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Comparisons of spot vs 24-h urine samples

for estimating salt intake

Chen Jia, Michelle A Millera, Antonella Veneziab,

Pasquale Strazzullob, and Francesco P Cappuccioa

a University of Warwick, W.H.O Collaborating Centre for Nutrition, Coventry, UKb Department of Clinical & Experimental Medicine, Federico II University of Naples, Italy

Collaboration Centre Centre of Excellence

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

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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

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DataStudy 1: Discovery sample

915 British men & women aged 40-59 (297 W, 326 B, 292 SA)

Study 2: Validation sample148 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.

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Methods

• Tanaka’s method24-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)

Predicted 24-h Ur Cr ~ Age, Height, Weight

• Arithmetic extrapolation method

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Methods

• Tanaka’s method

o Spearman’s rank Correlation and Quintile positions comparison

o Bland-Altman plot Graphical comparison

o Receiver Operating Characteristic curve & Sensitivity/Specificity >100 mmol/day as positive

• Arithmetic extrapolation method

Collaboration Centre Centre of ExcellenceResults Discovery sample – Correlation

by Tanaka’s and Arithmetic methods

Spearman's rank White African origin South Asian

Female 0.330 (<0.001) 0.055 (=0.436) 0.310 (<0.001)

Male 0.251 (=0.004) 0.166 (=0.065) 0.187 (=0.020)

Spearman's rank White African origin South Asian

Female 0.367 (<0.001) 0.116 (=0.102) 0.264 (=0.002)

Male 0.282 (=0.001) 0.221 (=0.013) 0.170 (=0.035)

Tanaka’s method vs Measured 24-h urinary sodium

Arithmetic method vs Measured 24-h urinary sodium

Collaboration Centre Centre of ExcellenceResults Discovery sample – Quintile positions

by Tanaka’s and Arithmetic methods

1 2 3 4 5 1 2 3 4 5 1 2 3 4 50

50

100

150

200

250

300Female

White African origin South Asian

1 2 3 4 5 1 2 3 4 5 1 2 3 4 50

50

100

150

200

250

300

350 1 2 3 4 5 1 2 3 4 5 1 2 3 4 50

50

100

150

200

250

300 MaleWhite African origin South Asian

1 2 3 4 5 1 2 3 4 5 1 2 3 4 50

50

100

150

200

250

300

350

EstimatedMeasured

Tan

aka

’s m

eth

od

Ari

thm

etic

met

ho

d

uri

nar

y s

od

ium

(m

mo

l/2

4h)

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Average of estimated and measured 24-h urinary sodium

Results Discovery sample – Bland-Altman plot

by Tanaka’s methodFemale Male

Est

ima

ted

- M

easu

red 95% CI

95% CI

Mean diff

Overestimation

Underestimation

Collaboration Centre Centre of ExcellenceResults Discovery sample – Bland-Altman plot

by Arithmetic methodlo

g(e

stim

ate

d)

- lo

g(m

eas

ure

d)

Average of log (estimated) and log(measured) urinary sodium

Female Male

Collaboration Centre Centre of ExcellenceResults

Discovery sample – ROC, sensitivity/specificityby Tanaka’s method

Note: Measured 24-h urinary sodium > 100 mmol/day is defined as positive

AUC=0.631Sensitivity=95.3%Specificity=6.3%

AUC=0.550Sensitivity=100%Specificity=0%

AUC=0.652Sensitivity=95.8%Specificity=8.5%

AUC=0.582Sensitivity=100%Specificity=0%

AUC=0.521Sensitivity=100%Specificity=0%

AUC=0.538Sensitivity=100%Specificity=0%

Sen

siti

vity

Fem

ale

Mal

e

1-specificity

White African origin South Asian

Collaboration Centre Centre of ExcellenceResults

Validation sample – Correlation & Quintile positions

1 2 3 4 50

100

200

300

400

500

uri

na

ry s

od

ium

(m

mo

l/24

h)

1 2 3 4 50

100

200

300

400

500

EstimatedMeasured

Tanaka’s method Arithmetic method

Spearman's rank Measured 24-h urinary sodiumTanaka’s method 0.499 (<0.001)Arithmetic method 0.329(<0.001)

Collaboration Centre Centre of ExcellenceResults

Validation sample – Bland-Altman plot & ROC, Sensitivity/specificity

Note: Measured 24-h urinary sodium > 100 mmol/day is defined as positive

AUC=0.649Sensitivity=100%Specificity=0%

AUC=0.576Sensitivity=85.5%Specificity=20.0%

ROC Curve ROC Curve

1-Specificity

Se

ns

itiv

ity

Average of methods

Tanaka’s method Arithmetic method

Est

imat

ed -

M

easu

red

Lo

g(e

stim

ated

/m

easu

red

)

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• 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

Conclusions

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Thank you