Comparison of three formulas to estimate 24-hour urinary sodium and potassium excretion in patients hospitalized in a hypertension unit

Accepted Manuscript Comparison of three formulas to estimate 24-hour urinary sodium and potassium excretion in patients hospitalized in a hypertension unit Piotr Jędrusik, MD, PhD, Bartosz Symonides, MD, PhD, Zbigniew Gaciong, MD, PhD PII:

S1933-1711(18)30078-0

DOI:

10.1016/j.jash.2018.03.010

Reference:

JASH 1148

To appear in:

Journal of the American Society of Hypertension

Received Date: 9 November 2017 Revised Date:

2 February 2018

Accepted Date: 20 March 2018

Please cite this article as: Jędrusik P, Symonides B, Gaciong Z, Comparison of three formulas to estimate 24-hour urinary sodium and potassium excretion in patients hospitalized in a hypertension unit Journal of the American Society of Hypertension (2018), doi: 10.1016/j.jash.2018.03.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1

Comparison of three formulas to estimate 24-hour urinary sodium and potassium excretion in patients

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hospitalized in a hypertension unit

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Short title: Comparison of three formulas to estimate 24-hour urine Na and K

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Piotr Jędrusik, MD, PhD ; Bartosz Symonides, MD, PhD ; Zbigniew Gaciong, MD, PhD

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Warsaw, Warsaw, Poland.

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Department of Internal Medicine, Hypertension and Vascular Diseases, Medical University of

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Conflicts of interest: None

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Word count: abstract 200, text with tables and figure captions 7152

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Number of tables: 3; number of figures: 2

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15 Corresponding author:

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Piotr Jędrusik, MD, PhD

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Department of Internal Medicine, Hypertension and Vascular Diseases

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Medical University of Warsaw

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Banacha 1a, 02 097 Warsaw, Poland

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Phone: +48 22 599 12 07; Fax: +48 22 599 18 28

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E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract

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Background: Measurements of 24-hour urinary sodium (24hrUNa) and potassium (24hrUK) excretion

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are useful in hypertensives but 24-hour urine collection may be difficult or unreliable. We compared

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three formulas (Tanaka, Kawasaki, PAHO) proposed to estimate 24hrUNa and 24hrUK based on spot

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urine measurements.

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Methods: We studied 382 patients admitted to a hypertension unit. Sodium, potassium, and creatinine

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levels were measured using standard laboratory methods in a morning urine sample, followed by 24-

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hour urinary collection. Agreement between estimated and measured 24hrUNa and 24hrUK was

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evaluated using the Pearson correlation and Bland-Altman plots.

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Results: Measured 24hrUNa was 158±75 mmol/d and 24hrUK was 54±24 mmol/d. The correlation

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coefficient was r=0.53 for estimated vs. measured 24hrUNa, r=0.69-0.73 for estimated vs. measured

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24hrUK (all P<0.001). The mean bias for 24hrUNa was significantly smaller for Tanaka (10.5 mmol/d)

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and PAHO (11.5 mmol/d) compared to Kawasaki formula (-29.9 mmol/d). The mean bias for 24hrUK

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was significantly smaller for Kawasaki (7.3 mmol/d) and PAHO (8.3 mmol/d) compared to Tanaka

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formula (16.5 mmol/d).

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Conclusion: Using a single morning urine sample, we found the PAHO formula to be the best for

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predicting mean 24hrUK and 24hrUNa in hospitalized hypertensive patients. However, precision and

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accuracy of all the evaluated formulas was inadequate.

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Key Words: spot urine measurement, 24-hour urinary sodium/potassium excretion, Tanaka formula,

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Kawasaki formula, PAHO formula, hypertension

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ACCEPTED MANUSCRIPT Introduction

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Measurements of 24-hour urinary sodium and potassium excretion are of value for several clinical

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purposes. Evaluation of 24-hour urinary sodium excretion is widely used to assess dietary sodium

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intake at both individual and population level, as 24-hour sodium excretion is considered a reasonably

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accurate measure of daily oral sodium intake [1]. Assessment of sodium intake is particularly useful in

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patients with hypertension in whom limitation of sodium intake is recommended as a non-drug

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therapeutic measure. This is even more important in patients with difficult-to-control or resistant

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hypertension, as measuring 24-hour urinary sodium excretion may give insight into patient compliance

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regarding dietary sodium restriction [2].

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Evaluation of urinary potassium excretion in hypertensives is mostly useful in those evaluated

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for primary hyperaldosteronism [3]. Although hypokalemia due to an increased urinary potassium

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excretion is not a constant finding in those patients [4], it is more common in the subset with an

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aldosterone-producing adrenal adenoma and may produce symptoms. In addition, hypokalemia may

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be unrelated to renal mechanisms, and thus confirmation of an increased urinary potassium loss may

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have a diagnostic value when evaluating hypokalemia and/or related symptoms.

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The reference diagnostic approach to evaluate urinary sodium and potassium excretion is

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based on 24-hour urinary collection which may be difficult to perform, as indicated by low response

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rates in large population studies [5]. In addition, the completeness of 24-hour urine collection may be

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unreliable, and no methods are available to identify incomplete samples [5], while the rates of

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incomplete collection may be as high as 30% [6].

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One approach to eliminate the need for 24-hour urine collection is to use spot urine samples

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which are much easier to collect. Spot urine measurements are routinely used to evaluate urinary

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protein excretion [7] and have also been employed to evaluate urinary sodium excretion and dietary

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salt intake at the population level [1,5]. Three formulas have been proposed to estimate 24-hour

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urinary excretion of both sodium and potassium based on spot urine measurements. Two formulas

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were developed by Kawasaki et al. [8] and Tanaka et el. [9] based on regression analysis performed in

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Asian population samples, and another simpler formula has been put forward by the Pan American

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Health Organization (PAHO) and the World Health Organization [10]. These formulas were used in a

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number of studies reported in the literature, mostly for the purpose of estimating 24-hour urinary

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sodium excretion in healthy subjects from the general population [11,12]. However, they were rarely

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systematically compared to each other, and no previous study compared all three formulas in

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hypertensive patients. The aim of our study was to compare estimates of 24-hour urinary sodium and potassium

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excretion using the Kawasaki, Tanaka and PAHO formulas based on morning spot urine

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measurements against the actual measured 24-hour urinary sodium and potassium excretion in

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unselected patients hospitalized in a hypertensive unit and evaluated in routine clinical settings.

