STANDARDIZING AND REPORTING RESULTS FROM MG2+ ISES, WITH SOME NOTES ON SAMPLE HANDLING

Mg2+ by ion-selective electrode (ISE) is a direct measure of the react ivity of magnesium ions in plasma, which may be clinically and physiol ogically more relevant than the concentration of total magnesium. The Mg2+-ISE will build up an electric potential which exactly matches the chemical potential of Mg2+ in the sample. Chemical potential (= chemi cal work per unit of Mg2+) has no absolute value and is difficult to v isualize. The results must be standardized, either to the magnesium co ncentration in a protein-free calibrator or to the concentration of to tal magnesium in plasma. The constant factor relationship will assure identical clinical discrimination, no matter how the Mg2+ -ISE results are reported. The general opinion of a conference held in Orlando, Fl orida in March of 1993 was to define free Mg2+ in plasma as the concen tration of magnesium in a saline standard with the same magnesium acti vity as the sample, and report it in SI units (mmol/L). The small diff erences in liquid junction potential and water concentration will prov ide a value for free Mg2+ in plasma of approximately 103% of the true concentration, or 96% of the true molality. Differences between plasma and interstitial fluid and Donnan equilibria across the vascular endo thelium will affect the result, so sampling should take place with the patient at rest to assure a stable plasma volume. Since heparin binds magnesium, it is preferable to use serum or plasma with a minimum of heparin which has been titrated with magnesium. Binding of Mg2+ depend s on pH, and pH of circulating plasma is not constant. Furthermore, pH may change during and after sampling. The Mg2+-ISE system should ther efore include pH and allow correction of free Mg2+ to pH 7.4.