Comparisons of spot vs 24-h urine samples for estimating salt intake

1. Comparisons of spot vs 24-h urine samples for estimating salt intake • Chen Jia, Michelle A Millera, AntonellaVeneziab, • Pasquale Strazzullob, and Francesco P Cappuccioa a University of Warwick, W.H.O Collaborating Centre for Nutrition, Coventry, UK b Department of Clinical & Experimental Medicine, Federico II University of Naples, Italy

2. Background • In steady state, most of Na eaten in a day is excreted through urine in the following 24-h • 24-h urinary Na excretion ‘gold standard’ for measuring sodium (salt) intake both in individuals and in populations • In large population studies 24-h collections often deemed inconvenient, hence alternative methods suggested (e.g. spot and timed urines) • Their reliability and reproducibility often disappointing

3. Aim • To assess reliability and reproducibility of estimations of 24-h urinary Na excretion through timed spot urines compared to 24-h Na output in two independent population samples including men and women from different ethnic groups

4. Data Study 1: Discovery sample 915 British men & women aged 40-59 (297 W, 326 B, 292 SA) Study 2: Validation sample 148 Italian White men aged 32-75 Na, Cr in “independent” 24-h and timed spot urines • Reliability and Reproducibility • Tanaka’s method† • Arithmetic extrapolation method †: Tanaka T et al. J Hum Hyper,2002;16, 97-103.

5. Methods • Tanaka’s method 24-h Ur Cr ≈ Predicted 24-h Ur Cr 24-h Ur Na/Cr  timed Ur Na/Cr 24-h Ur Na  timed Ur Na/Cr  Predicted 24-h Ur Cr 24-h Ur Na = timed Ur Na  timed Ur Vol  (24-h/timed Ur collection time) • Arithmetic extrapolation method Predicted 24-h Ur Cr ~ Age, Height, Weight

6. Methods • Tanaka’s method • Spearman’s rank Correlation and Quintile positions comparison • Bland-Altman plot • Graphical comparison • Receiver Operating Characteristic curve & Sensitivity/Specificity • >100 mmol/day as positive • Arithmetic extrapolation method

7. Results Discovery sample – Correlation by Tanaka’s and Arithmetic methods Tanaka’s method vs Measured 24-h urinary sodium Arithmetic method vs Measured 24-h urinary sodium

8. Results Discovery sample – Quintile positions by Tanaka’s and Arithmetic methods urinary sodium (mmol/24h) Tanaka’s method Arithmetic method

9. Results Discovery sample – Bland-Altman plotby Tanaka’s method Estimated - Measured Female Male 95% CI Mean diff 95% CI Overestimation Underestimation Average of estimated and measured 24-h urinary sodium

10. Results Discovery sample – Bland-Altman plotby Arithmetic method log(estimated) - log(measured) Female Male Average of log (estimated) and log(measured) urinary sodium

11. Results Discovery sample – ROC, sensitivity/specificityby Tanaka’s method Sensitivity White African origin South Asian Female AUC=0.652 Sensitivity=95.8% Specificity=8.5% AUC=0.550 Sensitivity=100% Specificity=0% AUC=0.631 Sensitivity=95.3% Specificity=6.3% Male AUC=0.538 Sensitivity=100% Specificity=0% AUC=0.521 Sensitivity=100% Specificity=0% AUC=0.582 Sensitivity=100% Specificity=0% 1-specificity Note: Measured 24-h urinary sodium > 100 mmol/day is defined as positive

12. Results Validation sample – Correlation & Quintile positions Tanaka’s method Arithmetic method

13. Results Validation sample – Bland-Altman plot & ROC, Sensitivity/specificity Tanaka’s method Arithmetic method Estimated - Measured Log(estimated/measured) Average of methods ROC Curve ROC Curve Sensitivity AUC=0.576 Sensitivity=85.5% Specificity=20.0% AUC=0.649 Sensitivity=100% Specificity=0% 1-Specificity Note: Measured 24-h urinary sodium > 100 mmol/day is defined as positive

14. Conclusions • Spot timed urinary Na does not provide reliable and reproducible estimates of 24-h urinary Na excretion • 24-h urinary collection for the measurement of urinary Na excretion remains the preferred tool for assessing salt intake

15. Thank you