Ionized magnesium in serum measured using a blood gas analyser: Agreement with whole blood, short-term stability and preliminary reference interval in healthy adults.

OBJECTIVES

Ionized magnesium (iMg) is the biologically active fraction of circulating magnesium, yet its use in routine practice remains limited. This study evaluated serum iMg measurement using a blood gas analyser, focusing on agreement with whole blood, short-term pre-analytical stability, and factors associated with iMg concentrations in healthy adults.

METHODS

In this cross-sectional study, 60 healthy individuals were included. iMg was measured in paired whole blood and serum samples using a blood gas analyser. Agreement between matrices was assessed using Spearman correlation, Passing-Bablok regression, and Bland-Altman analysis. Serum stability was evaluated at 4 and 24 h under refrigerated conditions. Multivariable linear regression was used to identify factors associated with iMg concentrations. A preliminary reference interval was estimated according to CLSI EP28-A3c.

RESULTS

Serum and whole blood iMg showed strong correlation (ρ = 0.727; p < 0.001) with minimal systematic bias, although variability between matrices was observed. Serum iMg remained within acceptable limits at 4 h but exceeded the maximum permissible instability at 24 h. The preliminary reference interval for whole blood iMg was 0.53-0.68 mmol/L, with a corresponding interval in serum of 0.49-0.71 mmol/L. In multivariable analysis, total magnesium was the only variable independently associated with iMg concentrations (β = 0.658; p < 0.001).

CONCLUSIONS

Serum iMg measured using a blood gas analyser may represent a feasible alternative matrix under controlled pre-analytical conditions. However, its limited stability requires timely analysis. These findings support a stepwise approach to magnesium assessment, in which iMg may be considered as a reflex test in patients with low or near-low total magnesium concentrations.