N. Foghandersen et O. Siggaardandersen, STANDARDIZING AND REPORTING RESULTS FROM MG2+ ISES, WITH SOME NOTES ON SAMPLE HANDLING, Scandinavian journal of clinical & laboratory investigation, 54, 1994, pp. 89-96
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.