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Am J Clin Nutr. 2013 Dec;98(6):1502-13. doi: 10.3945/ajcn.113.059436. Epub 2013 Sep 18.

Validity of predictive equations for 24-h urinary sodium excretion in adults aged 18-39 y.

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Division for Heart Disease and Stroke Prevention (MEC and CDG) and the Division of Nutrition, Physical Activity and Obesity (CGP), National Center for Chronic Disease Prevention and Health Promotion, and the Division of Laboratory Sciences, National Center for Environmental Health (CMP and KLC), CDC, Atlanta, GA; the Division of Health and Nutrition Examination Surveys, National Center for Health Statistics, CDC, Hyattsville, MD (C-YW and T-CC); the National Heart, Lung, and Blood Institute (CML) and Office of Dietary Supplements (CAS and CTS), NIH, Bethesda, MD; the Department of Statistics, Iowa State University, Ames, IA (ALC); the Department of Preventive Medicine, Northwestern University Feinberg School of Medicine, Chicago, IL (KL); and the Department of Epidemiology and Public Health, Imperial College of London and Medical Research Council-Health Protection Agency, Centre for Environment and Health, London, United Kingdom (PE).



Collecting a 24-h urine sample is recommended for monitoring the mean population sodium intake, but implementation can be difficult.


The objective was to assess the validity of published equations by using spot urinary sodium concentrations to predict 24-h sodium excretion.


This was a cross-sectional study, conducted from June to August 2011 in metropolitan Washington, DC, of 407 adults aged 18-39 y, 48% black, who collected each urine void in a separate container for 24 h. Four timed voids (morning, afternoon, evening, and overnight) were selected from each 24-h collection. Published equations were used to predict 24-h sodium excretion with spot urine by specimen timing and race-sex subgroups. We examined mean differences with measured 24-h sodium excretion (bias) and individual differences with the use of Bland-Altman plots.


Across equations and specimens, mean bias in predicting 24-h sodium excretion for all participants ranged from -267 to 1300 mg (Kawasaki equation). Bias was least with International Cooperative Study on Salt, Other Factors, and Blood Pressure (INTERSALT) equations with morning (-165 mg; 95% CI: -295, 36 mg), afternoon (-90 mg; -208, 28 mg), and evening (-120 mg; -230, -11 mg) specimens. With overnight specimens, mean bias was least when the Tanaka (-23 mg; 95% CI: -141, 95 mg) or Mage (-145 mg; -314, 25 mg) equations were used but was statistically significant when using the Tanaka equations among females (216 to 243 mg) and the Mage equations among races other than black (-554 to -372 mg). Significant over- and underprediction occurred across individual sodium excretion concentrations.


Using a single spot urine, INTERSALT equations may provide the least biased information about population mean sodium intakes among young US adults. None of the equations evaluated provided unbiased estimates of individual 24-h sodium excretion.


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