A study was conduced at Channing Laboratory, Department of Medicine, Brigham in collaboration with Women’s’ Hospital, Harvard Medical School, Boston, Massachusetts, USA, to assess the relationship between risk of stone formation and the levels of urinary factors. This study proves without doubt that results of a 24-hour urine collection are integral to the selection of the most appropriate intervention to prevent kidney stone recurrence.

Currently, the accepted definitions of normal urine values are not firmly supported by the literature. In addition, there is very little information about the relationship between risk of stone formation and the levels of urinary factors. Unfortunately, the majority of previous studies of 24-hour urine chemistries were limited by the inclusion of recurrent stone formers and inadequately defined controls.

The study was conducted on 807 men and women with a history of kidney stone disease and 239 without a history who were participants in three large ongoing cohort studies. Their 24 hour urine samples were collected and obtained. The participants were from the Nurses’ Health Study I (NHS 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).

24-hour urine analysis showed higher calcium excretion and lower urine volume in most cases, but urine oxalate and citrate did not differ. Among women, urine uric acid was similar in cases and controls but was lower in cases in men. The frequency of hypercalciuria was higher among the cases in Nurses’ Health Study I, NHS II, and HPFS.

The frequency of hyperoxaluria did not differ between cases and controls, but was three times more common among men compared with women. After adjusting for the other urinary factors, the relative risk of stone formation increased with increasing urine calcium levels and concentration in all three cohorts but not in a linear fashion. There was substantial variation in the relative risks for stone formation for the concentration of other urine factors within the different cohorts.

The traditional definitions of normal 24-hour urine values need to be reassessed, as a substantial proportion of controls would be defined as abnormal, and the association with risk of stone formation may be continuous rather than dichotomous. The 24-hour urine chemistries are important for predicting risk of stone formation, but the significance and the magnitudes of the associations appear to differ by age and gender.