Calculation of titratable acidity from urinary stone risk factors.

Using urine samples and standard solutions, this study demonstrates that the existing procedure for measuring titratable acidity in the urine is not reliable and may result in overestimates of up to 25%. The accuracy is affected by loss of CO2, the presence of uric acid crystals, and the precipitation of calciumphosphate phases during the titration. A method is presented for calculating titratable acidity, using a number of routinely-measured urine components and a computer program for calculating complex equilibria in the urine. The calculated titratable acidity is shown to be more reliable then the measured one. The results are compiled in a nomogram from which the titratable acidity can be directly read. When the parameters of urine pH, PO4 content and pCO2 are used, the accuracy of the nomogram is > 96% for urine samples with a pH value above 6.0 and > 89% for urine samples with a pH value below 6.0. For all samples, the accuracy is improved to > 97% when the nomogram using uric acid and citrate content is used in additionally.