THE NEED FOR PROTEIN CONTAINING QUALITY-CONTROL MATERIALS FOR BLOOD-PH AND ELECTROLYTE ANALYZERS

In clinical chemistry quality control refers to monitoring of precisio n and accuracy of the performance of analytical methods. Calibration s olutions and (matrixed) control solutions are used in transferring acc uracy between definitive method, reference method and field methods. F or this purpose aqueous (protein-free), protein-containing and serum-b ased types of quality control materials having different matrices are available. Here are presented differences in behaviour between aqueous (protein-free) and protein-containing materials. Potentiometry with a n electrochemical cell is an often used field method to determine pH a nd activity of electrolytes with an Ion-Selective Electrode (ISE). Bas ically, measured e.m.f.'s of calibrators and sample are translated int o the activity of the ionic species of the sample by means of the Nern st equation. Besides the standard e.m.f. (E(o)) of the electrochemical system, the measured e.m.f. includes the ISE-membrane e.m.f. (E(ISE)) and depends on electrolyte type and its concentration and E(Ij) depen ds on the composition and geometry of the salt bridge. Both E(ISE) and E(Ij) depend on the sample matrix. Protein-containing samples cause a negative bias on the e.m.f. at the Liquid junction and a positive bia s at the ISE. When E(ISE) and E(Ij) are compared to the mV span of the reference interval, the effects are large for Ca2+, Na+ and Cl-. Exac tly the same effects exist for H+, K+, Li+ and Mg2+ but the inaccuracy is less critical for these ions. It may be concluded that only protei n containing controls can detect an error that occurs only with measur ements on plasma specimens. In practice this means that calibration is checked with protein-free solutions, but that measurements in plasma are best checked with protein-containing solutions.