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8 Methods

We studied 382 patients admitted to our hypertensive unit in whom 24-hour urinary collection was

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performed as a routine diagnostic procedure. Details of the study protocol have been published

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elsewhere [13]. Briefly, all patients were evaluated clinically and underwent diagnostic tests deemed

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necessary to evaluate their blood pressure control, complications of hypertension, cardiovascular risk

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profile, and possible secondary causes of hypertension. A fasting urine sample was collected in the

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morning, followed by 24-hour urinary collection until the next morning. Standard instructions on how to

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collect urine over 24 hours were given to patients by the hospital staff, and no special oversight was

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employed by the personnel regarding the completeness of urine collection by the patients.

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The exclusion criteria included oliguria defined as 24-hour urine volume of < 400 mL, polyuria

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defined as 24-hour urine volume of > 3000 mL, and 24-hour urinary creatinine excretion of < 0.6 g/day,

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considered to indicate unreliable 24-hour urine collection (based on the average 24-hour urinary

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creatinine excretion of 14-26 mg/kg/day, or 0.6-2.0 g/day) [14].

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Urinary sodium, potassium, and creatinine levels in spot urine samples and 24-hour urine

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collections were measured using standard laboratory methods (Cobas Integra analyzers, Roche). The

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Modification of Diet in Renal Disease (MDRD) formula was used to estimate the glomerular filtration

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rate (GFR).

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Patient informed consent was not required, as all procedures performed in patients were a

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part of clinically indicated routine investigations, and all samples were anonymized. A non-

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interventional nature of the study was formally confirmed by the local Ethics Committee at our

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institution. The Tanaka, Kawasaki, and PAHO formulas were used to estimate 24-hour urinary sodium

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and potassium excretion based on sodium (Na), potassium (K), and creatinine levels measured in the

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spot urine sample. The Kawasaki formulas [8] to estimate 24-hour urinary sodium and potassium

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excretion are given as:

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Estimated 24-hour urinary Na (mmol) = 16.3 (spot urine Na (mmol/L)/spot urine creatinine (mmol/L) x

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24-hour urinary creatinine (mg))

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Estimated 24-hour urinary K (mmol) = 7.2 (spot urine Na (mmol/L)/spot urine creatinine (mmol/L) x 24-

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hour urinary creatinine (mg))

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0.5

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The Tanaka formulas [9] to estimate 24-hour urinary sodium and potassium excretion are given as:

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Estimated 24-hour urinary Na (mmol) = 21.98 (spot urine Na (mmol/L)/10 x urine creatinine (mg/dL)) x

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24-hour urinary creatinine (mg)

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Estimated 24-hour urinary K (mmol) = 7.59 (spot urine K (mmol/L)/10 x urine creatinine (mg/dL)) x 24-

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hour urinary creatinine (mg))

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)0.392

The formula endorsed by PAHO [10] is identical for sodium and potassium and given as:

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Estimated 24-hour urinary Na = (measured spot urine Na / measured spot urine creatinine) x 24-hour

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urinary creatinine

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Estimated 24-hour urinary K = (measured spot urine K / measured spot urine creatinine) x 24-hour

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urinary creatinine

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ACCEPTED MANUSCRIPT 1 We used the Pearson correlation coefficient and the Bland-Altman plots to evaluate the agreement

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between the measured 24-hour urinary sodium and potassium excretion and their estimations using

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the three formulas. The Bland-Altman approach [15,16] is based on calculating the mean difference

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between the two parameters being compared in the study population, in this case the measured and

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the estimated 24-hour urinary sodium or potassium excretion, also known as the mean bias, and

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plotting the individual differences between the two parameters against the individual mean of these

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two parameters. Thus, the individual means of the measured and estimated 24-hour urinary sodium or

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potassium excretion are plotted on the X axis of the presented Bland-Altman plots, and the individual

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differences between the two parameters, calculated by subtracting the estimated value from the

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measured one, are plotted on the Y axis. The mean bias was calculated as a mean of individual

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differences between the measured and estimated 24-hour urinary sodium or potassium excretion,

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calculated for study each participant. The 95% limits of agreement, indication the range where one can

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expect the error of individual estimation to be within in 95% of cases, were calculated as the mean

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bias ±1.96 standard deviation of the mean bias. Individual accuracy of the formulas was also

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evaluated as the percentage of estimated urinary sodium or potassium excretion values within 30% of

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the measured 24-hour urinary sodium or potassium excretion (P30).

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All statistical analyses were performed using the R software. Differences between the mean

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bias for the three formulas were compared using the Friedman rank sum test, a non-parametric

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version of one-way ANOVA with repeated measures. The 95% confidence intervals (CI) for the mean

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biases were calculated using the bootstrap method [17] (R software “boot” package, normal

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approximation). Statistical significance was set at P<0.05.

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Results

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The study population (n=382) included 152 men and 230 women at the mean age of 55±16 years

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(range 16-94 years). Detailed characteristics of the study population were reported elsewhere [13] and

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are summarized in Table 1. Sustained hypertension was diagnosed in 92% of patients, and 92% of

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patients received antihypertensive medications. The mean clinic blood pressure was 152±29/91±15

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mm Hg, and the mean ambulatory blood pressure was 132±18/78±12 mm Hg during the daytime and

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121±21/62±12 mm Hg during the night-time.

3 Table 1. Study population characteristics. Study population (N=382)

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Variable Gender (male/female)

152/230

55±16 (range 16-94)

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Age, mean ± SD (range) [years] Race/ethnicity - Caucasians 2

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Body mass index, mean ± SD [kg/m ] Hypertension Cardiovascular disease Diabetes

100% 28.9±5.4 92% 29% 16% 158±75

24-hour urinary potassium excretion, mean ± SD [mmol/d]

54±24

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24-hour urinary sodium excretion, mean ± SD [mmol/d]

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24-hour urinary creatinine excretion, mean ± SD [g/d] Serum creatinine level, mean ± SD [mg/dL] Estimated GFR < 60 mL/min/1.73 m

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1.23±0.49 0.95±0.48 17%

Clinic blood pressure, mean ± SD [mm Hg]

152±29/91±15

Ambulatory daytime blood pressure, mean ± SD [mm Hg]

132±18/78±12

Ambulatory nighttime blood pressure, mean ± SD [mm Hg]

121±21/62±12

Any hypertensive medication

92%

Loop diuretic or thiazide

45%

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ACCEPTED MANUSCRIPT Spironolactone

4.5%

Hydrochlorothiazide/amiloride

1.8%

ACE inhibitor/angiotensin receptor blocker

60%

Potassium supplementation

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2.2 ± 1.1

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Number of antihypertensive medications, mean ± SD

45%

GFR, glomerular filtration rate; SD, standard deviation.

