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

Abstract

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_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_1j depends on the composition and geometry of the salt bridge. Both E_ISE and E_1j 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 E_ISE and E_1j are compared to the mV span of the reference interval, the effects are large for Ca²⁺, Na⁺ and CI⁻. 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.

Key Words:

• Asymmetry potential
• blood gas/electrolytes analyzers
• ion-selective electrode
• liquid junction potential
• matrix effects
• protein-free vs. protein-containing reference material
• quality control