title: Variability in urinary oxalate measurements between six international laboratories.
abstract: BACKGROUND
  Hyperoxaluria is a major risk factor for kidney stone formation. Although urinary oxalate measurement is part of all basic stone risk assessment, there is no standardized method for this measurement.
  
  
  METHODS
  Urine samples from 24-h urine collection covering a broad range of oxalate concentrations were aliquoted and sent, in duplicates, to six blinded international laboratories for oxalate, sodium and creatinine measurement. In a second set of experiments, ten pairs of native urine and urine spiked with 10 mg/L of oxalate were sent for oxalate measurement. Three laboratories used a commercially available oxalate oxidase kit, two laboratories used a high-performance liquid chromatography (HPLC)-based method and one laboratory used both methods.
  
  
  RESULTS
  Intra-laboratory reliability for oxalate measurement expressed as intraclass correlation coefficient (ICC) varied between 0.808 [95% confidence interval (CI): 0.427-0.948] and 0.998 (95% CI: 0.994-1.000), with lower values for HPLC-based methods. Acidification of urine samples prior to analysis led to significantly higher oxalate concentrations. ICC for inter-laboratory reliability varied between 0.745 (95% CI: 0.468-0.890) and 0.986 (95% CI: 0.967-0.995). Recovery of the 10 mg/L oxalate-spiked samples varied between 8.7 ± 2.3 and 10.7 ± 0.5 mg/L. Overall, HPLC-based methods showed more variability compared to the oxalate oxidase kit-based methods.
  
  
  CONCLUSIONS
  Significant variability was noted in the quantification of urinary oxalate concentration by different laboratories, which may partially explain the differences of hyperoxaluria prevalence reported in the literature. Our data stress the need for a standardization of the method of oxalate measurement.
authors: Department of Internal Medicine, University of Texas, Southwestern Medical Center, Dallas, TX, USA.
authors: Naim M
authors: NM
authors: Maalouf
authors: Beverley
authors: B
authors: Adams Huet
authors: Andreas
authors: A
authors: Pasch
authors: John C
authors: JC
authors: Lieske
authors: John R
authors: JR
authors: Asplin
authors: Roswitha
authors: R
authors: Siener
authors: Albrecht
authors: A
authors: Hesse
authors: Jean-Marc
authors: JM
authors: Nuoffer
authors: Felix J
authors: FJ
authors: Frey
authors: John
authors: J
authors: Knight
authors: Ross P
authors: RP
authors: Holmes
authors: Joseph E
authors: JE
authors: Zerwekh
authors: Olivier
authors: O
authors: Bonny
mesh_terms: D019411:Clinical Laboratory Techniques / Q000592:standards; D006801:Humans; D007391:International Cooperation; D007753:Laboratories; D010070:Oxalates / Q000652:urine*; D015203:Reproducibility of Results
chemical_list: D010070:Oxalates