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Six patients (1.6%) had missing laboratory data and were not included in the analysis. In the

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overall study population, 41 (10.7%) patients met the exclusion criteria based on the adequacy of 24-

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hour urine collection (oliguria, polyuria, or inadequate urine collection) and were excluded from further

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analysis. The remaining patients formed the study group that included 335 subjects (135 men, 200

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women).

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Correlation between estimated and measured 24-hour urinary sodium and potassium excretion

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Following patient exclusions as per the above exclusion criteria (N=335), the Pearson correlation

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coefficients between the estimated and measured 24-hour urinary sodium excretion were r=0.53 for

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the Tanaka formula, r=0.53 for the Kawasaki formula, and r=0.53 for the PAHO formula (P<0.001 for

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all). For potassium, respective correlation coefficients were r=0.69, r=0.70, and r=0.73 (P<0.001 for all)

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(Figure 1).

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Figure 1. Correlations between the measured and estimated 24-hour urinary sodium (24hUNa) and

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potassium (24hUK) excretion in the study group (N=335). Upper panel: the Tanaka formula (r=0.53 for

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sodium, r=0.69 for potassium); middle panel: the Kawasaki formula (r=0.53 for sodium, r=0.70 for

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potassium); lower panel: the PAHO formula (r=0.53 for sodium, r=0.73 for potassium) (P<0.001 for

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all).

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ACCEPTED MANUSCRIPT Estimation of 24-hour urinary sodium and potassium excretion based on spot urine samples

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using the three formulas

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After patient exclusions as per the study exclusion criteria (N=335), the mean measured 24-hour

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urinary sodium excretion) was 160.3±67.6 mmol/d. The mean estimated 24-hour urinary sodium

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excretion was 149.2±36.6 mmol/d by the Tanaka formula, 189.7±58.6 mmol/d by the Kawasaki

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formula, and 148.3±88.4 mmol/d by the PAHO formula. The mean measured 24-hour urinary

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potassium excretion was 55.3±22.2 mmol/d. The mean estimated 24-hour urinary potassium excretion

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was 38.8±8.1 mmol/d by the Tanaka formula, 47.9±11.7 mmol/d by the Kawasaki formula, and

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47.0±24.2 mmol/d by the PAHO formula. The correlation coefficients for the correlations between 24-

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hour urinary sodium and potassium excretion estimated using the three formulas were very high

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(r=0.97-0.99 for the three formulas for sodium, r=0.98-0.99 for the three formulas for sodium; all

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P<0.001).

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For the estimation of 24-hour urinary sodium excretion, the mean bias (measured minus

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estimated 24-hour excretion) was significantly smaller (P<0.001) for the Tanaka (10.5 mmol/d, 95% CI

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for the mean bias 4.99 to 17.16; 95% limits of agreement -102 to 124 mmol/d) and PAHO formulas

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(11.5 mmol/d, 95% CI 3.44 to 20.21; 95% limits of agreement -142 to 165 mmol/d) compared to the

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Kawasaki formula (-29.9 mmol/d, 95% CI -36.48 to -22.85; 95% limits of agreement -151 to 91

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mmol/d) (Table 2 and Figure 2). The P30 values were 64.4% for the Tanaka formula, 51.2% for the

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PAHO formula, and 49.4% for the Kawasaki formula.

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For the estimation of 24-hour urinary potassium excretion, the mean bias was significantly

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smaller (P<0.001) for the Kawasaki (7.3 mmol/d, 95% CI for the mean bias 5.70 to 9.22; 95% limits of

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agreement -25 to 39 mmol/d) and PAHO formulas (8.3 mmol/d, 95% CI 6.34 to 10.43; 95% limits of

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agreement -28 to 44 mmol/d) compared to the Tanaka formula (-16.5 mmol/d, 95% CI 14.68 to 18.52;

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95% limits of agreement -18 to 51 mmol/d) (Table 2 and Figure 2). The P30 values were 71.3% for the

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Kawasaki formula, 60.9% for the PAHO formula, and 56.4% for the Tanaka formula.

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Figure 2. The Bland-Altman (B_A) plots showing the difference between measured and estimated 24-

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hour urinary sodium (Na; left) and potassium (K; right) excretion plotted against the mean 24-hour

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urinary sodium and potassium excretion by the two methods (mmol/d) in the study group (N=335).

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Upper panel: the Tanaka formula; middle panel: the Kawasaki formula; lower panel: the PAHO

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formula.

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ACCEPTED MANUSCRIPT 1 The regression lines on the Bland-Altman plots indicate that underestimation of 24-hour

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urinary sodium excretion by the Tanaka formula tended to increase with higher measured 24-hour

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urinary sodium excretion values, while with low 24-hour urinary sodium excretion, this formula actually

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overestimated 24-hour urinary sodium excretion (Figure 2, upper left). The same was true for the

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Tanaka and Kawasaki formulas for potassium (Figure 2, upper right and middle right), while the PAHO

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formula for sodium tended to overestimate 24-hour urinary sodium excretion with higher measured 24-

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hour urinary sodium excretion values (Figure 2, lower left).

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We also evaluated whether exclusion of patients with oliguria/polyuria/inadequate urine

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collection according to the study exclusion criteria has affected our results. When we included all

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patients with complete laboratory data (N=376), the results were materially similar (Table 2).

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Table 2. Summary of the results in the study group (with adequate 24-hour urine collection as per the

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study protocol) and all evaluable patients (before exclusions as per the study exclusion criteria). Study group (N=335; adequate 24-hour

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Formula

All evaluable patients (N=376)

urine collection as per the study protocol)

Mean bias** (95%

Pearson

Mean bias** (95%

correlation

limits of agreement)

correlation

limits of agreement)

coefficient*

[mmol/d]

coefficient*

[mmol/d]

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Pearson

24-hour urinary sodium excretion Tanaka

0.53 (P<0.001)

10.5 (-102 to 124)

0.50 (P<0.001)

9.5 (-113 to 132)

Kawasaki

0.53 (P<0.001)

-29.9 (-151 to 91)

0.50 (P<0.001)

-29.7 (-166 to 107)

PAHO

0.53 (P<0.001)

11.5 (-142 to 165)

0.42 (P<0.001)

8.1 (-196 to 212)

24-hour urinary potassium excretion

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ACCEPTED MANUSCRIPT 0.69 (P<0.001)

16.5 (-18 to 51)

0.69 (P<0.001)

15.6 (-21 to 52)

Kawasaki

0.70 (P<0.001)

7.3 (-25 to 39)

0.69 (P<0.001)

6.7 (-27 to 40)

PAHO

0.69 (P<0.001)

8.3 (-28 to 44)

0.69 (P<0.001)

8.0 (-27 to 43)

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*Estimated vs. measured 24-hour urinary excretion.

