CAN OVERNIGHT URINE REPLACE 24-HOUR URINE COLLECTION TO MEASURE URINARY CALCIUM IN EPIDEMIOLOGIC STUDIES

The present study investigated whether overnight urine collection can replace 24-hour urine collection to measure urinary calcium (Ca) excre tion in healthy individuals. One hundred healthy men (age 25-74 years) on their usual diet participated in the present study as part of an o ngoing epidemiological population-based survey. Two separate bags (day time and overnight bags) were given together with instructions to coll ect a complete 24-hour urine sample divided in to the daytime and the overnight specimen. Urinary Ca was analyzed by atomic absportion spect rophotometry and creatinine by the picric acid colorimetric method. Ov ernight urinary Ca was correlated to daytime (r = 0.618, p < 0.0001) a nd 24-hour urinary Ca (r = 0.898, p < 0.0001). In the 17 men found to have hypercalciuria (24-hour urinary Ca greater than or equal to 7.5 m mol), the overnight urinary Ca averaged 4.44 +/- 0.29 mmol/12 h (mean +/- SEM, range 2.35-6.38 mmol/12 h), indicating that an overnight urin ary Ca of < 2.35 mmol/12 h (< 60% of the overnight urinary Ca distribu tion) ruled out the possibility of finding hypercalciuria in 24-hour u rine. Fourteen of the seventeen hypercalciuric men had overnight urina ry Ca in the upper 20% of the distribution (greater than or equal to 3 .25 mmol/12 h). Hypercalciuria was found in 14 of the 19 men (73.7%) w ith an overnight urinary Ca of greater than or equal to 3.25 mmol/12 h , but only in 3 of the 81 men (3.7%) with an overnight urinary Ca of < 3.25 mmol/12 h. A substantial consistency was found in quantile analy ses of overnight and 24-hour data: no individual with urinary Ca in th e upper third of the overnight distribution had a 24-hour urinary Ca i n the lower third of the 24-hour urinary Ca distribution. Therefore, t he use of a single overnight urine collection appears to imply only a small risk in misclassifying hypercalciuric individuals. It can be con sidered a suitable procedure to measure urinary Ca excretion as a scre ening test in epidemiological population-based studies.