Gram stain of urethral discharge and nucleic acid amplification tests (NAATs) are the main laboratory investigations in an appropriate clinical setting.

• Urethritis is considered on the basis of any of the following:^[1][2][3]
  • Mucoid, mucopurulent, or purulent discharge on examination
  • Gram stain of urethral secretions demonstrating ≥2 WBC per field
  • Positive leukocyte esterase test on first-void urine or microscopic examination of sediment from a spun first-void urine demonstrating ≥10 WBC per high power field.^[4][5]

• Once the diagnosis based on any of the above mentioned methods is confirmed, further testing to establish the etiology by nucleic acid amplification tests (NAATs) is recommended.^[3]
• Nucleic acid amplification tests (NAATs) are recommended if the diagnostic criteria have not been met and the clinical suspicion for urethritis is high.^[3]

• The presence of PMNs without any visible organisms is consistent with nongonococcal urethritis.^[1]
• Presumed gonococcal infection is established by documenting the presence of WBC containing intracellular purple diplococci in gram stain.^[6]

• 
  Gram stain of urethral discharge. Note the numerous PMNs without intracellular bacteria in nongonococcal urethritis.
• 
  Gram stain of urethral discharge. Note the numerous PMNs with intracellular diplococcal bacteria in gonococcal urethritis.

These tests are more sensitive than cultures and retain high specificity. NAATs have replaced culture as the test of choice for identification of Chlamydia trachomatis, Neisseria gonorrhoeae, and Mycoplasma genitalium.^[7][8]