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Updated: Aug 8, 2026

Measurement of Extracellular Ion Fluxes Using the Ion-selective Self-referencing Microelectrode Technique
Published on: May 3, 2015
Variability within individuals of plasma ionic magnesium concentrations
Ian J Newhouse1, Kris P Johnson, William J Montelpare
1School of Kinesiology, Lakehead University, Thunder Bay, Ontario, Canada. Ian.Newhouse@Lakeheadu.ca
Background:
With the invention of the ion-selective electrode (ISE), ionic magnesium (iMg) is a common blood assay. This could be advantageous, as iMg is the biologically active form of Mg. There is some evidence that iMg has considerable within subject variability.
Results:
Individual ranges averaged.08 mmol/L (range.05 to.14). Coefficients of variation (CV) ranged from 3% to 7% (mean 4%) while analytical variation was determined to be 2.3%. Biological variability thus accounts for almost half of the variability, which is clinically significant, as 9 of the 13 subjects recorded at least one value below a reference range of.46 -.60 mmol/L. A significant within-day variation (p <.001) was noted, with differences between 7:00 and 10:00 as well as 10:00 and 22:00. Between day variations were not significant (p =.56).
Conclusions:
A plausible explanation of this data is that iMg has a circadian rhythm. Thus, cautious interpretation of single iMg values is warranted until future research determines the nature of iMg variability.
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