Variability of ionic magnesium

Abstract

With the invention o f the ion-selective electrode, ionic magnesium (iMg), the biologically active form of Mg, is a common blood assay. There is some evidence that iMg has considerable within subject variability. To further assess this, blood samples were collected from 13 healthy volunteers 6X/day (7:00 - 22:00, every 3 hours) for 3 consecutive days and analysed for iMg (NOVA Stat 8 Analyser). 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). A plausible explanation of this data is that iMg has a circadian rhythm, but a sinusoidal curve of best fit computed for each subject did not correlate highly to iMg (r=.20). Thus, cautious interpretation of iMg values is warranted until future research determines the nature of iMg variability.