Rationale and validation of predicting high sodium intake by spot urinary chloride in patients with chronic kidney disease.

Objective: To analyze the rationale and evaluate the validity of spot urinary chloride or derived formulas to predict high sodium intake in patients with chronic kidney disease (CKD).

Methods: We collected consecutive CKD patients at stages 1-4 who were admitted to our Nephrology department in a single center from January 01, 2014, to December 31, 2017, and tested spot and 24-hour urinary analysis on the same day. The feasibility of urinary chloride to predict urinary sodium was firstly analyzed by calculating their correlations. The validity of predicting excessive sodium intake by spot urinary sodium and chloride, two derived formulas based on spot urinary sodium or chloride, and our previous "CKDSALT" equation were accessed. We finally conducted Receiver operating characteristic (ROC) curves to compare their performance in detecting high sodium intake.

Results: All 5204 patients were eventually analyzed. In the derivation cohort (n = 2447), a strong positive linear correlation existed between urinary sodium and chloride in both spot urine (R = 0.804) and 24-hour urine samples (R = 0.905), and two predictive equations based on spot urinary sodium or chloride were derived. In the validation cohort (n = 2757), spot urinary sodium and chloride only showed "fair" performance. However, both urinary sodium and chloride equations had a "good" performance in ICC, Pearson's correlation, Bland-Altman plots, and ROC curves, while and CKDSALT equation showed the best performance.

Conclusions: Spot urinary chloride is a feasible method to predict and monitor high sodium intake in CKD patients, while a novel derived formula could elevate its diagnostic accuracy.