IFCC Guideline for sampling, measuring and reporting ionized magnesium in plasma

In vivo and in vitro relationship between ionized magnesium and ionized calcium

OBJECTIVES

The objective of this study was to determine the in vivo correlation of ionized magnesium (iMg) with ionized calcium (iCa), total calcium, albumin and pH. In addition, the analytical interference of iCa on iMg measurement on the Stat Profile Prime Plus (Nova Biomedical) and vice versa was defined.

METHODS

In vivo correlation of iCa, iMg and pH was studied in 238 paired blood gas samples of 109 different patients admitted to the intensive care unit. Albumin and total magnesium (tMg) were measured in heparinized plasma samples. Measurement of iMg was performed with the ion selective magnesium electrode (ISE) of the Stat Profile Prime Plus (Nova Biomedical) and iCa and pH were measured with a Rapid Point 500 blood gas analyzer (Siemens). Albumin, total calcium and total magnesium were analyzed with a Siemens Atellica CH. Analytical interference of iCa with iMg and vice versa was investigated using unbuffered saline solutions.

RESULTS

In the studied patient population, no significant correlations were observed between iMg and iCa, albumin, and pH. An inverse relationship was observed between iCa and Mg-ISE. For every 0.1 mmol/L change in iCa concentration, the iMg concentration deviated by 0.01 mmol/L at an iMg concentration of 0.5 mmol/L and by 0.013 mmol/L at an iMg concentration of 1.0 mmol/L. The measurement of iCa was not affected by iMg.

CONCLUSIONS

In vivo, no correlation was observed between iMg with iCa, albumin and pH. Interference of iCa on iMg measurement was noted, with a maximum deviation of ±0.02 mmol/L iMg across the reference range of iCa (1.15-1.32 mmol/L). Additionally, the iCa measurement was not affected by the iMg concentration.

The Stability of Analytes of Ionized Magnesium Concentration and Its Reference Range in Healthy Volunteers

This study aimed to determine the stability of refrigerated analytes of iMg concentration at different time intervals and to establish iMg reference range in a cohort of healthy Omani volunteers (≥18 years). The concentrations of iMg were measured using the direct ion-selective electrode technique. Pearson's and Lin's concordance correlation coefficients along with the Bland-Altman plot were used to assess the levels of agreement between iMg concentrations of fresh and refrigerated blood samples at different time intervals. The study included 167 volunteers (51% females) with a median age of 21 (range: 20-25) years. The median, 2.5th, and 97.5th percentiles for fresh iMg reference ranges were 0.55, 0.47, and 0.68 mmol/L, respectively. The overall agreement between the fresh and refrigerated iMg concentrations was poor (rho-c = 0.51; p < 0.001). However, according to Altman's definition, iMg concentrations of the refrigerated samples for a period of ≤1 h had an excellent correlation with the fresh iMg concentrations (Lin's rho-c = 0.80), with a small average bias difference of 0.009 (95%CI; -0.025-0.043). A cut-off refrigeration period within ≤1 h at 2-8 °C can be considered an alternate time frame for the gold standard measurement (fresh or within 0.5 h).

Estimating serum-ionized magnesium concentration in hemodialysis patients

INTRODUCTION

Cardiovascular mortality is significantly increased in kidney failure with replacement therapy (KFRT) patients, which is partly mediated by enhanced vascular calcification. Magnesium appears to have anticalcifying capabilities, and hypomagnesemia has been associated with increased mortality in KFRT patients. Ionized magnesium represents the biologically and physiologically active form. As serum ionized magnesium (Mg_ion ) is difficult to assess in clinical routine estimating equations derived from routinely assessed laboratory parameters could facilitate medical treatment.

METHODS

We developed equations to estimate serum Mg_ion using linear regression analysis in 191 hemodialysis (HD) patients. Reference test was measured ionized magnesium (Mg_ion ). As index tests, we chose estimated Mg_ion using total magnesium (Mg_tot ) and other laboratory and demographic variable candidates. Equations were internally validated, using 749 subsequent Mg_ion measurements.

FINDINGS

The median patient age was 65 years, 67.5% of the patients were male. Median (interquartile range [IQR]) measured Mg_ion was 0.64 [0.57, 0.72] mmol/L, 11 (6%) patients were hypo- (i.e., <0.45 mmol/L) and 127 (66%) were hypermagnesemic (>0.60 mmol/L). The final equation at the end of the development process included Mg_tot , serum ionized, and total calcium concentrations. In the validation dataset, bias (i.e., median difference between measured and estimated Mg_ion , -0.017 [-0.020, -0.014] mmol/L) and precision (i.e., IQR of bias 0.043 [0.039, 0.047] mmol/L) were small, 90% [88, 93] of estimated values were ±10% of measured values. The equation detected normomagnesemia with overall good diagnostic accuracy (area under the receiver-operating curve 0.91 [0.89, 0.93]).