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**Measured minus estimated 24-hour excretion.

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Tanaka

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As a sensitivity analysis, we stratified results in the study group by gender and diuretic use

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(Table 3). In the study group, the mean measured sodium excretion was 180 ± 70 mmol/d in men and

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147 ± 62 mmol/d in women. The mean measured potassium excretion was 63 ± 24 mmol/d in men

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and 50 ± 19 mmol/d in women.

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Table 3. Results in the study group (n=335) stratified by gender and diuretic use. Mean estimated

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Formula and subgroup

excretion [mmol/d]

Pearson correlation

Mean bias** (95%

coefficient*

limits of agreement) [mmol/d]

Tanaka – men

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24-hour urinary sodium excretion

157 ± 34

0.57

22.7 (-93 to 139)

144 ± 38

0.48

2.3 (-106 to 110)

Kawasaki – men

201 ± 55

0.57

-21.9 (-140 to 96)

Kawasaki – women

182 ± 60

0.48

-35.3 (-157 to 87)

PAHO – men

164 ± 87

0.57

15.5 (-129 to 160)

PAHO – women

138 ± 88

0.46

8.8 (-149 to 167)

Tanaka – diuretic

150 ± 38

0.56

17.3 (-98 to 132)

Tanaka – women

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ACCEPTED MANUSCRIPT 148 ± 34

0.52

6.0 (-106 to 118)

Kawasaki – diuretic

190 ± 60

0.56

-23.4 (-145 to 98)

Kawasaki – no diuretic

187 ± 53

0.52

-33.4 (-151 to 84)

PAHO – diuretic

150 ± 91

0.54

17.0 (-139 to 173)

PAHO - no diuretic

143 ± 78

0.53

24-hour urinary potassium excretion

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Tanaka – no diuretic

11.0 (-128 to 150)

42 ± 8

0.66

Tanaka – women

37 ± 7

0.68

13.1 (-16 to 43)

Kawasaki – men

52 ± 12

0.66

11.1 (-25 to 47)

Kawasaki – women

45 ± 11

0.68

4.8 (-23 to 33)

PAHO – men

55 ± 27

0.65

7.9 (-34 to 50)

PAHO – women

41 ± 20

0.67

8.6 (-23 to 40)

40 ± 8

0.69

17.9 (-17 to 52)

38 ± 8

0.68

15.5 (-18 to 49)

49 ± 11

0.69

8.4 (-24 to 41)

47 ± 12

0.68

6.6 (-25 to 38)

PAHO – diuretic

49 ± 23

0.69

8.6 (-26 to 44)

PAHO - no diuretic

45 ± 23

0.67

8.6 (-27 to 44)

Tanaka – no diuretic

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Tanaka – diuretic

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Tanaka – men

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Kawasaki – no diuretic

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*Estimated vs. measured 24-hour urinary excretion.

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**Measured minus estimated 24-hour excretion.

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21.5 (-17 to 60)

ACCEPTED MANUSCRIPT The mean estimated 24-hour urinary sodium excretion estimated with all three formulas was

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lower in women compared to men, similarly to the difference in the actual measured 24-hour urinary

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sodium excretion. The mean bias in men was lowest with the PAHO formula, and in women it was

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lowest with the Tanaka formula, followed by the PAHO formula. In addition, the mean bias was lower

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in women compared to men with the two best formulas, PAHO and Tanaka, while the vice versa was

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true for the Kawasaki formula.

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The mean estimated 24-hour urinary sodium excretion estimated with all three formulas was

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similar in those treated versus those not treated with diuretics. The mean bias in patients treated with

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diuretics was lowest with the PAHO and Tanaka formulas, and in patients not treated with diuretics it

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was lowest with the Tanaka formula, followed by the PAHO formula. In addition, with the two best

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formulas, PAHO and Tanaka, the mean bias was lower in patients not treated with diuretics compared

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to those treated with diuretics while the vice versa was true for the Kawasaki formula. Overall, the

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results regarding sodium were generally similar to those in the overall study group, with the PAHO and

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Tanaka formulas yielding lower mean bias values compared to the Kawasaki formula. However, all

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formulas for sodium showed some differences in the mean bias between men and women, and

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between diuretic users versus non-users.

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The mean estimated 24-hour urinary potassium excretion estimated with all three formulas

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was lower in women compared to men, again similarly to the difference in the actual measured 24-

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hour urinary potassium excretion. The mean bias in men was lowest with the PAHO formula, and in

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women it was lowest with the Kawasaki formula, followed by the PAHO formula. In addition, the mean

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bias was lower in women compared to men with the Tanaka and Kawasaki formulas but this difference

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between men and women was low for the PAHO formula.

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As was the case with sodium, the mean estimated 24-hour urinary potassium excretion

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estimated with all three formulas was similar in those treated versus those not treated with diuretics.

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The mean bias in patients treated with diuretics was lowest with the Kawasaki and PAHO formulas,

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and in patients not treated with diuretics it was lowest with the Kawasaki formula, followed by the

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PAHO formula. For all formulas, only small differences in the mean bias were found between patients

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not treated with diuretics and those treated with diuretics. Overall, the results regarding potassium

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were again generally similar to those in the overall study group, with the PAHO and Kawasaki

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formulas yielding lower mean bias values compared to the Tanaka formula. In addition, the Tanaka

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and Kawasaki formulas but not the PAHO formula showed some differences in the mean bias between

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men and women, while all formulas for potassium showed little differences between diuretic users

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versus non-users.

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5 Discussion

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In our study, we compared the three formulas that have been proposed to estimate both 24-hour

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urinary sodium and potassium excretion based on sodium and potassium level measurement in a spot

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urine sample, using urine sodium or potassium to creatinine ratio as a means to control for urinary

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electrolyte concentration. While most previous studies evaluating the value of spot urine

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measurements were undertaken in healthy, often relatively young populations, we studied

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hypertensive patients admitted to a specialist hypertension unit. Our study was performed in a typical

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in-hospital clinical environment using standard laboratory methods in unselected patients undergoing

14

routine clinical and diagnostic evaluation, without any special oversight over spot urine sampling and

15

24-hour urine collection. Thus, our findings are likely to represent the accuracy of estimating 24-hour

16

urinary sodium and potassium excretion that may be expected in routine clinical practice.

