The observation that the ionised calcium concentration is higher if measured in the original serum sample than in an ultrafiltrate is examined by means of a theoretical model. It is concluded that this difference is a predictable consequence of the Donnan equilibrium and is not a result of protein interference with the calcium-specific electrode.
References
1.
PayneRB.Clinically significant effect of protein concentration on ion-selective electrode measurements of ionized calcium. Ann Clin Biochem1982; 19: 233–7.
PedersenKO.Determination of calcium fractions of serum I. The separation of protein-bound and protein-free fractions by means of a simplified ultrafiltration technique. Scand J Clin Lab Invest1969; 24: 69–81.
4.
DonnanFG.Theorie der Membrangleichgewichte und Membranpotentiale bei Vorhandensein von nicht dialysierenden Elektrolyten. Ein Beitrag zur physikalisch-chemischen Physiologie. Zeitschrift für Elektrochemie und angewandte physikalisch Chemie1911; 17: 572–81.
5.
DefaresJGSneddonINWiseME.An introduction to the mathematics of medicine and biologyAmsterdam: North-Holland Publ Co, 1973.
6.
ThodeJWandrupJAasFSiggaard-AndersenO.Evaluation of a new semiautomatic electrode system for simultaneous measurement of ionized calcium and pH. Scand J Clin Lab Invest1982; 42: 407–15.
