms
Women (often asymptomatic): mucopurulent cervical discharge, intermenstrual or post-coital bleeding, dysuria
Conjunctival injection with mucopurulent discharge (neonate or adult inclusion conjunctivitis)
Classic findings
Sexually active young woman with intermenstrual or post-coital bleeding and mucopurulent cervicitis — chlamydia until proven otherwise. Test all sexually active women under 25 annually.
Differential diagnosis
Gonorrhea — More purulent discharge, concurrent in ~30%; treat for both empirically
Mycoplasma genitalium — Persistent urethritis/cervicitis after standard therapy; NAAT; macrolide resistance common
Trichomoniasis — Frothy, malodorous vaginal discharge, strawberry cervix; NAAT or wet mount
Co-test for gonorrhea at the same time and site (frequent coinfection)
Screen for syphilis and HIV; offer hepatitis screening as indicated
Wet mount for trichomoniasis if discharge
USPSTF recommends annual chlamydia screening for sexually active women <25 and older women with risk factors
Imaging
Pelvic ultrasound if PID with tubo-ovarian abscess suspected
Generally none required for uncomplicated infection
Diagnostic algorithm
Population/Site
First-line Therapy
Notes
Urogenital (non-pregnant)
Doxycycline 100 mg BID x 7 d
Preferred over azithromycin
Pregnancy
Azithromycin 1 g PO x 1
Doxycycline contraindicated; test of cure at 4 wk
Rectal
Doxycycline 100 mg BID x 7 d
Azithromycin inferior here
LGV (L1-L3)
Doxycycline 100 mg BID x 21 d
Inguinal/anal bubo; MSM
Neonatal
Erythromycin x 14 d (PO)
Counsel for pyloric stenosis risk
CDC 2021 chlamydia treatment by site and host.
Treatment
First-line
Uncomplicated urogenital chlamydia (CDC 2021):
• Doxycycline 100 mg PO BID × 7 days — preferred (more effective than azithromycin for rectal and pharyngeal infections)
• Azithromycin 1 g PO × 1 — alternative if adherence concerns or pregnancy (single dose, observed therapy)
Rectal chlamydia: doxycycline 100 mg PO BID × 7 days (azithromycin inferior)
LGV (L1-L3 serovars): doxycycline 100 mg PO BID × 21 days
Pregnancy: azithromycin 1 g PO × 1 (doxycycline contraindicated)
Neonatal conjunctivitis or pneumonia: erythromycin base or ethylsuccinate PO × 14 days (parents counseled about pyloric stenosis risk)
Ceftriaxone for gonorrhea + doxycycline for chlamydia — empiric treatment of concurrent gonorrhea is no longer routine without positive gonorrhea NAAT, but combination remains the empiric regimen when both organisms are likely (e.g., PID)
Second-line / adjunct
Levofloxacin 500 mg PO daily × 7 days — second-line, particularly for epididymitis
Expedited partner therapy (EPT) — provide medication or prescription to give to partners (legal in most states)
Test of cure NOT routinely recommended (except pregnancy at 4 weeks post-treatment)
Rescreen at 3 months for reinfection regardless of partner treatment
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