Kidney Research and Clinical Practice  
Estimating the urinary sodium excretion in patients with chronic kidney disease is not useful in monitoring the effects of a low-salt diet
Se-Yun Kim1, Yu Ho Lee1, Yang-Gyun Kim1, Ju-Young Moon1, Ho Jun Chin2, Sejoong Kim2, Dong Ki Kim3, Suhnggwon Kim3, Jung Hwan Park4, Sung Joon Shin5, Bum Soon Choi6, Chun Soo Lim7, Minjung Lee8, Sang-ho Lee1
1Department of Internal Medicine, Kyung Hee University Medical Center, Seoul, Korea
2Department of Internal Medicine, Seoul National University Bundang Hospital, Seongnam, Korea
3Department of Internal Medicine, Seoul National University Hospital, Seoul, Korea
4Department of Internal Medicine, Konkuk University School of Medicine, Seoul, Korea
5Department of Internal Medicine, Dongguk University Ilsan Hospital, Goyang, Korea
6Department of Internal Medicine, The Catholic University of Korea, Seoul St. Mary’s Hospital, Seoul, Korea
7Department of Internal Medicine, SMG-SNU Boramae Medical Center, Seoul, Korea
8Department of Clinical Pharmacology, School of Medicine, Kyung Hee University, Seoul, Korea
Correspondence to: Sang-ho Lee
Division of Nephrology, Kyung Hee University Hospital at Gangdong, 892 Dongnam-ro, Gangdong-gu, Seoul 05278, Korea. E-mail: lshkidney@khu.ac.kr
Received: August 10, 2017; Revised: August 21, 2018; Accepted: September 19, 2018; Published online: November 20, 2018.
© The Korean Society of Nephrology. All rights reserved.

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Purpose: Several epidemiologic studies have suggested that the urine sodium excretion (USE) can be estimated in lieu of performing 24-hour urine collection. However, this method has not been verified in patients with chronic kidney disease (CKD) or in an interventional study. The purpose of this study was to evaluate the usefulness of estimating USE in a prospective low-salt diet education cohort (ESPECIAL).
Methods: A new formula was developed on the basis of morning fasting urine samples from 228 CKD patients in the ESPECIAL cohort. This formula was compared to the previous four formulas in the prediction of 24-hour USE after treatment with olmesartan and low-salt diet education.
Results: Most previously reported formulas had low predictability of the measured USE based on the ESPECIAL cohort. Only the Tanaka formula showed a small but significant bias (9.8 mEq/day, P < 0.05) with a low correlation (r = 0.34). In contrast, a new formula showed improved bias (-0.1 mEq/day) and correlation (r = 0.569) at baseline. This formula demonstrated no significant bias (-1.2 mEq/day) with the same correlation (r = 0.571) after 8 weeks of treatment with olmesartan. Intensive low-salt diet education elicited a significant decrease in the measured USE. However, none of the formulas predicted this change in the measured urine sodium after diet adjustment.
Conclusion: We developed a more reliable formula for estimating the USE in CKD patients. Although estimating USE is applicable in an interventional study, it may be unsuitable for estimating the change of individual sodium intake in a low-salt intervention study.
Keywords: Chronic kidney disease, Low-salt diet, Sodium excretion


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