Chlamydia Ag This test detects a chlamydia infection, the most common bacterial sexually transmitted infection (STI) in the United States. A chlamydia test looks for the bacteria that cause the infection (Chlamydia trachomatis). For this test, you provide a urine sample. Or your provider takes a swab of fluid from your vagina, anus, throat or eye (depending on where the infection might be).