title: Analysis of the utility of stone gram stain in urolithiasis treated with percutaneous nephrolithotomy.
abstract: OBJECTIVE
  To define the sensitivity and specificity of stone gram stain for infected urolithiasis treated with percutaneous nephrolithotomy (PCNL).
  
  
  METHODS
  PCNL procedures performed at our institution were analyzed between January 2009 and May 2013. Stone fragments were sent in a sterile fashion for aerobic and fungal cultures. A gram stain and fungal smear were performed on the stones and reported within 24 hours of collection.
  
  
  RESULTS
  A total of 228 patients underwent 248 PCNLs. Of the 248 stones, 81 (33%) had a positive stone culture. Stone gram stain was positive in 31 cases and negative in 50. There were 167 negative stone cultures, and in these cases, gram stain was positive in 5 and negative in 162. The calculated sensitivity and specificity of stone gram stain were 38% and 97%. The positive and negative predictive values were 86% and 76%, respectively. In the subset of 16 patients with positive stone fungal cultures, fungal smear was performed in 12 and was positive in 4, giving fungal smear a sensitivity of 33%.
  
  
  CONCLUSIONS
  The results of this study suggest that stone gram stain cannot be relied on to detect a positive stone culture and may fail to detect up to 62% of infected stones. However, when positive, gram stain accurately predicts a positive stone culture in 86% of cases.
authors.affiliation: Department of Urology, Mayo Clinic, Rochester, MN.
authors.forename: Patrick A
authors.initials: PA
authors.lastname: Cockerill
authors.affiliation: Department of Urology, Mayo Clinic, Rochester, MN.
authors.forename: Marcelino E
authors.initials: ME
authors.lastname: Rivera
authors.affiliation: Department of Urology, Mayo Clinic, Rochester, MN. Electronic address: Krambeck.Amy@mayo.edu.
authors.forename: Amy E
authors.initials: AE
authors.lastname: Krambeck
mesh_terms: D000328:Adult; D000368:Aged; D000900:Anti-Bacterial Agents / Q000627:therapeutic use; D001424:Bacterial Infections / Q000175:diagnosis / Q000209:etiology; D015331:Cohort Studies; D005260:Female; D005500:Follow-Up Studies; D005658:Fungi / Q000302:isolation & purification; D005840:Gentian Violet / Q000032:analysis; D006090:Gram-Negative Bacteria / Q000302:isolation & purification; D006094:Gram-Positive Bacteria / Q000302:isolation & purification; D006801:Humans; D007669:Kidney Calculi / Q000175:diagnosis / Q000382:microbiology* / Q000473:pathology / Q000601:surgery*; D008297:Male; D008826:Microbial Sensitivity Tests; D008875:Middle Aged; D009403:Nephrostomy, Percutaneous / Q000009:adverse effects / Q000379:methods*; D010619:Phenazines / Q000032:analysis; D011183:Postoperative Complications / Q000517:prevention & control; D011237:Predictive Value of Tests; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D016896:Treatment Outcome; D014552:Urinary Tract Infections / Q000175:diagnosis / Q000209:etiology / Q000517:prevention & control*
chemical_list: D000900:Anti-Bacterial Agents; C035863:Gram's stain; D010619:Phenazines; D005840:Gentian Violet