Sodium and potassium measurements: direct potentiometry and flame photometry

Data from the 1979 and part of the 1980 College of American Pathologists (CAP) Chemistry Survey Program have been reviewed in order to compare the direct potentiometric measurement of sodium and potassium with flame-emission photometric measurements. As may have been predicted from earlier intralaboratory evaluations, the results of the CAP interlaboratory comparison confirm that flame-emission photometry of the two electrolytes yields lower mean concentrations. Reasons for this difference lie in the physicochemical aspects of the sample and methods. In Survey specimens, the magnitude of difference was rarely one of clinical significance, but the documented increase in the use of direct electrode measurements in laboratories warrants informing clinicians of potentially significant differences in selected patients.