Measurement of urinary total 13C and 13C urea by isotope ratio mass spectrometry after administration of lactose [13C]-ureide

A method of measuring total C-13 excreted in urine after oral administration of lactose [C-13]-ureide was developed using isotope ratio mass spectrometry, Furthermore, a method to measure C-13 urea excreted in the urine was developed. Each urine sample collected over a 24 hour period, after administration of the tracer dose, was analysed for both total C-13 and C-13 urea. Combustion of the dried urine samples allowed measurement of the total C-13 content, Treatment of urine samples with urease (EC 3.5.1.5) and analysis by isotope ratio mass spectrometry of the CO2 evolved allowed measurement of C-13 urea in the urine sample. The total C-13 and C-13 urea contents of each urine sample, obtained throughout the protocol, were compared to total C-13 and C-13 urea contents of a urine sample taken before the test. This allowed calculation of the fraction of tracer incorporated into urea and the fraction of tracer excreted in total. Analyses showed that approximately 15% of the dose administered, in terms of C-13, was recovered in the urine over the sampling period. Further analysis for urinary C-13 urea showed that less than 1% of the label was incorporated into urea excreted over the sampling period. Copyright (C) 1999 John Wiley & Sons, Ltd.