Usefulness of Overnight Urines in Population Groups

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Abstract
Twenty-four hour urine collections are difficult to obtain and evaluate in free-living population groups. The Na-Cr ratio in single daytime voidings failed to predict 24-hr urine Na excretion and was an unsatisfactory substitute for 24-hr urine collections. The sodium, potassium, and calcium excretion rates during the sleeping hours as measured in the first voided morning urine, however, were strongly correlated with 24-hr values, as were the excretion rates for the remaining portion of the day among young adult female subjects. (The correlation was Somewhat less impressive among the small number of males studied.) The greater measurement imprecision inherent in the overnight specimen can be partially compensated for by the use of larger population samples because of the relative ease with which the overnight urine specimen can be obtained. The use of overnight collections has an added advantage of standardizing recumbency and limitation of activity. Our present evidence suggests that among healthy females an overnight urine specimen is of value in estimating cation excretion among population groups. Unfortunately, the degree to which a single day's excretion reflects the usual or habitual excretion of these ions is not currently known.