Measurement of ionised magnesium in HEPES-buffered serum samples

The objective of the research is to develop a method of adjusting the pH of serum samples to 7.4, thus allowing measurement of ionised magnesium (Mg2+) at physiological pH without the need to maintain samples under anaerobic conditions. Levels of Mg2+ HEPES-buffered samples were compared with results obtained when serum samples were buffered with CO2, and the Mg2+ level at pH 7.4 calculated from the log form of the Siggaard-Andersen equation. The HEPES buffering technique was also used to measure Mg2+ in serum samples from sheep where circulating levels of Mg2+ were elevated by infusion of MgCl2. The mean Mg2+ values obtained when samples (n=6) were buffered with CO2 or 50 or 100 mM HEPES were not significantly different from each other (mean±S.E.M.; 0.675±0.019, 0.684±0.015, and 0.669±0.015, respectively). When MgCl2 was infused into sheep (n=6), a significant rise in serum Mg2+ was found (mean±S.E.M.; basal 0.613±0.045 mM; peak 1.450±0.095 mM) when using the HEPES-buffering method for sample pre-treatment (final HEPES concentration was 50 mM).