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In contrast to some previous studies, we used both the correlation coefficient and the Bland-

18

Altman method, which is better suited to evaluate the agreement between two measurement methods

19

[11,12]. Specifically, the Bland-Altman plot allows to visualize the difference between the newly

20

introduced method (in our case, formula-based estimation of 24-hour urinary excretion from a spot

21

urine sample measurement) and the reference method (measurement of the actual 24-hour urinary

22

excretion) over the whole range of the actual values of the evaluated parameter. The Bland-Altman

23

method provides several parameters of different value for the assessment of populations or

24

individuals. The mean difference between the two measurements, or the mean bias, is the most

25

important parameter at the population level, e.g., when comparing populations or evaluating temporal

26

population trends, while the 95% limit of agreement indicates the imprecision of estimates at the

27

individual level. Another important parameter is the slope of the regression line on the Bland-Altman

28

plot, showing whether the formula remained similarly precise at the lower and upper end of the 24-

29

hour urinary excretion range.

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ACCEPTED MANUSCRIPT Although correlations between the measured and estimated 24-hour urinary excretion in our

2

study were similar for all three formulas for both sodium (r=0.53 for all three formulas) and potassium

3

(r=0.69 to 0.70), and they were within the range previously reported in the literature [1,8,9], analysis of

4

the Bland-Altman plots revealed important differences between these formulas. Regarding potassium,

5

the mean bias was similar for the PAHO and Kawasaki formulas, and underestimation of 24-hour

6

urinary potassium excretion by these formulas was in the range of 7 to 8 mmol/d. However, in contrast

7

to the PAHO formula, both Tanaka and Kawasaki formulas tended to underestimate high urinary

8

potassium excretion and overestimate low potassium excretion. Thus, the PAHO formula may be

9

generally considered the most precise of the three evaluated formulas for estimating 24-hour urinary

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potassium excretion in our study.

Regarding sodium, the mean bias was lowest for the PAHO and Tanaka formulas

12

(underestimation by about 11 mmol/d), with 95% limits of agreement somewhat wider for the PAHO

13

formula. The Kawasaki formula was the least precise as it overestimated 24-hour urinary sodium

14

excretion by as much 30 mmol/l. Similarly to potassium, the Tanaka formula underestimated high

15

urinary sodium excretion and overestimated low sodium excretion. The PAHO formula overestimated

16

urinary sodium excretion at its higher levels, but this overestimation was numerically lower than

17

underestimation by the Tanaka formula, and Kawasaki formula was characterized by constant

18

overestimation of 24-hour urinary sodium excretion over its whole range. In summary, when estimating

19

urinary sodium excretion, we found the both the Tanaka and the PAHO formula were characterized by

20

similarly low bias. The PAHO formula was somewhat more imprecise at the individual level, while the

21

Tanaka formula became clearly less precise at the lower and upper end of the 24-hour urinary sodium

22

excretion range compared to the PAHO formula. For instance, when 24-hour urinary sodium excretion

23

was 100 mmol/d and below, or 200 mmol/d and above, the mean bias by the Tanaka formula became

24

significant, with major overestimation and underestimation of urinary sodium excretion, respectively,

25

while the bias by the PAHO formula was much lower. The Kawasaki formula did not become less

26

precise at the lower and upper end of the 24-hour urinary sodium excretion range but had the highest

27

bias of all three formulas. Thus, for estimating 24-hour urinary sodium excretion at the population

28

level, the PAHO and Tanaka formulas may yield similar average population estimates, but in subjects

29

with elevated urinary sodium excretion, its underestimation by the Tanaka formula was clearly

30

numerically higher compared to overestimation by the PAHO formula.

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ACCEPTED MANUSCRIPT In addition to the assessment of mean group (population) values of 24-hour urinary excretion,

2

the evaluated formulas would be obviously much more attractive from the clinical point of view if they

3

provided reasonably accurate estimates in individual subjects, perhaps allowing individual clinical

4

decisions to be made based on spot urine measurements. In this regard, our study provided data not

5

only on the usefulness of the evaluated formulas at the population level, but also on their precision and

6

adequacy in individual patients. Our findings showed that at the individual level, all formulas were not

7

particularly precise based on the width of the 95% limits of agreements on the Bland-Altman plots. We

8

also introduced another measure of individual accuracy, the percentage of estimated urinary sodium

9

or potassium excretion values within 30% of the measured 24-hour urinary sodium or potassium

10

excretion (P30) [18]. We believe that this 30% threshold of individual error of excretion estimation

11

could be considered a reasonably good measure of the usefulness of the given formula from the

12

clinical perspective, although this parameter was previously only reported in some studies.

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The Tanaka formula yielded the highest percentage of estimated urinary sodium excretion

14

values within 30% of the measured 24-hour urinary sodium excretion (64%, compared to about 50%

15

for the Kawasaki and PAHO formulas). Regarding potassium, the highest percentage of estimated

16

excretion values within 30% of the measured 24-hour urinary potassium excretion was obtained with

17

the Kawasaki formula (71%, compared to 56-61% for the Tanaka and PAHO formulas). These results

18

indicate that even with the best formula, the individual error of estimating 24-hour urinary sodium

19

excretion would exceed 30% in 36% of subjects, and the individual error of estimating 24-hour urinary

20

potassium excretion would exceed 30% in 29% of subjects. Thus, all three formulas were rather

21

inadequately accurate for individual clinical decisions, although their accuracy for potassium was

22

somewhat higher that for sodium.

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Our findings of inadequate individual precision and accuracy of the evaluated formulas,

23 24

especially for sodium, are generally consistent with mostly similarly negative findings of previous

25

studies, although most other studies evaluated these formulas only for sodium and in generally healthy

26

subjects [19,20]. Two reviews that evaluated estimating 24-hour urinary sodium excretion based on

27

spot urine measurements [1,5] also concluded that while spot urinary sodium may provide adequate

28

mean estimates at the population level, it is a poor predictor of individual 24-hour urinary sodium

29

excretion.

18

ACCEPTED MANUSCRIPT Some other studies used P30 as the measure of individual accuracy of the evaluated

2

formulas, but only for sodium and in populations different than our hypertensive subjects, such as

3

patients with chronic kidney disease (CKD). In the study by Dougher et al. [18], P30 values calculated

4

for four formulas for sodium (INTERSALT, Tanaka, Kawasaki, and Nerbass) in CKD patients were

5

50% to 57%. The authors concluded that all four equations had poor precision and accuracy for the

6

estimation of individual 24-hour sodium excretion. In another study in healthy Chinese subjects [21]

7

which also did not support the use of spot urine to estimate 24-hour urinary sodium excretion at the

8

individual level, accuracy was evaluated based on the proportions of relative differences >40%

9

between the measured and estimated 24-hour urinary sodium excretion.

