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

Abstract

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

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

Design: 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.

Results: 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.

Conclusions: 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.

Trial registration: ClinicalTrials.gov NCT01631240.

Figures

FIGURE 1.

Mean bias in predicted minus measured 24-h urinary sodium excretion based on the same day by prediction equation and by timing of the spot urine collection in adults aged 18–39 y. The mean bias in predicted 24-h sodium excretion by using each equation is shown for a single urine specimen collected in the morning (▪), in the afternoon (◆), in the evening (▴), and overnight (•). The 95% CIs for the mean bias are shown by the vertical lines surrounding each mean. INTERSALT, International Cooperative Study on Salt and Blood Pressure.

FIGURE 2.

Mean bias in predicted minus measured 24-h urinary sodium excretion based on the same day by prediction equation and by timing of the spot urine collection in adults aged 18–39 y within the following race-sex subgroups: black men (A), black women (B), other race men (C), and other race women (D). The mean bias in predicted 24-h sodium excretion by using each equation is shown for a single urine specimen collected in the morning (▪), in the afternoon (◆), in the evening (▴), and overnight (•). The 95% CIs of the mean bias are shown by the vertical lines surrounding each mean. Among other race men, the lower limit of the 95% CI of the mean bias in predicted 24-h sodium excretion with the use of Mage equations with overnight specimens was −843 mg. INTERSALT, International Cooperative Study on Salt and Blood Pressure.

FIGURE 3.

Bland-Altman plots of the relative bias (difference in agreement) between measured 24-h urinary sodium excretion on one day and predicted 24-h urinary sodium excretion on the same day based on the INTERSALT equation (27) and spot urinary sodium collected in the morning (A), in the afternoon (B), in the evening (C), and overnight (D). The relative bias for each individual is the predicted minus the measured 24-h urinary sodium excretion divided by the mean of the predicted and measured 24-h urinary sodium multiplied by 100 and is plotted against the mean of the predicted and measured 24-h urinary sodium excretion. The solid black line represents the mean relative difference (bias). The dashed lines represent the 95% limits of agreement of the mean relative difference ± 1.96 × SD. INTERSALT, International Cooperative Study on Salt and Blood Pressure.

FIGURE 4.

Bland-Altman plots of the relative bias (difference in agreement) between measured 24-h urinary sodium excretion on one day and predicted 24-h urinary sodium excretion on the same day based on the Tanaka equation (30) and spot urinary sodium collected in the morning (A), in the afternoon (B), in the evening (C), and overnight (D). The relative bias for each individual is the predicted minus the measured 24-h urinary sodium excretion divided by the mean of the predicted and measured 24-h urinary sodium multiplied by 100 and is plotted against the mean of the predicted and measured 24-h urinary sodium excretion. The solid black line represents the mean relative difference (bias). The dashed lines represent the 95% limits of agreement of the mean relative difference ± 1.96 × SD.

FIGURE 5.

Bland-Altman plots of the relative bias (difference in agreement) between measured 24-h urinary sodium excretion on one day and predicted 24-h urinary sodium excretion on the same day based on the Kawasaki equation (29) and spot urinary sodium collected in the morning. The relative bias for each individual is the predicted minus the measured 24-h urinary sodium excretion divided by the mean of the predicted and measured 24-h urinary sodium multiplied by 100 and is plotted against the mean of the predicted and measured 24-h urinary sodium excretion. The solid black line represents the mean relative difference (bias). The dashed lines represent the 95% limits of agreement of the mean relative difference ± 1.96 × SD.

FIGURE 6.

Bland-Altman plots of the relative bias (difference in agreement) between measured 24-h urinary sodium excretion on one day and predicted 24-h urinary sodium excretion on the same day based on the Mage equation (20, 21) and spot urinary sodium collected in the morning (A), in the afternoon (B), in the evening (C), and overnight (D). The relative bias for each individual is the predicted minus the measured 24-h urinary sodium excretion divided by the mean of the predicted and measured 24-h urinary sodium multiplied by 100 and is plotted against the mean of the predicted and measured 24-h urinary sodium excretion. The solid black line represents the mean relative difference (bias). The dashed lines represent the 95% limits of agreement of the mean relative difference ± 1.96 × SD.