Multiple overnight urine collections may be used for estimating the excretion of electrolytes and creatinine.

Abstract

We studied the excretion of sodium, potassium, calcium, magnesium, and creatinine in overnight and 24-h urines collected over a period of seven consecutive days from 28 Dutch boys, ages eight and nine years. The correlation coefficients for the relation between true mean overnight and true mean 24-h excretion ranged between 0.94 and 1.00 for sodium, calcium, magnesium, and creatinine and was equal to 0.61 for potassium. Generally the within-person coefficients of variation for overnight excretions (range, 33-52%) were greater than for 24-h excretions (range 16-35%). We conclude that overnight collections may replace 24-h collections in the case of young boys, but that generally more overnight than 24-h urine specimens are required to achieve a similar degree of precision. The exact number of urine specimens required varies with the aims of the study.