Studies to identify the basis for an alkaline urine pH in patients with calcium hydrogen phosphate kidney stones

Background. Patients with CaHPO4 kidney stones belong to a diagnostic category that has a high urine pH as its common feature. Our objective was to provide a new clinical approach to examine the basis for this high pH. Methods. The study group consisted of 26 CaHPO4 stone formers and 28 normal volunteers. Urine was collected q2h plus an overnight sample to identify patients with a urine pH > 6.5 for 12/24 h. Urine ammonium (UNH4), sulphate (USO4) and citrate were measured and diet net alkali was calculated. Results. Of the 26 patients, 13 had persistently alkaline urine. In 7/13, UNH4 (68 ± 13 mEq/day) and USO4 (57 ± 7 mEq/day) were both high. In 6/13 patients, UNH4 was the usual 31 ± 3 mEq/day; in 4/6, UNH4/USO4 was 0.9 ± 0.1; the cause of the alkaline urine pH seemed to be a dietary alkali load because the rise in urine pH was episodic and coincided with a high net diet alkali load and peak citrate excretion rates. The remaining two patients had a high UNH4/USO4 (2.2 and 1.6). Citrate excretion was very low in the male, but not in the female patient. Conclusions. There are heterogeneous causes for a persistently high urine pH. Two of the patients had a possible molecular basis: the lesion could be a low proximal convoluted tubule cell pH in the male and an increased entry of NH3 into the late distal nephron in the female.