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

Background: Patients with CaHPO(4) 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 CaHPO(4) 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 (U(NH4)), sulphate (U(SO4)) and citrate were measured and diet net alkali was calculated.

Results: Of the 26 patients, 13 had persistently alkaline urine. In 7/13, U(NH4) (68 +/- 13 mEq/day) and U(SO4) (57 +/- 7 mEq/day) were both high. In 6/13 patients, U(NH4) was the usual 31 +/- 3 mEq/day; in 4/6, U(NH4)/U(SO4) 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 U(NH4)/U(SO4) (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 NH(3) into the late distal nephron in the female.