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Original article
Sensitivity of Gram stain in the diagnosis of urethritis in men
  1. Mª Angeles Orellana1,
  2. Mª Luisa Gómez-Lus2,
  3. David Lora3
  1. 1Laboratorio C.E.P, Pontones, Madrid, Spain
  2. 2Departamento de Microbiología Clínica I, Universidad Complutense de Madrid, Madrid, Spain
  3. 3Clinical Research Unit, IMAS12-CIBERESP, Hospital 12 de Octubre, Madrid, Spain
  1. Correspondence to Dr Mª Angeles Orellana Miguel, C/Camino Viejo de Leganés n 36, Madrid 28019, Spain; m.a.orellana{at}


Background Acute urethritis is among the most common types of sexually transmitted diseases in men. The diagnosis usually requires microscopic evidence of urethritis, but sometimes urethral pathogens are detected in asymptomatic men without such evidence. The aims of this study were to assess the sensitivity of Gram stain in men with urethral symptoms and to relate it to the microorganisms isolated.

Methods Between January 2006 and December 2007, 491 urethral samples were analysed. The authors assessed the presence of leukocytes by Gram stain and tested specifically for Chlamydia trachomatis, Ureaplasma urealyticum, Mycoplasma hominis and Trichomonas vaginalis, as well as analysing the results of conventional culture.

Results The percentages of positive samples as a function of Gram category were two or less polymorphonuclear leukocytes (PMNLs)/high-power field (HPF) 25% (92/364), three to four PMNLs/HPF 32% (18/57) and five or more PMNLs/HPF 54% (38/70). Classing samples with more than two PMNLs/HPF as positive, the sensitivity, specificity and positive likelihood ratio for Gram stain were 38% (95% CI 30 to 46), 79% (95% CI 75 to 84) and 1.8 (95% CI 1.4 to 2.4), respectively. On the other hand, taking as positive five or more PMNLs/HPF, the sensitivity, specificity and positive likelihood ratio for Gram stain were 26% (95% CI 18 to 33), 91% (95% CI 87 to 94) and 2.7 (95% CI 1.8 to 4.2), respectively. The sensitivity of Gram stain to Neisseria gonorrhoeae, Chlamydia trachomatis and Ureaplasma urealyticum were 80% (95% CI 64 to 96), 23% (95% CI 8 to 39) and 11% (95% CI 2 to 20), respectively.

Conclusion The low sensitivity of Gram stain means that negative results do not exclude the presence of urethritis in symptomatic patients.

  • Sexually transmitted infection
  • urethritis
  • sensitivity
  • gram stain
  • bacteriology
  • chlamydia infection
  • bacteriuria
  • antimicrobial resistance
  • diagnosis
  • statistics
  • epidemiology (clinical)
  • biostatistics

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  • Competing interests None.

  • Provenance and peer review Not commissioned; externally peer reviewed.