Magnesium fractions in serum of healthy individuals and CAPD patients, measured by an ion-selective electrode and ultrafiltration

Objectives: Validation and comparison of a magnesium ion-selective electrode (ISE) with a cation-exchange resin technique, followed by determination of all magnesium fractions in serum of healthy volunteers and continuous ambulatory peritoneal dialysis (CAPD) patients. Design and Methods: The analytical aspect has been studied by measuring the influence of complexing agents on the fraction ionized magnesium (friMg2+). A theoretical approximation of friMg2+, based on mass equilibria and complexation constants, was calculated and compared with the measurements. Results: ISE measurements showed good agreement with theory. Reference values of the ionized, protein-bound, and complexed magnesium tractions were (mean ± SD) 0.65 ± 0.04, 0.27 ± 0.04, and 0.08 ± 0.03, respectively. Fractions obtained in the CAPD group were 0.62 ± 0.04, 0.22 ± 0.05, and 0.16 ± 0.05, respectively, and differed significantly from the values of the reference population. Conclusions: All known serum magnesium parameters can be established by a combination of ultrafiltration, atomic absorption spectrometry, and ISE measurements. Unknown complexing compounds most probably account for the increased fraction of complexed magnesium in the serum of CAPD patients.