An analysis of measured and calculated calcium quantities in serum

Abstract

Potentiometrically measured ionized calcium (Ca²⁺) was compared with total and albumin corrected S-Calcium and calculated Ca²⁺ in serum samples from blood donors and patients, for whom a request of S-Calcium had been made. Total S-Calcium, Ca²⁺ and S-Albumin in donors conformed with a simple mass law equation with a mean K_caAlb = 95 l/mol, whereas apparent albumin binding affinity rose with decreasing S-Albumin in patients (due to rise of other calcium binding proteins) with resulting overestimation of albumin corrected and particularly of calculated Ca²⁺. The frequency and severity of hypoalbuminaemia in patients make calculated Ca²⁺ unreliable and albumin corrected S-Calcium the clinically most satisfactory routinely practicable substitute for direct Ca²⁺ measurement.

Key Words:

• multivariate analysis
• potentiometry
• protein binding
• reference ranges
• selective ion electrodes
• serum albumin
• serum calcium
• serum ionized calcium