Characterization of a new ion selective electrode for ionized magnesium in whole blood, plasma, serum, and aqueous samples

Altura BT et al. Characterization of a New Ion Selective Electrode for Ionized Magnesium in Whole Blood, Plasma, Serum, and Aqueous Samples. Scand J Clin Lab Invest 1994; 54, Suppl 217: 21-36. Results from a novel ion selective electrode (ISE) for ionized magnesium (Mg²⁺) correlate well with atomic absorption spectroscopy on aqueous solutions containing from 0.1-3.0 mmol MgCl₂/L. Day to day precision (coefficient of variation) of the electrode on protein-based controls is <4%; aqueous-based controls <6% The new ISE is selective for Mg²⁺ with a selectivity constant for Ca²⁺ (K_MgCa) of 8 × 10-². Adding pathophysiologic concentrations of Cd²⁺, Ca²⁺, Cu²⁺, Fe³⁺, K⁺, Na⁺, or Zn²⁺ to serum and aqueous solutions gave negligible to minimal changes in measured Mg²⁺. Ligand binding studies in aqueous solution indicate that pathophysiologic concentrations of different anions (e.g. heparin, lactate, bicarbonate, phosphate, acetate and sulfate) bind to Mg²⁺, effectively reducing its concentration in solution. Likewise, silicon (as either found in Vacuutainer tubes or as chlorosilane) failed to exert any significant effect on measured Mg²⁺. Addition of Intralipid^®(up to 500 mg/dL) gave negligible to minimal changes in Mg²⁺. Mg²⁺ measurements on whole blood, plasma, and serum for a given human subject's samples are virtually identical, at least within the reference range for Mg²⁺. Typically, Mg²⁺ is 71% of TMg, but varies from subject to subject; i.e. Mg²⁺ cannot be predicted from TMg. Clinical studies revealed that the Mg²⁺/TMg ratio could be remarkably consistent in sequential samples (e.g., throughout the course of coronary bypass surgery) taken from one patient, but that this ratio could differ dramatically from the ratio in sequential samples taken from another. Mg²⁺ is held within a narrow range (0.53-0.67mmol/L) in normal, healthy subjects when compared to TMg (0.70-0.96 mmol/L).