Recommendations for measurement of and conventions for reporting sodium and potassium by ion-selective electrodes in undiluted serum, plasma or wholeblood

Ion-selective electrodes (ISEs) respond to ion-activity and therefore do no t sense substance concentration directly. However, it is recognized that so dium and potassium in plasma will continue to be expressed for clinical pur poses in terms of substance concentration (mmol/l). A convention is propose d whereby for routine clinical purposes results of ISE measurements of sodi um and potassium in undiluted plasma should be reported in terms of substan ce concentration (mmol/l). In specimens with normal concentrations of plasm a water, total CO2, lipids, protein and pH, the values will concur with the total substance concentration as determined for example by flame atomic em ission spectrometry (FAES) or ISE measurements on diluted samples. In speci mens with abnormal concentrations of plasma water, the results will differ. However, under these circumstances, measurements of sodium and potassium b y ISE in the undiluted sample will more appropriately reflect the activity of sodium and potassium and are therefore clinically more relevant than the determination in diluted samples. Detailed recommendations are made about practical procedures to achieve this. The recommended name for this quantit y is the substance concentration of ionized sodium or ionized potassium in plasma, as opposed to total sodium or total potassium determined by, e.g. F AES, or ISE measurements on diluted samples.