DISCUSSION

Mg_ion can be estimated from equations containing routinely assessed laboratory variables with high accuracy and good overall performance. These equations might simplify the assessment of ionized magnesium levels in the individual hemodialysis patients and help the treating physician to guide the overall treatment.

Magnesium: Biochemistry, Nutrition, Detection, and Social Impact of Diseases Linked to Its Deficiency

Magnesium plays an important role in many physiological functions. Habitually low intakes of magnesium and in general the deficiency of this micronutrient induce changes in biochemical pathways that can increase the risk of illness and, in particular, chronic degenerative diseases. The assessment of magnesium status is consequently of great importance, however, its evaluation is difficult. The measurement of serum magnesium concentration is the most commonly used and readily available method for assessing magnesium status, even if serum levels have no reliable correlation with total body magnesium levels or concentrations in specific tissues. Therefore, this review offers an overview of recent insights into magnesium from multiple perspectives. Starting from a biochemical point of view, it aims at highlighting the risk due to insufficient uptake (frequently due to the low content of magnesium in the modern western diet), at suggesting strategies to reach the recommended dietary reference values, and at focusing on the importance of detecting physiological or pathological levels of magnesium in various body districts, in order to counteract the social impact of diseases linked to magnesium deficiency.

Alterations of Mg2+ After Hemorrhagic Shock

Hemorrhagic shock is generally characterized by hemodynamic instability with cellular hypoxia and diminishing cellular function, resulting from an imbalance between systemic oxygen delivery and consumption and redistribution of fluid and electrolytes. Magnesium (Mg) is the fourth most abundant cation overall and second most abundant intracellular cation in the body and an essential cofactor for the energy production and cellular metabolism. Data for blood total Mg (tMg; free-ionized, protein-bound, and anion-bound forms) and free Mg²⁺ levels after a traumatic injury are inconsistent and only limited information is available on hemorrhagic effects on free Mg²⁺ as the physiologically active form. The aim of this study was to determine changes in blood Mg²⁺ and tMg after hemorrhage in rats identifying mechanism and origin of the changes in blood Mg²⁺. Hemorrhagic shock produced significant increases in blood Mg²⁺, plasma tMg, Na⁺, K⁺, Cl^-, anion gap, partial pressures of oxygen, glucose, and blood urea nitrogen but significant decreases in RBC tMg, blood Ca²⁺, HCO₃^-, pH, partial pressures of carbon dioxide, hematocrit, hemoglobin, total cholesterol, and plasma/RBC ATP. During hemorrhagic shock, K⁺, anion gap, and BUN showed significant positive correlations with changes in blood Mg²⁺ level, while Ca²⁺, pH, and T-CHO correlated to Mg²⁺ in a negative manner. In conclusion, hemorrhagic shock induced an increase in both blood-free Mg²⁺ and tMg, resulted from Mg²⁺ efflux from metabolic damaged cell with acidosis and ATP depletion.

The Importance of Magnesium in the Human Body: A Systematic Literature Review

Magnesium, the second and fourth most abundant cation in the intracellular compartment and whole body, respectively, is of great physiologic importance. Magnesium exists as bound and free ionized forms depending on temperature, pH, ionic strength, and competing ions. Free magnesium participates in many biochemical processes and is most commonly measured by ion-selective electrode. This analytical approach is problematic because complete selectivity is not possible due to competition with other ions, i.e., calcium, and pH interference. Unfortunately, many studies have focused on measurement of total magnesium rather than its free bioactive form making it difficult to correlate to disease states. This systematic literature review presents current analytical challenges in obtaining accurate and reproducible test results for magnesium.

