title: Using Hounsfield unit measurement and urine parameters to predict uric acid stones.
abstract: OBJECTIVE
  To determine the in vivo ability to predict uric acid stone composition by Hounsfield units (HU) with the addition of urine parameters.
  
  
  METHODS
  We reviewed all consecutive stones sent for analysis during a 4-year period from our institution for patients with an in-house computed tomography (CT) scan within the prior 6 months and urinalysis within the prior week. CT scans were independently reviewed by a radiologist blinded to stone composition.
  
  
  RESULTS
  Of the 507 patients with stones sent for analysis, 235 met the criteria for inclusion. Analysis showed 212 stones were predominantly calcium-based, and 22 were predominantly uric acid in composition. There was a significant difference between calcium stones and uric acid stones in mean HU (890 ± 20 vs 484 ± 44; P <.01) and urine pH (6.4 ± 0.8 vs 5.1 ± 0.2; P <.01). Receiver operating characteristic curve evaluation gave optimal predictive values of HU ≤494 (rounded to 500) and pH of ≤5.5 to predict uric acid stones. The combination of HU and pH criteria resulted in a sensitivity of 86% and a specificity of 98%, with a positive predictive value of 80%, which increased to 90% if we limited to stones >4 mm.
  
  
  CONCLUSIONS
  Uric acid stones show a significant difference in HU and urine pH from calcium stones, and the use of both criteria is superior to either separately. For a stone >4 mm, a HU ≤500 and pH ≤5.5 has a positive predictive value of 90% for uric acid composition. Our prediction model gives a straightforward tool that can be easily measured to predict a uric acid stone.
authors.affiliation: Division of Urology, Albany Medical College, Albany, NY, USA. spettes@mail.amc.edu
authors.forename: Sara
authors.initials: S
authors.lastname: Spettel
authors.forename: Paras
authors.initials: P
authors.lastname: Shah
authors.forename: Kiran
authors.initials: K
authors.lastname: Sekhar
authors.forename: Allen
authors.initials: A
authors.lastname: Herr
authors.forename: Mark D
authors.initials: MD
authors.lastname: White
mesh_terms: D002118:Calcium / Q000032:analysis; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D012372:ROC Curve; D016037:Single-Blind Method; D014057:Tomography, X-Ray Computed; D014527:Uric Acid / Q000032:analysis*; D014545:Urinary Calculi / Q000737:chemistry / Q000000981:diagnostic imaging* / Q000652:urine*; D014556:Urine / Q000737:chemistry
chemical_list: D014527:Uric Acid; D002118:Calcium