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.