Spot urine sodium measurements do not accurately estimate dietary sodium intake in chronic kidney disease

Abstract

Background: Sodium intake influences blood pressure and proteinuria, yet the impact on long-term outcomes is uncertain in chronic kidney disease (CKD). Accurate assessment is essential for clinical and public policy recommendations, but few large-scale studies use 24-h urine collections. Recent studies that used spot urine sodium and associated estimating equations suggest that they may provide a suitable alternative, but their accuracy in patients with CKD is unknown.

Objective: We compared the accuracy of 4 equations [the Nerbass, INTERSALT (International Cooperative Study on Salt, Other Factors, and Blood Pressure), Tanaka, and Kawasaki equations] that use spot urine sodium to estimate 24-h sodium excretion in patients with moderate to advanced CKD.

Design: We evaluated the accuracy of spot urine sodium to predict mean 24-h urine sodium excretion over 9 mo in 129 participants with stage 3-4 CKD. Spot morning urine sodium was used in 4 estimating equations. Bias, precision, and accuracy were assessed and compared across each equation.

Results: The mean age of the participants was 67 y, 52% were female, and the mean estimated glomerular filtration rate was 31 ± 9 mL · min(-1) · 1.73 m(-2) The mean ± SD number of 24-h urine collections was 3.5 ± 0.8/participant, and the mean 24-h sodium excretion was 168.2 ± 67.5 mmol/d. Although the Tanaka equation demonstrated the least bias (mean: -8.2 mmol/d), all 4 equations had poor precision and accuracy. The INTERSALT equation demonstrated the highest accuracy but derived an estimate only within 30% of mean measured sodium excretion in only 57% of observations. Bland-Altman plots revealed systematic bias with the Nerbass, INTERSALT, and Tanaka equations, underestimating sodium excretion when intake was high.

Conclusion: These findings do not support the use of spot urine specimens to estimate dietary sodium intake in patients with CKD and research studies enriched with patients with CKD. The parent data for this study come from a clinical trial that was registered at clinicaltrials.gov as NCT00785629.

Keywords: 24-hour urine; cardiovascular disease; chronic kidney disease; hypertension; sodium intake; spot urine.

© 2016 American Society for Nutrition.

Figures

FIGURE 1

Equations used to estimate 24-h sodium excretion from spot urine sodium samples. yrs, years.

FIGURE 2

Bland-Altman plots depicting bias and precision of spot sodium–based estimating equations relative to mean 24-h urine sodium excretion. Solid lines represent the overall mean of the difference (estimated − measured 24-h sodium excretion); dashed lines represent 95% limits of agreement (mean ± 1.96 SDs); n = 129. INTERSALT, International Cooperative Study on Salt, Other Factors, and Blood Pressure.