Relationships between blood Mg2+ and energy metabolites/enzymes after acute exhaustive swimming exercise in rats

Magnesium (Mg) plays a central role in neuronal activity, cardiac excitability, neuromuscular transmission, muscular contraction, vasomotor tone, and blood pressure, all of which are significantly related to physical performance. To date, the available data about detection of blood total Mg (tMg; free-ionized, protein-bound, and anion-complex forms) are inconsistent, and there is limited information on blood free-ionized Mg (Mg(2+)) in relation to physical exercise. The aim of this study was to determine the biochemical changes related to energy metabolism after acute exhaustive swimming exercise (AESE) in rats in an attempt to correlate the role of blood Mg(2+) with metabolites/enzymes related to energy production. After AESE, blood Mg(2+), tMg, K(+), partial pressure of carbon dioxide, lactate, total protein (T-PRO), high-density lipoprotein (HDL), creatinine (CRE), blood urea nitrogen (BUN), uric acid (UA), alanine aminotransferase (ALT), aspartate aminotransferase (AST), alanine phosphatase (ALP), lactate dehydrogenase (LDH), and creatinine kinase (CK) were significantly increased, whereas pH, partial pressure of oxygen, oxygen saturation, the Mg(2+)/tMg and Ca(2+)/Mg(2+) ratios, HCO3 (-), glucose, triglyceride (TG), and low-density lipoprotein (LDL) were significantly decreased. During AESE, lactate, T-PRO, albumin, AST, ALP, LDH, CK, CRE, BUN, and UA showed significant positive correlations with changes in blood Mg(2+), while glucose, TG, and LDL correlated to Mg(2+) in a negative manner. In conclusion, AESE induced increases in both blood Mg(2+) and tMg, accompanied by changes in blood metabolites and enzymes related to energy metabolism due to increased metabolic demands and mechanical damages.

Hemoglobin oxygen affinity in patients with cystic fibrosis

In patients with cystic fibrosis lung damages cause arterial hypoxia. As a typical compensatory reaction one might expect changes in oxygen affinity of hemoglobin. Therefore position (standard half saturation pressure P₅₀st) and slope (Hill’s n) of the O₂ dissociation curve as well as the Bohr coefficients (BC) for CO₂ and lactic acid were determined in blood of 14 adult patients (8 males, 6 females) and 14 healthy controls (6 males, 8 females). While Hill’s n amounted to approximately 2.6 in all subjects, P₅₀st was slightly increased by 1mmHg in both patient groups (controls male 26.7±0.2, controls female 27.0±0.1, patients male 27.7±0.5, patients female 28.0±0.3 mmHg; mean and standard error, overall p<0.01). Main cause was a rise of 1–2 µmol/g hemoglobin in erythrocytic 2,3-biphosphoglycerate concentration. One patient only, clearly identified as an outlier and with the mutation G551D, showed a reduction of both P₅₀st (24.5 mmHg) and [2,3-biphosphoglycerate] (9.8 µmol/g hemoglobin). There were no differences in BCCO₂, but small sex differences in the BC for lactic acid in the controls which were not detectable in the patients. Causes for the right shift of the O₂ dissociation curve might be hypoxic stimulation of erythrocytic glycolysis and an increased red cell turnover both causing increased [2,3-biphosphoglycerate]. However, for situations with additional hypercapnia as observed in exercising patients a left shift seems to be a more favourable adaptation in cystic fibrosis. Additionally when in vivo PO₂ values were corrected to the standard conditions they mostly lay left of the in vitro O₂ dissociation curve in both patients and controls. This hints to unknown fugitive factors influencing oxygen affinity.

Ion Selective Electrodes (ISEs) and interferences--a review

Ion Selective Electrodes (ISEs) are used to measure some of the most critical analytes on clinical laboratory and point-of-care analysers. These analytes which include Na(+), K(+), Cl(-), Ca(2+), Mg(2+) and Li(+) are used for rapid patient care decisions. Although the electrodes are very selective, they are not free of interferences. It is important for laboratories to have an understanding of the type and extent of interferences in order to avoid incorrect clinical decisions and treatment.

Determination of unbiased selectivity coefficients using pulsed chronopotentiometric polymeric membrane ion sensors

A new procedure for the determination of selectivity coefficients of neutral carriers using pulsed chronopotentiometric ion selective sensors (pulstrodes) is established. Pulstrode membrane which lacks an ion-exchanger suppresses the zero current ion flux, allowing a Nernstian response slope for even highly discriminated ions. Unlike previously developed methods, unbiased selectivity remains unaltered even with the exposure to the primary ion solution for prolonged time. Studies with potassium-, silver-, and calcium-selective electrodes reveal that pulstrodes yield the same or slightly favorable unbiased selectivity coefficients than reported earlier. In contrast to alternative methods for the determination of unbiased selectivity, this technique offers a unique simplicity and reliability. Therefore the new procedure promises to be a valuable additional tool for the characterization of unbiased selectivity coefficients for the ISEs.