Electrode Measurement of Glucose and Urea in Undiluted Samples

The development of ion-selective electrodes (ISEs) for elec-trolyte measurements necessitates a re-evaluation of the biological and clinical interpretation of a result. In pathologi-cal situations (e.g., hyperlipidemia and hyperproteinemia) direct potentiometry is the method of choice for ion measure-ments in blood. However, the “plasma water effect ” exists also in normal samples, requiring new reference values for physiological ranges. A compromise between medical and instrumentation workers retained the old reference values (flame photometry for Na and K) by introducing correction factors into the ISE instruments, so that the results for direct ISE and flame photometry are the same for “normal ” sam-ples. Analyses of “abnormal ” samples will reveal biases between the two methods. Now, a new generation of elec-trodes for assaying additional metabolites reopens the issue