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The three formulas evaluated in our study were previously studied mostly in healthy subjects,

11

and relatively few studies compared these formulas to each other, especially in regard to both sodium

12

and potassium. The Tanaka and Kawasaki formulas were initially validated in healthy Japanese

13

populations [8,9]. However, estimates of 24-hour urinary sodium excretion using the Kawasaki and

14

Tanaka formulas were shown in some studies to be inadequate in non-Asian populations. In a multi-

15

ethnic study performed in the United Kingdom and Italy, Ji et al. reported that estimates of urinary

16

sodium excretion using the Tanaka formula showed limited agreement with the measured 24-hour

17

urinary sodium excretion [22]. These authors also found low correlations in various ethnic groups and

18

both genders, ranging from only r=0.05 in women of African origin to r=0.36 in white women. In

19

addition, in that study the Tanaka formula tended to underestimate sodium excretion at its higher

20

levels, and overestimate sodium excretion at its lower levels, similarly to our study. Based on available

21

evidence, Ji et al. concluded that the Tanaka method performs inconsistently in different populations.

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The Tanaka formula was also found to be imprecise in the study by Hooft van Huysduynen et

22 23

al. [23] in young healthy Danish women aged 19-26 years. In that study, the mean 24-hour urinary

24

sodium excretion estimated using the Tanaka method was overestimated by on average 21.2 mmol/d,

25

and potassium excretion was overestimated by 13.6 mmol/d. Based on these results, the authors

26

concluded that it was not possible to accurately predict 24-hour urinary sodium and potassium

27

excretion using the Tanaka method.

28

The Kawasaki formula was developed for the second morning sample and not the fasting

29

morning (overnight) sample, which might affect its performance when used with different timing of spot

19

ACCEPTED MANUSCRIPT urine sampling. In our study, the Kawasaki formula indeed performed worse for sodium but not for

2

potassium, at least regarding the mean bias. However, in many studies it was used for less precisely

3

defined “morning” samples, including first voiding (i.e., overnight spot sample) [18,20]. In the study by

4

Cogswell at al. [19] in healthy black and non-black U.S. population, the Kawasaki formula was indeed

5

used for second morning sample and despite this it performed worst out of the four evaluated formulas

6

for sodium. In contrast, in the study by Zhou et al. [21] in healthy Chinese subjects, the Kawasaki

7

formula was better than the Tanaka and INTERSALT formulas for sodium, despite being used for

8

loosely defined morning urine samples. These differences in the performance of the Kawasaki formula

9

may be thus more related to the geographical/racial origin of the study population than to whether the

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second morning sample was used or not.

In a New Zealand study performed in healthy subjects aged 18-65 years [12], the PAHO

12

formula was found to be more accurate for estimating 24-hour urinary sodium excretion than the

13

Kawasaki and Tanaka formulas. In that study, all formulas overestimated urinary sodium excretion,

14

and the mean bias was 5.9 mmol/d for the PAHO formula, 6.6 mmol/d for the Tanaka formula, and

15

48.5 mmol/d for the Kawasaki formula. Correlation with the measured 24-hour urinary sodium

16

excretion was 0.61 for the PAHO formula, 0.58 for the Tanaka formula, and 0.56 for the Kawasaki

17

formula. The study protocol allowed spot urine to be collected by the participants at any time within the

18

24-hour urinary collection period. Similarly to our study, the Tanaka formula in the New Zealand study

19

tended to underestimate sodium excretion at higher levels of excretion, and overestimate it at lower

20

levels of excretion, the PAHO formula overestimated sodium excretion at higher levels of excretion,

21

and the Kawasaki formula was the most biased one but showed no major under- or overestimation at

22

lower and higher levels of excretion. The authors suggested that the observed error of the Kawasaki

23

formula may be attributed to such factors as the difference in body size between Japanese and New

24

Zealand populations.

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The only study that directly compared the Tanaka and Kawasaki formulas for both potassium

25 26

and sodium was reported by Mente et al. [24]. In this international study that included 1,083 individuals

27

aged 35-70 years from the general population in 11 countries and evaluated morning fasting urine

28

samples, the Kawasaki formula performed better than the Tanaka formula for both potassium and

29

sodium, but the PAHO formula was not evaluated in this study. The Kawasaki formula overestimated

20

ACCEPTED MANUSCRIPT 24-hour urinary sodium excretion by on average 13.6 mmol/l, and the Tanaka formula underestimated

2

it by on average 23.8 mmol/l, with the correlation coefficients of 0.71 and 0.54, respectively. Both

3

formulas underestimated potassium excretion: the mean bias was 11.8 mmol/d for the Kawasaki

4

formula and 20.7 mmol/d for the Tanaka formula, with the correlation coefficients of 0.55 and 0.36,

5

respectively.

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The usefulness of the PAHO formula in healthy subjects in the general population was

7

evaluated in various ethnic groups and geographical regions of the world. In the study by Doenyas-

8

Barak et al. [25] performed in a small group of healthy subjects in Israel, 24-hour urinary sodium and

9

potassium excretion was estimated based on four scheduled spot urine samples collected over 24

10

hours, using an approach that may be considered a modification of the PAHO formula. Urinary sodium

11

excretion was overestimated on average by 10 mmol/d (95% limits of agreement -75 to 54 mmol/d),

12

and potassium excretion was underestimated by 11 mmol/d (95% limits of agreement -33 to +55

13

mmol/d). Based on these results, the authors concluded that their approach was acceptable as a

14

convenient method for estimation of 24-hour urinary sodium and potassium excretion.

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Mizéhoun-Adissoda et al. [26] used the second morning spot urine sample and the PAHO

16

formula to estimate 24-hour urinary sodium and potassium excretion in a healthy African population

17

aged 25-64 years. In their study, the PAHO formula overestimated the actual 24-hour urinary

18

potassium excretion by 13.4 mmol/d (95% limits of agreement -53 to +23 mmol/d), and 24-hour urinary

19

sodium excretion by 21.7 mmol/d (95% limits of agreement -478 to 439 mmol/d). The authors

20

concluded that the evaluated approach was acceptable for estimating 24-hour urinary sodium and

21

potassium excretion in a black population, but the limits of agreement for the mean difference for

22

sodium were too large for this method to be useful at in individual level.

