The need for protein containing quality control materials for blood pH and electrolyte analyzers

In clinical chemistry quality control refers to monitoring of precision and accuracy of the performance of analytical methods. Calibration solutions and (matrixed) control solutions are used in transferring accuracy between definitive method, reference method and field methods. For this purpose aqueous (protein-free), protein-containing and serum-based 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 an electrochemical cell is an often used field method to determine pH and activity of electrolytes with an Ion-Selective Electrode (ISE). Basically, measured e.m.f.'s of calibrators and sample are translated into the activity of the ionic species of the sample by means of the Nernst equation. Besides the standard e.m.f. (E(zero)) of the electrochemical system, the measured e.m.f. includes the ISE-membrane e.m.f. (EISE) and depends on electrolyte type and its concentration and Eij depends on the composition and geometry of the salt bridge. Both EISE and Eij depend on the sample matrix. Protein-containing samples cause a negative bias on the e.m.f. at the liquid junction and a positive bias at the ISE. When EISE and Eij are compared to the mV span of the reference interval, the effects are large for Ca2+, Na+ and Cl-. Exactly 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 protein containing controls can detect an error that occurs only with measurements 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.