Serum sodium concentration and the progression of established chronic kidney disease

Background Higher serum sodium concentration has been reported to be a risk factor for the development of incident chronic kidney disease (CKD), but its relationship with the progression of established CKD has not been investigated. We hypothesised that increased serum sodium concentration is a risk factor for estimated glomerular filtration rate (eGFR) decline in CKD. Methods This was a retrospective cohort study using data collected over a 6-year period, with baseline data obtained during the first 2 years. We included patients known to our renal service who had had a minimum of three blood tests every 2 years and an eGFR of <60 mL/min/1.73 m(2) at baseline. Exclusion criteria were renal replacement therapy, diabetes mellitus, heart failure and decompensated liver disease. A multiple linear regression model investigated the relationship between baseline serum sodium and eGFR decline after adjustment for confounding factors. Results 7418 blood results from 326 patients were included. There was no relationship between serum sodium concentration and estimated glomerular filtration rate at baseline. After multivariable adjustment, a 1 mmol/L increase in baseline serum sodium was associated with a 1.5 mL/min/1.73 m(2) decline in eGFR during the study period (95% CI 0.9, 2.0). A reduction in eGFR was not associated with significant changes in serum sodium concentration over 6 years. Conclusion Higher serum sodium concentration is associated with the progression of CKD, independently of other established risk factors. Conversely, significant alterations in serum sodium concentration do not occur with declining kidney function.