Is the estimation of daily urinary sodium excretion in patients with chronic kidney disease sufficiently accurate in clinical practice?

Abstract

To the Editor Assessment of dietary salt intake is essential for hypertension management. Several calculation formulas using spot urine samples have been reported for the estimation of daily urinary sodium excretion [1–3]. Recently, Imai et al. [4] have reported the validity of Kawasaki’s and Tanaka’s equations for the estimation of daily urinary sodium excretion in chronic kidney disease (CKD) patients. They concluded that Tanaka’s equation is sufficiently accurate for use. We further examined the accuracy of these calculated estimates. We enrolled 182 CKD patients (116 men, 66 women, mean age 63 ± 14 years, mean height 162 ± 8 cm, mean weight 62 ± 12 kg) who were continuously treated at our department. They provided 24-h urine samples at regular intervals. The mean baseline serum creatinine level was 2.28 ± 1.80 mg/dl (range 0.47–8.77), and the mean estimated glomerular filtration rate was 36.5 ± 24.3 ml/min/ 1.73 m (range 4.8–122.8; CKD stages 1, 2, 3, 4, and 5; n = 6, 24, 61, 59, and 32, respectively). Participants collected 24-h urine, the first morning spot urine, and the casual daytime spot urine on the same day, and sodium and creatinine values were measured in each sample. We calculated 24-h urinary sodium excretion from sodium and creatinine values in the spot urine using Kawasaki’s equation [1] and Tanaka’s equation [2], respectively. We compared those estimated sodium excretion values (morning K-Na, morning T-Na, daytime K-Na, and daytime T-Na) with measured sodium excretion in 24-h urine (measured Na). The mean values of morning K-Na, morning T-Na, daytime K-Na, daytime T-Na, and measured Na were 208 ± 55, 161 ± 34, 163 ± 57, 132 ± 36, and 146 ± 53 mEq, respectively (Fig. 1). Morning K-Na and daytime K-Na were significantly greater than measured Na. The correlations between estimated excretion and measured excretion were significant (morning K-Na: r = 0.56, morning T-Na: r = 0.55, daytime K-Na: r = 0.36, daytime T-Na: r = 0.35; all p \ 0.0001). The numbers of patients whose differences between estimated sodium excretion by each equation and measured sodium excretion were within a range of ±34 mEq (equivalent to 2 g of salt) were 35 (19.2 %) for morning K-Na and 91 (50.0 %) for morning T-Na, 71 (39.0 %) for daytime K-Na and 96 (52.7 %) for daytime T-Na, respectively. The mean values of estimated creatinine excretion by Kawasaki’s equation, those by Tanaka’s equation, and measured creatinine excretion were 1170 ± 311, 1164 ± 276, and 1070 ± 331 mg, respectively (Kawasaki’s, Tanaka’s vs. measured, p \ 0.0001). The correlations between estimated creatinine excretion by each equation and measured creatinine excretion were significant (Kawasaki’s, r = 0.66; Tanaka’s, r = 0.62; both p \ 0.0001). Originally, Kawasaki’s study used second morning urine and Tanaka’s study used casual daytime urine. The present study shows that the correlation coefficients between estimated sodium excretion using morning urine and measured sodium excretion were greater than those between estimated sodium excretion using daytime urine and measured sodium excretion. On the other hand, the differences between estimated sodium excretion using Tanaka’s equation and measured sodium excretion were smaller than those between estimated sodium excretion using Kawasaki’s equation and measured sodium excretion. However, the differences could not be ignored even using Tanaka’s equation. Such T. Okada (&) A. Hayashi T. Nakao Department of Nephrology, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan e-mail: t-okada@tokyo-med.ac.jp