[Reevaluation of optimum extraction condition for urinary organic acids using a stable isotope dilution technique]

We reevaluated the optimum conditions for analysis of 13 urinary organic acids using solvent extraction and GC/MS by the stable isotope dilution technique. The acids analyzed were uracil, and lactic, oxalic, 3-hydroxybutyric, succinic, fumaric, glutaric, adipic, pyroglutamic, 2-ketoglutaric, orotic, sebacic, and citric acids. Analytical recovery and accuracy for 13 organic acids ranged from 90 to 107% and from 1.6 to 13.7%, respectively. The optimal pH for most organic acids was 1-2, while that for oxalic and citric acids was 0.25, and that for 2-ketoglutaric and orotic acids was 0.5. The presence of urinary albumin decreased the extraction rates of organic acids, especially of orotic and citric acids; in slight albuminuria (0.5 g/l) the extraction recovery (the extraction rate with albumin/that without albumin) of orotic and citric acid was 38% and 67%, respectively, and in more marked albuminuria (5 g/l), 6% and 26%. Membrane pretreatment with Centricon-3 improved these extraction rates under the condition of albuminuria. Dehydration with the desiccant agent decreased urinary acid extraction rates, especially of uracil and orotic and citric acids. The extraction rates of these three organic acids was decreased in albuminuria and by the desiccant agent. Accurate quantitative analysis of urinary organic acids by the stable isotope dilution technique is necessary for routine examination conducted in clinical laboratories.