Influence of air exposure and storage condition on serum ionized magnesium level.

The aim of this study was to evaluate whether reporting serum level of ionized magnesium (iMg) is appropriate when affected by various conditions such as exposure to air and delayed measurement. Serum levels of pH, iMg and normalized magnesium (nMg, normalized or adjusted concentration of iMg to pH 7.40) from 28 inpatients were measured at intervals of 3 min after exposing the samples to air at room temperature. Serum from 30 inpatients was stored in closed tubes at 4 degrees C and -20 degrees C and iMg and nMg levels were measured after 2 days. It was found that serum iMg and nMg concentrations exposed to air were decreased by 0.0023 mmol/l and 0.0001 mmol/l per minute, respectively. nMg did not display any significant changes compared with iMg at 0 min, whereas iMg showed significant changes, which exceeded between-day precision. For the stored serum, only iMg of serum at -20 degrees C showed no statistically significant changes (p = 0.169). It is concluded that to report the result as iMg, the sample should be kept anaerobically, and if exposed to air, the result should be reported as nMg. For storage, iMg of serum kept anaerobically at -20 degrees C is reliable.