The role of the diurnal variation of urinary pH in determining stone compositions.

The diurnal variation of urinary pH and urinary excretion of stone-forming constituents were investigated on 79 stone patients. Composition of the stones was analyzed by means of infrared spectrophotometry. In uric acid stone patients urinary pH was low during the 24-hour period. In mixed calcium oxalate-calcium phosphate stone patients urinary pH was low in the early morning but high in the daytime and at night. In magnesium ammonium phosphate stone patients urinary pH was high throughout the entire day. On the other hand, urinary excretion of calcium was high in the mixed calcium oxalate-calcium phosphate stone patients and low in uric acid or magnesium ammonium phosphate stone patients. These findings suggested that the diurnal variation of urinary pH and urinary excretion of calcium might have an important role in determining stone composition.