First morning urine pH in the diagnosis of renal tubular acidosis with nephrolithiasis.

Few reports of patients with renal stones compare fasting first morning urine pH with the lowest urine pH after a single oral dose of 0.1 g/kg ammonium chloride. We studied these values in 110 individuals, including 96 stone formers and 14 non-stone forming controls. We hypothesized that fasting first morning urine pH > or = 6.10 is abnormal. Nine of ten patients who had urine pH > or = 5.25 after NH4Cl, had pH > or = 6.10 for both a single and a second first morning urine, giving a sensitivity of 90%. In contrast, a single fasting first morning urine pH > or = 6.10 was found in 19 of 100 individuals with pH < 5.25 after ammonium chloride. However, only 4 of these 100 individuals had abnormal fasting first morning pH > or = 6.10 on two occasions, giving a specificity of 96% for the two urines. Useful prediction of urine pH > or = 5.25 after ammonium chloride occurs when two fasting first morning urines have pH > or = 6.10.