Role of the diurnal variation of urinary pH and urinary calcium in urolithiasis: A study in outpatients

Abstract Background: We previously reported that the urinary excretion of calcium and the diurnal variation of urinary pH were important for stone formation in hospitalized inpatients with hospital standard diet. Because almost all urinary stones are formed in outpatients with ambulatory free diet, it is essential to investigate these factors in outpatients. Methods: We examined the urinary excretion of calcium, oxalate, uric acid, phosphate and magnesium in 96 male outpatients and 142 male inpatients. We also studied the relationship between the stone composition and the diurnal variation of urinary pH in 32 male outpatients (five uric acid stones (UA), 13 pure calcium oxalate stones (CaOX) and 14 mixed calcium oxalate and calcium phosphate stones (CaOX–CaP)) and 53 male inpatients (nine UA, 15 CaOX and 29 CaOX–CaP). Results: There was a significant difference in the urinary excretion of calcium among outpatients with UA, CaOX and CaOX–CaP (133 ± 96 vs 219 ± 97 vs 268 ± 102 mg per day, P < 0.05). In outpatients with UA, urinary pH was constantly low throughout the entire day. In contrast, outpatients with CaOX and those with CaOX–CaP had diurnal variation of urinary pH that was low in the early morning, followed by elevation in the daytime and was lowered in the night. The pHs in the early morning, afternoon and night were significantly higher in outpatients with CaOX–CaP than in those with CaOX. Conclusion: The diurnal variation of urinary pH and the urinary calcium are important for stone formation.