Validation and comparison of three formulae to estimate sodiumand potassiumexcretion froma singlemorning fasting urine comparedto 24-hmeasures in 11countries

ABSTRACT

Background and objectives: Although 24-h urinarymeasure to estimate sodium and potassium excretion isthe gold standard, it is not practical for large studies. Wecompared estimates of 24-h sodium and potassiumexcretion from a single morning fasting urine (MFU) usingthree different formulae in healthy individuals.Methods: We studied 1083 individuals aged 35–70 yearsfrom the general population in 11 countries. A 24-h urineand MFU specimen were obtained from each individual. Asubset of 448 individuals repeated the measures after 30–90 days. The Kawasaki, Tanaka, and INTERSALT formulaewere used to estimate urinary excretion from a MFU specimen.Results: The intraclass correlation coefficient (ICC)between estimated and measured sodium excretion washigher with Kawasaki (0.71; 95% confidence interval, CI0.65–0.76) compared with INTERSALT (0.49; 95% CI0.29–0.62) and Tanaka (0.54; 95% CI 0.42–0.62)formulae (P<0.001). For potassium, the ICC was higherwith the Kawasaki (0.55; 95% CI 0.31–0.69) than theTanaka (0.36; 95% CI 0.07 to 0.60; P<0.05) formula(no INTERSALT formula exists for potassium). The degreeof bias (vs. the 24-h urine) for sodium was smaller withKawasaki (R313 mg/day; 95% CI R182 to R444)compared with INTERSALT ( 872 mg/day; 95% CI 728to 1016) and Tanaka ( 548 mg/day; 95% CI 408 to 688) formulae (P<0.001 and P»0.02, respectively).Similarly for potassium, the Kawasaki formula provided thebest agreement and least bias. Blood pressure correlatedmost closely and similarly with the 24-h and Kawasakiestimates for sodium compared with the other twoformulae.Conclusion: In a diverse population, the Kawasaki formulais the most valid and least biased method of estimating24-h sodium excretion from a single MFU and is suitablefor population studies.