Risk factors of calcium stone formation in patients with primary Sjögren's syndrome.

Distal renal tubular acidosis (dRTA), which occurs in patients with primary Sjögren's syndrome (SS), is a risk factor for the development of urolithiasis. Twenty-seven patients with SS were evaluated with respect to biochemical risk factors of calcium stone formation. Sixteen had no history of urolithiasis (group 1) whereas 11 had such a history (group 2). The stone composition was known for seven of the patients, and calcium phosphate was the major stone constituent in all of them. dRTA was present in all patients in group 2, and in 7 of the 16 patients in group 1. Hypocitraturia was common in both groups, and the urinary excretion of citrate did not differ between the two groups. There was a higher urinary excretion of calcium and urate in group 2 and this group also had a higher urine volume. The risk of forming a urine supersaturated with calcium oxalate (CaOx) expressed in terms of AP(CaOx)index(s), which is an approximate estimate of the ion-activity product of CaOx calculated for a 24-h urine volume of 1500 ml, was higher in stone formers. A similarly derived estimate of the ion-activity product of calcium phosphate, AP(CaP)index(s), was calculated for a urine pH of 7. Although AP(CaP)index(s) was not significantly higher in group 2, there was a good correlation between AP(CaP)index(s) and AP(CaOx)index(s). We conclude that the urine composition in patients with SS, dRTA and urolithiasis is similar to that of other stone-forming patients with dRTA, and recurrence preventive therapy can be designed as for these patients.