Invited commentary: Quantifying salt in urine--a complex solution.

Reduction of dietary sodium intake has been identified as a priority to reduce the worldwide burden of hypertension and cardiovascular disease. Dietary sodium intake is most precisely ascertained by using timed urine collection. Casual urine sodium measurements are relatively easy to perform, but their relationship to timed urine sodium measurements is unclear. In this issue of the Journal, Brown et al. (Am J Epidemiol. 2013;177(11):1180-1192) report the development and validation of equations to estimate 24-hour urine sodium excretion from casual urine samples. Their study included a large number of participants on 2 continents, a well-collected gold standard, separate discovery and validation samples, and relevant covariates. The resulting equations represent the best available methods to estimate dietary sodium intake from casual urine samples. However, the study is limited by evidence of a suboptimal model fit, restriction to people 20-59 years of age in North America and Europe, and exclusion and adjustment that further limit external validity. In addition, individual-level correlations of estimated and measured 24-hour urine sodium excretion were modest. Properly applied, the results will facilitate tracking of dietary sodium intake within populations over time and identification of communities for which dietary sodium restriction is most likely to be beneficial. Further work is needed to extend estimation to additional populations and improve individual-level assessment.