Standardizing and reporting results from Mg²⁺ ISEs, with some notes on sample handling

Abstract

Fogh-Andersen N, Siggaard-Andersen O. Standardizing and Reporting Results from Mg²⁺ ISEs, with Some Notes on Sample Handling. Scand J Clin Lab Invest 1994; 54, Suppl 217: 89-96.

Mg by ion-selective electrode (ISE) is a direct measure of the reactivity of magnesium ions in plasma, which may be clinically and physiologically 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 (= chemical work per unit of Mg²⁺) has no absolute value and is difficult to visualize. The results must be standardized, either to the magnesium concentration in a protein-free calibrator or to the concentration of total 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, Florida in March of 1993 was to define free Mg²⁺ in plasma as the concentration of magnesium in a saline standard with the same magnesium activity as the sample, and report it in SI units (mmol/L). The small differences in liquid junction potential and water concentration will provide 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 endothelium 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⁺ depends on pH, and pH of circulating plasma is not constant. Furthermore, pH may change during and after sampling. The Mg²⁺-ISE system should therefore include pH and allow correction of free Mg²⁺ to pH 7.4.

Key Words:

• Accuracy
• erythrocyte effect on the liquid junction potential
• heparin
• ionized magnesium
• pH effect
• pH correction
• protein effect
• residual liquid junction potential