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Analyzers with ion-selective electrodes (ISEs) for ionized magnesium (iMg) should yield comparable and unbiased results for iMg. This IFCC guideline on sampling, measuring and reporting iMg in plasma provides a prerequisite to achieve this goal [in this document, “plasma” refers to circulating plasma and the forms in which it is sampled, namely the plasma phase of anticoagulated whole blood (or “blood”), plasma separated from blood cells, or serum]. The guideline recommends measuring and reporting ionized magnesium as a substance concentration relative to the substance concentration of magnesium in primary aqueous calibrants with magnesium, sodium, and calcium chloride of physiological ionic strength. The recommended name is “the concentration of ionized magnesium in plasma”. Based on this guideline, results will be approximately 3% higher than the true substance concentration and 4% lower than the true molality in plasma. Calcium ions interfere with all current magnesium ion-selective electrodes (Mg-ISEs), and thus it is necessary to determine both ions simultaneously in each sample and correct the result for Ca2+ interference. Binding of Mg in plasma is pH-dependent. Therefore, pH should be measured simultaneously with iMg to allow adjustment of the result to pH 7.4. The concentration of iMg in plasma may be physiologically and clinically more relevant than the concentration of total magnesium. Furthermore, blood-gas analyzers or instruments for point-of-care testing are able to measure plasma iMg using whole blood (with intact blood cells) as the sample, minimizing turn-around time compared to serum and plasma, which require removal of blood cells. Clin Chem Lab Med 2008;46:21–6. [ABSTRACT FROM AUTHOR]

In current clinical practice, plasma and blood glucose are used interchangeably with a consequent risk of clinical misinterpretation. In human blood, glucose is distributed, like water, between erythrocytes and plasma. The molality of glucose (amount of glucose per unit water mass) is the same throughout the sample, but the concentration is higher in plasma, because the concentration of water and therefore glucose is higher in plasma than in erythrocytes. Different devices for the measurement of glucose may detect and report fundamentally different quantities. Different water concentrations in the calibrator, plasma, and erythrocyte fluid can explain some of the differences. Results for glucose measurements depend on the sample type and on whether the method requires sample dilution or uses biosensors in undiluted samples. If the results are mixed up or used indiscriminately, the differences may exceed the maximum allowable error for glucose determinations for diagnosing and monitoring diabetes mellitus, thus complicating patient treatment. The goal of the International Federation of Clinical Chemistry and Laboratory Medicine, Scientific Division, Working Group on Selective Electrodes and Point of Care Testing (IFCC-SD-WG-SEPOCT) is to reach a global consensus on reporting results. The document recommends reporting the concentration of glucose in plasma (in the unit mmol/L), irrespective of sample type or measurement technique. A constant factor of 1.11 is used to convert concentration in whole blood to the equivalent concentration in plasma. The conversion will provide harmonized results, facilitating the classification and care of patients and leading to fewer therapeutic misjudgments. Clin Chem Lab Med 2006;44:1486–90. [ABSTRACT FROM AUTHOR]

The proposed recommendation for measuring and reporting chloride in undiluted plasma or blood by ion-selective electrodes (ISEs) will provide results that are identical to chloride concentrations measured by coulometry for standardized normal plasma or blood samples. It is applicable to all current ISEs dedicated to chloride measurement in undiluted samples that meet the requirements. However, in samples with reduced water concentration, results by coulometry are lower than by ion-selective electrode due to volume displacement. The quantity measured by this standardized ISE procedure is called the ionized chloride concentration. It may be clinically more relevant than the chloride concentration as determined by coulometry, photometry or by ISE after dilution of the sample. [ABSTRACT FROM AUTHOR]

All analyzers with ion-selective electrodes for ionized magnesium (iMg) should yield comparable and unbiased results. The prerequisite to achieve this goal is to reach consensus on sampling, measurement and reporting. The recommended guidelines for sampling, measurement and reporting iMg in plasma (“plasma” refers to circulating plasma and the forms in which it is sampled: the plasma phase of anticoagulated whole blood, plasma separated from blood cells, or serum) or blood, referring to the substance concentration of iMg in the calibrants, will provide results for iMg that are approximately 3% greater than its true concentration, and 4% less than its true molality. Binding of magnesium to proteins and ligands in plasma and blood is pH-dependent. Therefore, pH should be simultaneously measured to allow adjustment of iMg concentration to pH 7.4. The substance concentration of iMg may be physiologically and consequently clinically more relevant than the substance concentration of total magnesium. [ABSTRACT FROM AUTHOR]