title: Liquid chromatography-tandem mass spectrometry determination of oxalate in spot urine.
abstract: BACKGROUND
  For assessment of total oxalic acid (OX) status, reliable quantification of OX in both urine and plasma is important. For urine, but not plasma, a commercial kit is available. We have recently described a LC-MSMS method for OX in plasma. The aim of the present study was to evaluate the usefulness of this assay for urine. We also wanted to evaluate if 24 h urine collection could be substituted by OX/creatinine-ratio (U-OX/crea) in spot-urine, and establish precursory reference intervals for U-OX/crea in children and adults.
  
  
  METHODS
  Acidified urines were analysed and relevant validation parameters assessed. Diurnal excretion patterns were investigated in nine healthy volunteers on self-chosen diets. For method comparison, 29 urine samples were analysed with both the present method and a commercial urine-oxalate kit. Precursory reference values for U-OX/crea in children and adults (N=103, 1 month-76 years) were calculated.
  
  
  RESULTS
  The within-batch coefficient of variation (CV) was 2.5% and a relative recovery of 97% in urine spiked with 5-200 micromol/L OX was found. The LC-MSMS method gave 7.9% higher OX values compared to the kit. No significant diurnal pattern of U-OX/crea was observed. U-OX/crea in children decreases with age, with no gender dependency. In adults no age variation was found, but females had somewhat higher U-OX/Crea compared to males.
  
  
  CONCLUSIONS
  The LC-MSMS method has proven useful for urinary OX quantification. Random spot-urine samples can be used. Age-dependent reference limits for U-OX/crea must be applied in children, in contrast to adults.
authors.affiliation: Department of Medical Biochemistry, Oslo University Hospital Rikshospitalet, Norway. katja.elgstoen@rikshospitalet.no
authors.forename: Katja B P
authors.initials: KB
authors.lastname: Elgstoen
authors.forename: Berit
authors.initials: B
authors.lastname: Woldseth
authors.forename: Kari
authors.initials: K
authors.lastname: Hoie
authors.forename: Lars
authors.initials: L
authors.lastname: Morkrid
mesh_terms: D000328:Adult; D017677:Age Distribution; D000368:Aged; D000375:Aging / Q000652:urine; D002853:Chromatography, Liquid / Q000379:methods*; D002940:Circadian Rhythm; D003404:Creatinine / Q000652:urine; D005260:Female; D006262:Health; D006801:Humans; D008297:Male; D008875:Middle Aged; D010070:Oxalates / Q000652:urine*; D012015:Reference Standards; D015203:Reproducibility of Results; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D010070:Oxalates; D003404:Creatinine