Systematic review of studies comparing 24-hour and spot urine collections for estimating population salt intake.

OBJECTIVE: To examine the usefulness of urine sodium (Na) excretion in spot or timed urine samples to estimate population dietary Na intake relative to the gold standard of 24-hour (h) urinary Na.
METHODS: An electronic literature search was conducted of MEDLINE (from 1950) and EMBASE (from 1980) as well as the Cochrane Library using the terms "sodium," "salt," and "urine." Full publications of studies that examined 30 or more healthy human subjects with both urinary Na excretion in 24-h urine and one alternative method (spot, overnight, timed) were examined.
RESULTS: The review included 1 380 130 participants in 20 studies. The main statistical method for comparing 24-h urine collections with alternative methods was the use of a correlation coefficient. Spot, timed, and overnight urine samples were subject to greater intra-individual and interindividual variability than 24-h urine collections. There was a wide range of correlation coefficients between 24-h urine Na and other methods. Some values were high, suggesting usefulness (up to r = 0.94), while some were low (down to r = 0.17), suggesting a lack of usefulness. The best alternative to collecting 24-h urine (overnight, timed, or spot) was not clear, nor was the biological basis for the variability between 24-h and alternative methods.
CONCLUSIONS: There is great interest in replacing 24-h urine Na with easier methods to assess dietary Na. However, whether alternative methods are reliable remains uncertain. More research, including the use of an appropriate study design and statistical testing, is required to determine the usefulness of alternative methods.