A Urethral Swab is done to test for STDs that cause Urethritis. This includes testing for Gonorrhea, Chlamydia, NSU (Mycoplasma, Ureaplasma, Trichomonas). To collect the sample, a cotton swab is then gently inserted about three-fourths inch (2 centimeters) into the urethra and turned. To get a good sample, the test should be done at least 2 hours after urinating.