Clinical Pathology: General Principles, Lab Management, Clinical Chemistry

• Measuring ionized (i.e., “free”) magnesium (Mg) may be useful in certain conditions, such as inflammatory bowel disease, cardiac surgery, stroke, head injury, renal dysfunction, alcoholism, liver disease, and eclampsia. However, in contrast to total Mg (i.e., protein bound, complexed, and free), ionized Mg is not often measured. The relationship between total Mg and ionized Mg is weak and is influenced by factors such as albumin.

• Ionized Mg can be measured in serum using ion-selective electrodes (ISEs). These are available from several manufacturers and interferences are model specific.

• Current ISEs for Mg have insufficient selectivity for Mg over calcium (Ca). Free Ca is simultaneously determined and used chemometrically with the signal from the Mg electrode to calculate free Mg concentrations.

• Many of the factors that affect the free Ca levels also affect the free Mg levels.

• Certain silicones, other tube additives, and zinc heparin interfere with free Mg determinations.

• Thiocyanate from smoking (and from foods in the diet, such as cabbage) leads to falsely low free Mg levels when measured using some ISE models.

Rehak NN, Cecco SA, Niemela JE, Elin RJ: Thiocyanate in smokers interferes with the Nova magnesium ion-selective electrode. Clin Chem 1997;43(9):1595–1600.

Johansson M, Whiss PA: Weak relationship between ionized and total magnesium in serum of patients requiring magnesium status. Biol Trace Elem Res 2007;117:13–21.

Burtis C, Ashwood ER, Bruns DE, eds: Tietz Textbook of Clinical Chemistry and Molecular Diagnosis. 4th ed. St. Louis:Elsevier Health Sciences, 2006: 1909–1912.

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