Experimental and computer modelling speciation studies of the effect of pH and phosphate on the precipitation of calcium and magnesium salts in urine

Background: pH and phosphate concentration are the major determinants of precipitation in urine of the salts of calcium and magnesium. This study aims to model the process of salt precipitation and establish whether the acidification of urine samples is necessary for the accurate measurement of calcium and magnesium in a clinical laboratory setting. Methods: Urine samples were collected from 21 patients, aliquots were taken from each patient sample and the pH was adjusted to cover the range 2.0-10.0. The analytical and biological variation for each analyte was established and used to calculate percentage changes and critical differences. The critical difference was used to assess whether there was a significant difference between acidified and un-acidified samples. The JESS ( Joint Expert Speciation System) thermodynamic computer-modelling program was used to predict the distribution of salt species formed with varying pH values and phosphate levels in simulated urine. Results: The results showed that at a pH greater than 6.5, measured calcium, magnesium and phosphate significantly decreased as a result of precipitation (p < 0.0001), although the critical difference was generally not exceeded. Computer modelling showed that both pH and phosphate concentration affected the distribution of salt species formed, as well as the precipitation patterns of calcium and magnesium phosphates. Overall, calcium phosphate precipitation tends to predominate at lower phosphate concentrations and at pH values below about 6.5, while both calcium and magnesium phosphate precipitation occur at higher phosphate concentrations and pH values greater than 6.5. Conclusions: For accurate analysis of these analytes in urine, the pH should be routinely measured and acidification should be undertaken prior to analysis if the pH is greater than 6.5. Based on the findings of this study, acidification or the lack of it does not result in a clinically significant change in calcium, magnesium and phosphate measured in urine. This study also predicted the likely salt species formed at varying urinary pH values and phosphate concentrations.