What it is (overview)

A Chlamydia antigen test is a diagnostic test used to detect evidence of infection with Chlamydia trachomatis, one of the most common causes of sexually transmitted infection (STI). “Antigen” refers to a part of the germ (bacteria) that the test is designed to identify. If chlamydia antigens are found in the sample, it suggests an active genital infection.

The test is typically performed on a urine sample (often a first-catch/first-void urine) or a swab test taken from the cervix (in many people with a vagina), vagina, urethra, rectum, or throat, depending on symptoms and sexual exposure. The goal is to diagnose chlamydia early, because many people have no symptoms but can still transmit the infection and develop complications affecting reproductive health.

What results may mean:

Positive (detected) means chlamydia antigens were found, which is consistent with a current chlamydia infection and usually leads to antibiotic treatment and partner notification/testing. Negative (not detected) means chlamydia antigens were not found in that sample. A negative result does not always rule out infection if the sample was collected too soon after exposure, from the wrong site, or if the bacterial amount is low. If symptoms continue or exposure risk is high, your clinician may recommend repeat testing or another diagnostic test.

When & why it's usually done

This chlamydia diagnostic test may be ordered when a person has symptoms of an STI, has had a possible exposure, or needs routine screening. Chlamydia is common and frequently silent, so testing is important even without symptoms.

Symptoms that may prompt testing include:

Burning with urination, increased urinary frequency, unusual vaginal or penile discharge, pelvic or lower abdominal pain, pain during sex, bleeding between periods or after sex, testicular pain or swelling, rectal pain/discharge/bleeding, or sore throat after oral sex. Because symptoms overlap with other infections (such as gonorrhea, urinary tract infection, or vaginitis), testing helps identify the cause and guide treatment.

Common reasons for testing include:

Routine STI screening (especially for sexually active adolescents and young adults), a new or multiple sexual partners, condomless sex, a partner diagnosed with chlamydia or another STI, prior history of STIs, or testing during pregnancy to reduce the risk of pregnancy and newborn complications. Clinicians may also test after a sexual assault, or when evaluating infertility or chronic pelvic pain where untreated chlamydia could be a contributing factor.

Why it matters: Untreated chlamydia can lead to serious complications such as pelvic inflammatory disease (PID), infertility, ectopic pregnancy, and chronic pelvic pain. In people with testes, it can cause epididymitis and may affect fertility. Early detection and treatment can prevent long-term reproductive health problems and reduce transmission to partners.

Common diseases related to it

• Chlamydia trachomatis infection (urogenital chlamydia)
• Cervicitis (inflammation of the cervix)
• Urethritis (inflammation of the urethra)
• Pelvic inflammatory disease (PID)
• Epididymitis
• Proctitis (rectal infection)
• Pharyngitis due to STI exposure (throat infection)
• Infertility related to prior untreated chlamydia
• Ectopic pregnancy risk related to tubal damage from prior infection
• Neonatal conjunctivitis or pneumonia (when infection is present during delivery)

Health goals where it may help

• Early detection and treatment of a sexually transmitted infection (STI)
• Protecting reproductive health and reducing risk of infertility
• Preventing complications such as pelvic inflammatory disease (PID) and chronic pelvic pain
• Safer-sex and partner-protection planning, including testing after exposure
• Routine sexual health screening for people with new or multiple partners
• Pregnancy planning and prenatal screening to reduce risks to parent and baby
• Reducing spread of genital infections through timely diagnosis and follow-up testing when recommended