This test is looking for evidence of infection by the bacterium Chlamydia trachomatis. Chlamydia is the most common bacterial sexually transmitted disease (STD) in the United States. About 75% of infected women and 50% of infected men have no symptoms; some may experience only mild symptoms. For women, symptoms, if they occur, include bleeding between menstrual periods and after sexual intercourse, abdominal pain, painful intercourse, and an abnormal vaginal discharge. For men, symptoms include pus or milky discharge from the penis and inflammation of the prostate (prostatitis) or of the rectal area (proctitis). Both sexes can experience painful or frequent urination.