MEASUREMENT OF FREE MAGNESIUM IN BLOOD, SERUM AND PLASMA WITH AN ION-SENSITIVE ELECTRODE

The fraction of total magnesium bound to protein and other substances depends upon the pH. pH-dependency of ionized free magnesium (iMg(2+)) in serum is expressed by the Siggaard-Andersen equation: iMg(2+) (pH) = iMg(2+) (7.4) . 10(x)((7.4-pH)). During preparation of serum or pla sma, considerable pH changes occur which have to be corrected on the b asis of the above mentioned equation. For pH correction of iMg(2+), x < 0.1 has so far been used. However, this is correct only for new Mg2-sensitive electrodes. During the lifetime of Mg2+ electrodes used in the ''Microlyte Magnesium'' (Kone Instruments, Finland) x increases an d x = 0.2 was found to be a suitable approximation for most of the lif etime. By instantaneous iMg(2+) measurements in whole blood samples pH changes and the uncertainty of x can be avoided. Dilution of blood by intravenous infusions decreases x nearly proportionally to the decrea se of protein concentration in blood. Various methodological influence s such as temperature and delay time before centrifugation, storage of serum and venous occlusion were studied. The circadian rhythm of iMg( 2+) was found to be considerably more pronounced than that of total Mg and was negatively correlated to changes of free fatty acids. To avoi d variations of iMg(2+) due to circadian changes, blood collection sho uld be carried out between 6 and 10 a. m. The normal range of iMg(2+) in blood of 179 healthy subjects was found to be between 0.46 and 0.60 mmol/l and the quotient of free and total Mg between 0.59 and 0.71. T he accuracy of ''Microlyte Magnesium'' (Kone Instruments, Finland) is sufficient in a wide range of iMg(2+).