INTERNATIONAL CONSENSUS ON THE STANDARDIZATION OF SODIUM AND POTASSIUM MEASUREMENTS BY ION-SELECTIVE ELECTRODES IN UNDILUTED SAMPLES

The International,Federation of Clinical Chemistry (IFCC) and the Nati onal Committee for Clinical Laboratory Standards (NCCLS) are about to recommend to adjust sodium and potassium measurements by ion-selective electrodes in undiluted samples to the amount of substance concentrat ion in the sample as determined, e.g., by flame atomic emission spectr ometry. The adjustment is only valid in case of normal standardized se ra (or plasma), implying ''normal'' water concentration (normal concen tration of proteins, lipids or other macromolecules), ''normal'' bindi ng of the pertinent electrolytes and ''normal'' coefficient of activit y. If these criteria are not met, results obtained by ''adjusted'' ISE 's will differ from total molar concentration. That is: in individual samples of patients results from ISE's and total molar concentration w ill differ unpredictably. It forced IFCC to propose new quantities for the measurements by adjusted ISE's: ionized sodium and ionized potass ium. The reference interval for ionized sodium and ionized potassium i s identical to the pertinent reference interval for molar concentratio n of total sodium and total potassium, but it is in contrast independe nt from water concentration and valid, e.g., in hypoproteinaemia as we ll. as in hyperlipaemia or hyperproteinaemia. Accuracy control of ioni zed sodium and ionized potassium based on reference method values is h ampered by abnormal water concentration and inadequate properties of t he matrix of many control sera. Alternative approaches how to report m easurements by ISE's in undiluted samples, such as activity or free mo lal concentration are discussed with their pros and cons regarding acc uracy control by reference method values. The need for appropriate con trol materials with a matrix similar to native human sera is stressed.