Objective: To survey the urinary risk factors associated with recurrent calcium stone and the contribution of renal tubular acidosis to the prevalence of recurrent calcium stone formation in Thai recurrent stone formers.
Material and Method: There were 86 consecutive recurrent calcium stone formers. Three-day dietary record, serum biochemical parameters, first morning urine pH, and two 24-hour urine collections were obtained from each subject. Urinary risk factors for calcium stone formation were determined from the average of the 2-day urine collection. Normal controls were 34 subjects matched for aged, sex, and weight, and without a history of renal stone formation.
Results: Seven patients (8.1%) were diagnosed as incomplete renal tubular acidosis (iRTA). Among the 79 idiopathic calcium stone formers (ISF), 69.6%, 15.2%, 10.1%, 7.2% and 1.3% of patients were hypocitraturia, hypercalciuria, low urinary volume, hyperuricosuria and hyperoxaluria, respectively. The common combinations of risk factors were hypocitraturia plus low urine output (8.9%) or plus hypercalciuria (7.6%). There were significant differences between ISF and normal controls in urinary oxalate excretion (0.16±0.01 vs 0.12±0.01, p<0.05), urinary calcium/citrate ratio (4.49±0.50 vs 2.83±0.34, p<0.01) and ion activity product for calcium oxalate stone (0.46±0.03 vs 0.33±0.03, p<0.05). Urinary citrate in ISF varied directly with net alkaline absorption (r = 0.34, p<0.005) and urinary potassium (r = 0.54, p<0.001). There were significant correlations between urinary calcium excretion and both sodium excretion (r = 0.42, p<0.001) and urea excretion (r = 0.41, p<0.001) in ISF. There were seven (8.1%) with incomplete renal tubular acidosis. Patients with iRTA tended to have less urinary citrate and higher calcium/citrate ratio than did ISF, but hypercalciuria was uncommon.
Conclusions: Hypocitraturia was the most common urinary risk factor found in Thai recurrent idiopathic calcium stone formers followed by hypercalciuria and low urinary volume. Almost one-fourth of the stone formers had multiple risk factors. Hypocitraturia might result from low potassium and low alkaline intake. iRTA was common among recurrent calcium stone formers. Determination of morning urine pH should be a part of the investigations for urinary risk factors to avoid overlooking the diagnosis of iRTA.

Keywords: Urolithiasis, Renal stone, Hypocitraturia, Hypercalciuria, Hyperoxaluria, Hyperuricosuria