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 μmol/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. Conclusion. 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.

Key Words::

• Oxalate
• creatinine
• urine
• reference values
• primary hyperoxaluria

Acknowledgements

Many thanks are due to Anna Greta Bakken, Oslo University Hospital Ullevaal, for her contribution to the method comparison work and to Egil Jellum for scientific advice and discussions.

Declaration of interest: We have had no involvements that might raise the question of bias in the work reported or in the conclusions, implications, or opinions stated.