Quantification of orotic acid in dried filter-paper urine samples by stable isotope dilution

A rapid, sensitive, and specific method for quantification of orotic acid from dried filter-paper urine samples is described. The method involves stable isotope dilution with 1,3-[15N2]orotic acid analysis by gas chromatography-mass spectrometry. The assay is sufficiently sensitive to be used with solvent extraction techniques commonly used for urinary organic acid analysis. Extraction efficiencies of both native and isotopic orotic acid from dried filter paper and from water were 31% and 28%, respectively. The concentration of orotic acid in dried filter-paper urine specimens from 50 healthy controls was 1.1 +/- 0.67 (mean +/- SD) mmol/mol of urinary creatinine. The same 50 urine samples, analyzed directly from a 5-mL aliquot of liquid urine, gave values of 0.93 +/- 0.51. The correlation coefficient between the results obtained by the two different collection methods was 0.87. Age-related reference values in filter-paper samples are also reported. The concentrations, which are normalized to urinary creatinine, decrease with age. This method is applicable to rapid screening for urea cycle disorders and may also be used for carrier testing of ornithine transcarbamylase deficiency.