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Only few studies were designed to assess the usefulness of these formulas in hypertensive

23 24

patients. Recently, Han et al. evaluated the usefulness of the Tanaka and Kawasaki formulas to

25

estimate 24-hour urinary sodium excretion based on morning and late afternoon/evening urine

26

samples in 222 untreated Chinese hypertensive patients [27]. Best results were obtained with the

27

Kawasaki formula used for morning fasting urine samples, which underestimated sodium excretion by

28

only 2.1 mmol/d but with 95% limits of agreement of more than ±100 mmol/d, similarly to our study

29

(correlation coefficient 0.64). In contrast, the Kawasaki formula used for afternoon samples

21

ACCEPTED MANUSCRIPT overestimated sodium excretion by as much as 84.5 mmol/d, with the correlation coefficient of only

2

0.17. The Tanaka formula overestimated sodium excretion for both morning and afternoon samples,

3

with the mean bias of 30.1 and 21.1 mmol/d, respectively, and the correlation coefficient of 0.38 and

4

0.26, respectively. Han et al. excluded patients on antihypertensive medications, with CKD, and those

5

with suspected secondary forms of hypertension. In our study, we included treated hypertensive

6

patients and those with CKD or secondary hypertension. Previously, Imai et al. [28] reported that the

7

Tanaka formula used for morning urine samples was accurate enough for estimating sodium excretion

8

in patients with CKD, and Kawamura et al. [29] found that the Kawasaki formula worked well for

9

estimating sodium excretion from morning urine samples in patients with hypertension taking

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antihypertensive drugs.

As far as we are aware of, no previous study compared the three formulas for estimating 24-

12

hour urinary sodium and potassium excretion based on spot urine measurements in hypertensives. In

13

addition, the Bland-Altman approach has not been used previously to evaluate the diagnostic precision

14

of estimating 24-hour urinary potassium excretion in hypertensives based on spot urine potassium

15

measurements.

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Spot urine-based estimates of urinary sodium or potassium excretion cannot be expected to

17

reflect actual 24-hour urinary excretion with a very high degree of precision as spot urine

18

measurements reflect urinary excretion over only last few hours, and urinary sodium and potassium

19

excretion may fluctuate depending on multiple factors such as dietary intake, patient activity and

20

posture, renal and urinary system function, and neurohormonal influences [1,30]. In addition, urinary

21

creatinine level, which is a factor in all the formulas we evaluated, may also fluctuate. Although

22

individual creatinine excretion is considered to be relatively stable, it may change in relation to such

23

factors as dietary protein intake and exercise [27]. Due to this variation over 24 hours, the timing of

24

spot urine sampling may affect the estimates of 24-hour urinary sodium and potassium excretion. Han

25

et al. [27] confirmed that the Kawasaki formula worked poorly for afternoon samples in hypertensives,

26

while the usefulness of the Tanaka formula was similar for morning and afternoon samples, which is

27

consistent with the fact that the Tanaka formula was developed based on convenience spot urine

28

specimens collected at various times of the day. Some other studies have also identified particular

29

times for optimal collection of spot urine samples, for example Mann and Gerber [11] concluded that a

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ACCEPTED MANUSCRIPT 1

late afternoon spot urine was best for predicting 24-hour urinary sodium excretion using the PAHO

2

formula. The optimal timing of spot urine sampling in relation to the period of 24-hour urinary collection

4

has also been debated. As highlighted by Ji et al. [1], most studies compared 24-hour urine collection

5

data with data derived from partial collections or spot urine samples collected during the same 24-hour

6

period rather than independently of it. Only some authors [11] tested the relationship between spot

7

and 24-hour urine using spot urine samples collected over a different 24-hour period, based on the

8

notion that the appropriate validation test would be between a 24-hour sample and another sample

9

independent of the 24-hour collection to avoid spurious intercorrelations. In contrast, an argument for

10

using spot urine collected within the period of 24-hour urine collection is that due to a well-documented

11

day-to-day variability of electrolyte intake and excretion, spot urine is less likely to reliably estimate

12

another day’s excretion. McLean et al [12] examined whether using two spot urine samples, one within

13

the 24-hour urine collection period, and one independent of it, improved the reliability of the estimate

14

but found no advantage over estimates using a single spot urine result. In our study, spot urine

15

samples were obtained in the morning, at the same time when 24-hour urine collection was initiated.

16

Similar approach was also used in some other studies [21,26] and it also reflected the diagnostic

17

routine in our hospital unit, where both single urine samples are collected and 24-hour urine

18

collections are usually started in the morning. It should also be noted that all factors affecting the

19

variability of urinary sodium, potassium, and creatinine excretion may be expected to be of a lesser

20

importance when a study compares various formulas for estimating 24-hour urinary excretion that are

21

based on the same variables, as they are likely affected by these factors in a similar way.

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Regarding the generalizability of our findings, we believe that our study population was in

22 23

general representative for hypertensive patients admitted to our unit, with the case mix mostly

24

including patients with difficult-to-control hypertension and those evaluated for suspected secondary

25

hypertension. The patients were not selected in any way, as we tried to include everyone that had both

26

routine urinalysis and 24-hour urine collection ordered at the same time. Regarding the

27

representativeness of our study population at a wider level, e.g. national, this may perhaps be judged

28

based on such parameters as the severity of hypertension (expressed as the mean number of

29

antihypertensive drugs per patient) and the rates of concomitant conditions (cardiovascular disease,

23

ACCEPTED MANUSCRIPT diabetes). Our study population had higher cardiovascular risk compared to the general population or

2

the overall hypertensive population in Poland, with higher rates of cardiovascular disease and diabetes

3

compared to the national estimates [31,32]. In a large POSTER survey on poorly controlled

4

hypertension in Poland [33], cardiovascular disease and diabetes rates and the mean number of

5

antihypertensive medications per patient were similar to our study. Finally, it should be noted that all

6

our patients were Caucasians. Thus, our results are likely not representative for other racial and ethnic

7

groups.

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In summary, our findings are likely generalizable to a Caucasian hypertensive population that

9

is seen by hypertension specialists or admitted to specialized hypertension units due to such problems

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as difficult-to-control hypertension or suspected secondary hypertension.

