Twenty-four-hour urine chemistries and the risk of kidney stones among women and men

Background. Results of a 24-hour urine collection are integral to the selec tion of the most appropriate intervention to prevent kidney stone recurrenc e. However, the currently accepted definitions of normal urine values are n ot firmly supported by the literature. In addition, little information is a vailable about the relationship between risk of stone formation and the lev els of urinary factors. Unfortunately, the majority of previous studies of 24-hour urine chemistries were limited by the inclusion of recurrent stone formers and poorly defined controls. Methods. We obtained 24-hour urine collections from 807 men and women with a history of kidney stone disease and 239 without a history who were partic ipants in three large ongoing cohort studies: the Nurses' Health Study I (N HS I; mean age of 61 years), the Nurses' Health Study II (NHS II; mean age of 42 years), and the Health Professionals Follow-up Study (HPFS; mean age of 59 years). Results. Mean 24-hour urine calcium excretion was higher and urine volume w as lower in cases than controls in NHS I (P less than or equal to 0.01), NH S II (P less than or equal to 0.13) and HPFS (P less than or equal to 0.01) , but urine oxalate and citrate did not differ. Among women, urine uric aci d was similar in cases and controls but was lower in cases in men (P = 0.06 ). The frequency of hypercalciuria was higher among the cases in NHS I (P = 0.26), NHS II (P = 0.03), and HPFS (P = 0.02), but 27, 17, and 14% of the controls, respectively, also met the definition of hypercalciuria. The freq uency of hyperoxaluria did not differ between cases and controls, but was t hree times more common among men compared with women. After adjusting for t he other urinary factors, the relative risk of stone formation increased wi th increasing urine calcium levels and concentration in all three cohorts b ut not in a linear fashion. Compared with individuals with a urine calcium concentration of <75 mg/L, the relative risk of stone formation among those with a urine calcium concentration of <greater than or equal to>200 mg/L f or NHS I was 4.34 (95% CI, 1.59 to 11.88), for NHS II was 51.09 (4.27 to 61 1.1), and for HPFS was 4.30 (1.71 to 10.84). There was substantial variatio n in the relative risks for stone formation for the concentration of other urine factors within the different cohorts. Conclusions. The traditional definitions of normal 24-hour urine values nee d to be! reassessed, as a substantial proportion of controls would be defin ed as abnormal, and the association with risk of stone formation may be con tinuous rather than dichotomous. The 24-hour urine chemistries are importan t for predicting risk of stone formation, but the significance and the magn itudes of the associations appear to differ by age and gender.