The relation was investigated of urinary sodium to potassium ratio in first morning voided urine (spot urine) to urinary stone disease in 3,625 men and women aged 25 to 74 years participating in the baseline examination of the Gubbio Population Study. History of urinary stone disease (excretion of stone, and/or radiographic or ultrasonic evidence of urinary stone, and/or operation for urinary stone removal) was reported by 127 individuals (3.50%). Prevalence of urinary stone disease was lower in women than in men (2.59 and 4.58%, P < 0.001) and positively related to age (P < 0.001). Compared to nonstone formers, stone formers (N = 127) had higher urinary sodium to potassium ratio (P < 0.01), with similar plasma potassium and sodium concentration. In both sexes, urinary stone disease was positively related (P < 0.001) to sodium to potassium ratio: quartile analysis of this ratio showed that prevalence of stone formers in quartile 4 compared to quartile 1 was 3.33 times higher in women (P < 0.005, 95% confidence interval 1.36/8.60) and 2.71 times higher in men (P < 0.004, 95% confidence interval 1.35/5.93). In multiple logistic regression, urinary stone disease was significantly related to age, sex, and urinary sodium to potassium ratio (P < 0.01), controlled for other possible confounders, with or without exclusion of stone formers with plasma creatinine >1.20 mg/dl. In an alternative model, with urinary sodium to potassium ratio not included, urinary stone disease was positively related to urinary sodium to creatinine ratio (P < 0.001) and weakly (P = 0.079) related inversely to urinary potassium to creatinine ratio. The data indicate that high sodium to potassium ratio in the urine is related to urinary stone disease.
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