title: Stone size limits the use of Hounsfield units for prediction of calcium oxalate stone composition.
abstract: OBJECTIVE
  To evaluate the role of stone size in predicting urinary calculus composition using Hounsfield units on noncontrasted computed tomography (CT) scan.
  
  
  METHODS
  A retrospective review was performed for all patients who underwent ureteroscopy or percutaneous nephrolithotomy during a 1-year period, had a stone analysis performed, and had CT imaging available for review. All CT scans were reviewed by a board-certified radiologist. Variables evaluated included age, sex, body mass index, stone size, stone location, Hounsfield units (HUs), and stone composition.
  
  
  RESULTS
  We identified a total of 91 patients (41 men and 50 women) with CT imaging and stone analysis available for review. Stone analysis showed 41 calcium oxalate monohydrate (CaOxMH), 13 calcium oxalate dihydrate, 29 calcium phosphate, 5 uric acid, 2 struvite, and 1 cystine stone. Average age was 46 years, and average body mass index was 32 kg/m2. Measured HUs varied significantly with size for CaOxMH and calcium oxalate dihydrate stones (P values <.05), but not for calcium phosphate stones (P = .126). Using a CaOxMH identification value of 700-1000 HUs, 28 of 41 stone compositions (68%) would not have been correctly identified, including all 10 (100%) small (<5 mm) stones, 13 of 22 (59%) medium (5-10 mm) stones, and 5 of 9 large (>10 mm) stones (55%).
  
  
  CONCLUSIONS
  For calcium stones, the ability of CT HUs to predict stone composition was limited, likely due to the mixed stone composition. Within a cohort of CaOxMH stone formers, measured HUs varied linearly with stone size. All stones <5 mm were below thresholds for CaOxMH composition.
authors.affiliation: Department of Urology, University of Kentucky, Lexington, KY.
authors.forename: Gregory
authors.initials: G
authors.lastname: Stewart
authors.affiliation: Department of Urology, University of Kentucky, Lexington, KY.
authors.forename: Lewis
authors.initials: L
authors.lastname: Johnson
authors.affiliation: Department of Radiology, University of Kentucky, Lexington, KY.
authors.forename: Halemane
authors.initials: H
authors.lastname: Ganesh
authors.affiliation: Department of Surgery, University of Kentucky, Lexington, KY.
authors.forename: Daniel
authors.initials: D
authors.lastname: Davenport
authors.affiliation: Department of Urology, University of Kentucky, Lexington, KY.
authors.forename: Woodson
authors.initials: W
authors.lastname: Smelser
authors.affiliation: Department of Urology, University of Kentucky, Lexington, KY.
authors.forename: Paul
authors.initials: P
authors.lastname: Crispen
authors.affiliation: Department of Urology, University of Kentucky, Lexington, KY. Electronic address: lewis.johnson@uky.edu.
authors.forename: Ramakrishna
authors.initials: R
authors.lastname: Venkatesh
mesh_terms: D000328:Adult; D000368:Aged; D002129:Calcium Oxalate / Q000032:analysis*; D005260:Female; D006801:Humans; D007669:Kidney Calculi / Q000737:chemistry* / Q000000981:diagnostic imaging / Q000473:pathology*; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed; D014514:Ureteral Calculi / Q000737:chemistry* / Q000000981:diagnostic imaging / Q000473:pathology*; D055815:Young Adult
chemical_list: D002129:Calcium Oxalate