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 ele ctrode (ISE) with a cation-exchange resin technique, followed by deter mination 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 infl uence of complexing agents on the fraction ionized magnesium (friMg(2)). A theoretical approximation of friMg(2+), based on mass equilibria and complexation constants, was calculated and compared with the meas urements. Results: ISE measurements showed good agreement with theory. Reference values of the ionized, protein-bound, and complexed magnesi um fractions were (mean +/- SD) 0.65 +/- 0.04, 0.27 +/- 0.04, and 0.08 +/- 0.03, respectively. Fractions obtained in the CAPD group were 0.6 2 +/- 0.04, 0.22 +/- 0.05, and 0.16 +/- 0.05, respectively, and differ ed significantly from the values of the reference population. Conclusi ons: All known serum magnesium parameters can be established by a comb ination of ultrafiltration, atomic absorption spectrometry, and ISE me asurements. Unknown complexing compounds most probably account for the increased fraction of complexed magnesium in the serum of CAPD patien ts.