Validation of the equations for estimating daily sodium excretion from spot urine in patients with chronic kidney disease

Background: Measuring sodium excretion in a 24-h urine collection is the most reliable method of estimating salt intake, but it is not applicable to all patients. As an alternative, equations for estimating Na excretion from Japanese by a spot urine sample were created, but they have not been validated in patients with chronic kidney disease (CKD), which are frequently associated with nocturia and medication. Methods: We enrolled 136 patients with CKD and collected both 24-h urine and the first morning urine. Na excretion was estimated from the first morning urine by Kawasaki's equation, which was originally used for the second morning urine, and Tanaka's equation, which is applied for spot urine samples taken at any time from 9 am to 7 pm. We evaluated the two equations for bias, RMSE and accuracy within 30 and 50% of the measured Na excretion. Results: Bias, RMSE and accuracy within 30% of the estimated Na excretion were 48 ± 69 and 2 ± 69 mmol/day, 84 and 69 mmol/day, and 35 and 49% using Kawasaki's equation and Tanaka's equation, respectively. Na excretion in the first morning urine was accurately estimated by Tanaka's equation, but it was overestimated by Kawasaki's equation. Nocturia and medication such as diuretics and ACE inhibitor or angiotensin receptor blocker did not affect the accuracy with which Na excretion was estimated by Tanaka's equation substantially. Conclusion: Tanaka's equation for estimating Na excretion from the first morning urine in patients with CKD is accurate enough for use in clinical practice.