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The determination of sodium and potassium in serum will be performed in the near future mainly by ion-selective electrodes. When the samples are highly diluted before measurement, a demonstrably accurate value of the electrolyte concentration in serum can be obtained. Accuracy control by using the pertinent reference methods of the National Institute of Standards and Technology (NIST) is well established for this purpose. In undiluted samples a potential is measured by ion-selective electrodes, which is dependent on the relative molal activity of the electrolyte, from which free molal concentration can be estimated. Usually, the total molal concentration of e.g. sodium is about 1.5% higher than that of free sodium, as it includes portions bound to e.g. carbonate and proteins. Accuracy control is hampered, because reference methods for either relative molal activity or free molal concentration are not yet available. Reference method values for total molal concentration will differ systematically from an accurate value for free molal concentration. When ion-selective electrodes are calibrated by using "normal" sera, accuracy control can be based on the reference method of the NIST; but this is valid only for a very narrow, at best "normal", concentration range of proteins and lipids, assuming, of course, that binding is normal. The greater the variation of the concentration of macromolecules from normal (lower or higher), the greater the difference between the values obtained by the two methods. Calibration of ion-selective electrodes by using sera (this is the least desirable approach for calibration) requires the introduction of a new unit of measurement, which is not compatible with the rational system of quantities and units.(ABSTRACT TRUNCATED AT 250 WORDS)

Reference Methods for the Determination of Sodium, Potassium, pH and Blood Gases with Ion-Selective Electrodes