• Daniel Dante Yeh, Nalin Chokengarmwong, Yuchiao Chang, Liyang Yu, Colleen Arsenault, Joseph Rudolf, Elizabeth Lee-Lewandrowski, and Kent Lewandrowski.
• Massachusetts General Hospital, Department of Surgery, Division of Trauma, Emergency Surgery, and Surgical Critical Care, United States. Electronic address: dxy154@miami.edu.
• J Crit Care. 2017 Dec 1; 42: 147-151.

PurposeIonized fraction (iMg) is the physiologically active form of magnesium (Mg); total Mg may not accurately reflect iMg status. Erroneously "low" Mg levels may result in unnecessary repetitive testing.Materials And MethodsFrom 11/2015 to 01/2016, patients ordered for Mg from a pilot ICU also had iMg tested. Weighted kappa statistic was used to assess agreement between Mg categories (low, normal, high). Predictors of unnecessary repeated Mg testing and repletion using data were explored through logistic regression models using GEE techniques to account for repeated measurements in both bivariate and multivariable analyses.ResultsThere were 470Mg/iMg paired measurements from 173 patients. The weighted kappa statistic was 0.35 (95%CI 0.27-0.43) indicating poor agreement in assessment of magnesium status. Of the 34Mg samples reported as "low", only 6 (18%) were considered "low" using concurrent iMg testing. In the multivariable models, history of atrial fibrillation (aOR=1.61, 95%CI 1.16-2.21, p=0.004) and concomitant metoclopramide (aOR=1.71, 95%CI 1.03-2.81, p=0.036) were significant predictors of unnecessary repeat Mg testing.ConclusionsIn the surgical ICU, categorical agreement (low, normal, high) was poor between Mg and iMg. Over 80% of "low" total Mg values are erroneous and may result in unnecessary additional measurements and repletion.Copyright © 2017 Elsevier Inc. All rights reserved.

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