A candidate reference method for coupled sodium-water determination in human serum

A candidate reference method is described for coupled sodium-water determination based on ion-exchange sodium separation from the serum matrix followed by gravimetry as Na2SO4, and serum water determination by means of microwave evaporation. For sera with normal sodium and water contents, the mean relative standard deviation is 0.6% (0.8 mmol/l). Mean inaccuracy for the coupled sodium-water determination is -0.3% (0.4 mmol/l). The candidate reference method can be considered a reference method because the reference method value did not differ significantly from the definitive value, there is no known source for interferences or bias, and misinterpretation due to abnormal protein or lipid levels is excluded because serum sodium is determined on a plasma water basis. Sodium concentrations determined by the candidate reference method are used for comparing field methods with the candidate reference method. If the resulting regression equation is used in the calibration procedure, good correlation between all (in)direct field methods and the candidate reference method is ensured, and accurate results are produced. Results of proficiency testing show a good correlation between (in)direct field methods and the candidate reference method, because sera with approximately normal water contents are used.