A method of controlling diffusive sodium transport from the dialysate solution to the blood of a patient undergoing hemodialysis treatment or from the blood to the dialysate solution includes calculating SNa+average, an average of the patient's historic serum sodium concentrations, and estimating SNa+estimated, the patient's pre-dialysis serum sodium concentration, based on the average of the patient's measured historic pre-dialysis serum sodium concentrations, SNa+measured. The method enables adjusting DNa+, the sodium concentration of the dialysate solution, based on the average of the patient's historic serum sodium concentrations, SNa+average, if needed, and performing the dialysis treatment of the patient using a dialysate solution containing a sodium concentration DNa+.