[Measurement errors in determination of electrolyte concentrations with ion selective electrodes in the presence of inorganic or metabolizable anions].

Ion-selective electrodes (ISE) from various manufacturers were tested with regard to their accuracy in determining the concentrations of sodium, potassium and calcium in aqueous solutions when uni- or polyvalent, inorganic or organic anions are being present in physiological concentrations. The results obtained by all measuring instruments using ISE revealed that the presence of anions such as acetate, citrate, malate, lactate, phosphate or sulfate, either in physiological or therapeutic concentrations, may lead to considerable errors when determining the concentrations of sodium and calcium. A certain caution is therefore recommended in order to minimize the risk when using ISE for the electrolyte diagnosis of infusion solutions, blood derivatives, dialysates and urine samples.