Ionized Magnesium in Serum and Ultrafiltrate: pH and Bicarbonate Effect on Measurements with the AVL 988-4 Electrolyte Analyzer

Abstract

In blood, free (“ionized”) magnesium (Mg2) is in equilibrium with complexed (protein-bound, and organic or inorganic complexed) species. This equilibrium is influenced by pH and by protein and ligand concentration, both in vivo and in vitro. The phenomenon is well known for calcium; pH influences the serum concentration of free calcium ion (S-cCa2) through the competition between hydrogen ions and calcium ions toward binding sites of protein. Fogh-Andersen (1) proposed the relationship d pcCa2/ d pH, correcting the measured S-cCa2 to the standard pH of 7.4, that, although controversial, is widely used in commercial analyzers (2)(3). Recently, synthetic neutral carriers for the determination of free magnesium ion concentration (cMg2) have been developed (4)(5)(6)(7), the selectivity of which toward calcium allows the chemometric correction of the free magnesium measurement in the presence of pathophysiological calcium concentrations (8)(9). Magnesium bound to protein represents 30–35% of the total magnesium ion concentration in serum (6)(7)(8)(10), and, as with calcium, an empirical correction of measured serum free magnesium ion concentration (S-cMg2) to a standard pH can be proposed. The slope of the regression log S-cMg2 vs pH is ∼−0.1 (6)(11), half that for S-cCa2. In the S-cMg2 range of 0.31–0.76 mmol/L, we found an average value of −0.117 (8), which was reproducible during the life-span of the tested electrodes. By contrast, Ising et al. (12), evaluating the performance of the Kone Microlyte Magnesium assay (magnesium ionophore ETH 5220), found a dependence of the d pcMg2/ d pH relation on the electrode life-time. According to these authors, the built-in value of −0.07 was incorrect, and could produce error in the normalized value up to …