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13

Various factors potentially affecting urinary sodium, potassium, and creatinine excretion, such as

14

dietary intake and the daily level of exercise, were not controlled for but these would affect the three

15

formulas equally, and in-hospital variability of these factors is likely reduced in comparison to

16

outpatient settings. Either over- or undercollection of 24-hour urine by the study subjects cannot be

17

ruled out. We were unable to use the para-aminobenzoic acid (PABA) method to evaluate

18

completeness of urine collection [5] for logistic and financial reasons (provision of PABA pills to

19

patients, laboratory costs). However, we attempted to minimize the error related to possible inaccurate

20

24-hour urine collection by excluding patients with 24-hour urine output or urinary creatinine excretion

21

beyond the set limits, as defined in the exclusion criteria.

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Our calculations were based on a single 24-hour urine collection and single morning spot urine

22 23

sample in each participant. As we decided to choose the only three formulas that could be used for

24

both sodium and potassium, we did not consider other formulas reported in the literature that were

25

developed only for sodium, such as the INTERSALT formula, the Danish formula, the SH2 formula

26

from Singapore, and others [18,23,34-36]. Another formula is the Mage formula, developed to be used

27

with NHANES study urine specimens, which is essentially the same as the PAHO formula, only with

28

conversion factors added due to different measurement units used for sodium and creatinine [19].

24

ACCEPTED MANUSCRIPT Except for the INTERSALT formula, all other formulas developed for sodium were either used only in

2

single studies or developed for specific subsets, such as the Nerbass formula for CKD patients [18].

3

The INTERSALT formula, despite its relative popularity in the previous studies that compared various

4

formulas for sodium, did not fare consistently better that the other formulas we chose. For example, in

5

a study in healthy U.S. subjects [19], the Mage/PAHO formula performed equally well as the

6

INTERSALT formula for first morning (overnight) urine samples, and both formulas were better than

7

the Tanaka and Kawasaki formulas.

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The three formulas we evaluated require input about 24-hour urinary creatinine excretion,

9

either measured or estimated. Our calculations were based on the measured 24-hour creatinine

10

excretion, similarly to the approach used in other studies [11,12]. However, 24-hour urinary creatinine

11

excretion must also be derived indirectly in order to truly to eliminate the need for a 24-hour urine

12

collection. Currently available methods to estimate 24-hour urinary creatinine excretion may also have

13

some limitations [22].

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14 Summary and conclusions

16

In our study, we used three formulas to estimate 24-hour urinary sodium and potassium excretion

17

based on spot urine measurements in hypertensive patients. Using a single morning urine sample, we

18

found the PAHO formula to be the best for predicting the mean 24-hour urinary potassium excretion

19

and perhaps also the mean 24-hour urinary sodium excretion in hypertensive adults in an inpatient

20

facility. When estimating 24-hour urinary sodium excretion, the PAHO and Tanaka formulas yielded

21

similar average population estimates, but in subjects with elevated urinary sodium excretion, its

22

underestimation by the Tanaka formula was numerically higher compared to overestimation by the

23

PAHO formula, and the Tanaka formula overestimated sodium excretion in subjects with low values of

24

24-hour urinary sodium excretion. Similarly, in our study population the Kawasaki formula for

25

potassium was characterized by a similar mean bias compared to the PAHO formula but in contrast to

26

the latter, the Kawasaki formula tended to underestimate potassium excretion at higher levels of its 24-

27

hour urinary excretion, and overestimate it at lower levels of excretion. In addition, we found that the

28

Kawasaki formula was clearly inferior for estimating 24-hour urinary sodium excretion, and the Tanaka

29

formula was clearly inferior for estimating 24-hour urinary potassium excretion compared to the other

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ACCEPTED MANUSCRIPT 1

formulas. However, all the evaluated formulas were characterized by low precision and accuracy for

2

estimating individual 24-hour urinary sodium or potassium excretion.

3 4

References

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comparing 24-hour and spot urine collections for estimating population salt intake. Rev Panam Salud

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Publica 2012; 32: 307-315.

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2. Agarwal R. Resistant hypertension and the neglected antihypertensive: sodium restriction. Nephrol

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5. McLean RM. Measuring population sodium intake: a review of methods. Nutrients 2014, 6, 4651-

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6. Mente A, O’Donnell MJ, Yusuf S. Measuring sodium intake in populations: simple is best? Am J

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ACCEPTED MANUSCRIPT 10. WHO/PAHO Regional Expert Group for Cardiovascular Disease Prevention through Population-

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Wide Dietary Salt Reduction. Protocol for population level sodium determination in 24-hour urine

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samples. Geneva: World Health Organization, 2010.

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13. Jędrusik P, Symonides B, Wojciechowska E, Gryglas A, Gaciong Z. Diagnostic value of potassium

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14. Chernecky CC, Berger BJ. Laboratory tests and diagnostic procedures. St. Louis: Saunders

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15. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of

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clinical measurement. Lancet 1986; 327(8476): 307-310.

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standard method is misleading. Lancet 1995; 346: 1085-1087.

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17. Carpenter J, Bithell J. Bootstrap confidence intervals: when, which, what? A practical guide for

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18. Dougher CE, Rifkin DE, Anderson CAM, Smits G, Persky MS, Block GA, et al. Spot urine sodium

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measurements do not accurately estimate dietary sodium intake in chronic kidney disease. Am J Clin

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Nutr 2016; 104: 298-305.

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19. Cogswell ME, Wang C-Y, Chen T-C, Pfeiffer CM, Elliott P, Gillespie CD, et al. Validity of predictive

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equations for 24-h urinary sodium excretion in adults aged 18–39 y. Am J Clin Nutr 2013; 98: 1502-

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20. Kelly C, Geaney F, Fitzgerald AP, Browne GM, Perry IJ. Validation of diet and urinary excretion

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derived estimates of sodium excretion against 24-h urine excretion in a worksite sample. Nutr Metab

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ACCEPTED MANUSCRIPT 21. Zhou L, Tian Y, Fu J-J, Jiang Y-Y, Bai Y-M, Zhang Z-H, et al. Validation of spot urine in predicting

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24-h sodium excretion at the individual level. Am J Clin Nutr 2017; 105: 1291–1296.

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ACCEPTED MANUSCRIPT Highlights for the manuscript Ref. No.: JASH-D-17-00245 “Comparison of three formulas to estimate 24-hour urinary sodium and potassium excretion in patients hospitalized in a hypertension unit”

• We compared 3 spot urine equations to estimate 24-h urinary Na and K excretion.

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• The study was performed in 382 hospitalized hypertensives.

• The PAHO equation was best for predicting mean 24-h urinary Na and K excretion.

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• At the individual level, the precision and accuracy of all equations was inadequate.