Sensors zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA and Act uat or s B, I I (1993) 263-271 263 Optodes in. clinical chemistry: potential and limitations* U. E. Spichiger, D. Freiner, E. Bakker, T. Rosatzin and W. Simon Department of Organic Chemistry, Swiss Federal Instilu!e of Technology (ETH), Vniversit&trasse 16, CH-8092 Zkich (Switzerland) Abstract Total ion concentrations of sodium,potassium, calcium and chloride have been measured reversibly by ion-selective optode membranes with comparable selectivity to potentiometric systems. The results of quality-control samples analysed by the selective optode membranes show a good agreement with the assigned values and a reasonable reproducibility, though for quantification of sodium and potassium the unknown concentrations are evaluated in a first trial by a standard addition method. All measurements are referred to a pair of blank membranes in a reference cell to compensate for interferences induced by the diluted plasma sample. The efficiency of the evaluated values with regard to the medical interpretation encompasses the performance standard of the analytical procedure as well as the intra- and interindividual variation of the subjects studied. The required performance characteristics of a sensor are constituted dilhzrently, depending on the mode of application and the medical question to be answered. The type of optodes described may be tailored to a large extent to adapt them to the required performance boundary conditions. This will bc discussed with respect to other optical sensor schemes. Introduction Disorders of the water balance are frequently seen in clinical medicine if water losses exceed replacement or vice versa [ 11. The partition and exchange of elec- trolytes are tightly connected to water balance apart from the specific regulating mechanism. Unfortunately there exists no analytical method which quantitates water activity, although metabolic events are severely affected by the relative and absolute concentrations of electrolytes. Consequently quantitication of total elec- trolyte concentrations amounts to 20% of all require- ments in a clinical chemical laboratory. Nevertheless, the truly biologically relevant fraction is the active ion fraction of electrolytes, evaluated in the sample directly. The ion activity represents the activity of ions in the aqueous phase, independent of the concentration and specific volume of larger fractions of further con- stituents (proteins, lipids). Considerable errors may occur if concentration values are interpreted naively without regard to the volume or concentration of so- lutes as proteins and lipids in serum or whole blood [2-41. This misinterpretation is especially relevant for critically ill patients. Meanwhile discussions on ‘What should be measured and what should be reported’ are going on [5]. To save the reliability of the interpretation of total concentration values, the analysis of a profile of analytes, e.g., total protein or albumin concentration, as well as a lipid profile is recommended. As a refer- *Keynote address. 0925-4005/93/$6.00 ence, about 45% of the total calcium concentration is complexed to proteins. The free calcium-ion concentra- tion, i.e., the biologically active fraction of calcium, is in equilibrium with the protein concentration and the pH. The complexation of calcium to plasma proteins is a reversible process which was described by a thernio- dynamic equilibrium between the protein concentration [Prq related to pH and temperature of the patient, the calcium-ion concentration [C!a2+ln and the calcium- protein complex [Ca,Pr]@“+z), where Z& is the equi- librium constant [6,7]: and Ca,Pr(2” +Z)+2nH+--tnCa2++H,pr’2”+*) (2) n is the stoichiometry of the divalent ion complexed to a protein molecule and z is the mean charge of proteins in the physiological medium, assuming a mean isoelec- tric point (IP) of 4.96 [l]. For total concentration measurements in uirro the pH is decreased to a value of pH 3.5 to distinguish the calcium ions from the com- plexes. Determinations of total electrolyte conccntra- tions in diluted samples are implemented in clinical analysers with high sample throughput. Recommended methods are ion-selective electrode (ISE) measurements for sodium, potassium and chloride in the diluted sam- ple [8,9], flame atomic emission spectroscopy (FAES) for total sodium and potassium, atomic absorption spectroscopy for calcium and magnesium concentration [ 10,l l] and spectrophotometric methods based on ion @ 1993 - Elsevier Sequoia. All rights reserved