Relative Supersaturation of 24-Hour Urine and Likelihood of Kidney Stones

PURPOSE: The relative supersaturation of calcium oxalate, calcium phosphate and uric acid is used clinically in kidney stone prevention. The magnitude of the association between relative supersaturation and stone risk requires further quantification. MATERIALS AND METHODS: We performed a cross-sectional study using 24-hour urine collections from the NHS (Nurses' Health Study) I and II, and HPFS (Health Professional Follow-up Study) cohorts to quantify the association between the relative supersaturation of calcium oxalate, calcium phosphate and uric acid, and the likelihood of stone formation. RESULTS: The OR of being a stone former was 5.85 (95% CI 3.40-10.04) in NHS I, 6.38 (95% CI 3.72- 11.0) in NHS II and 6.95 (95% CI 3.56-13.6) in HPFS for the highest category of calcium oxalate relative supersaturation compared with less than 1.0. The OR of being a stone former was 1.86 (95% CI 0.94-3.71) in NHS I, 4.37 (95% CI 2.68-7.10) in NHS II and 3.59 (95% CI 2.04-6.31) in HPFS for the highest category of calcium phosphate relative supersaturation compared with less than 1.0. For uric acid relative supersaturation the OR of being a stone former was 4.30 (95% CI 2.34-7.90) in NHS I and 2.74 (95% CI 1.71-4.40) in NHS II for the highest relative supersaturation category compared with less than 1.0. In HPFS the uric acid relative supersaturation was not significantly associated with the likelihood of stone formation. CONCLUSION: The likelihood of being a stone former increases with higher relative supersaturation of calcium oxalate and calcium phosphate in men and women, and with higher relative supersaturation of uric acid in women.