Formula-led methods using first morning fasting spot urine to assess usual salt intake: a secondary analysis of PURE study data

Objectives: Observational studies that assess the relationship between salt intake and long-term outcomes require a valid estimate of usual salt intake. The gold-standard measure in individuals is sodium excretion in multiple nonconsecutive 24-h urines. Multiple studies have demonstrated that random spot urine samples are not valid for estimating usual salt intake; however, some researchers believe that fasting morning spot urine samples produce a better measure of usual salt intake than random spot samples. Methods: We have used publicly available data from a PURE China validation study to compare estimates of usual salt intake from morning spot urine samples and four published formulae with mean of two 24-h urine samples (reference). We estimated the mean and 95% confidence interval of absolute and relative error for each formula-led method and the degree to which estimates were able to be classified into the correct quartile of intake. Bland-Altman plots were used to test the level of agreement. Results: The results show that compared with the reference method, all formulae-led estimates from spot urine collections have high error rates: both random and systematic. This is demonstrated for individual estimates, as well as by quartiles of reference salt intake. This study conclusively demonstrates the unsuitability of morning spot urine formula-led estimates of usual salt intake. Conclusion: Our findings support international recommendations to not conduct, fund, or publish research studies that use spot urine samples with estimating equations to assess individuals’ salt intake in association with health outcomes.