Ionized and not total magnesium as a discriminating biomarker for hypomagnesaemia in continuous venovenous haemofiltration patients

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.

Magnesium matters: unveiling hidden risks in kidney transplant patients through total and ionized magnesium profiling

Background

In kidney transplant (KT) patients, magnesium (Mg2+) deficiency is widespread. It is often encountered early after KT, may persist longer, and is frequently promoted by calcineurin inhibitors (CNIs) and tubular leakage. Studies demonstrated an association between post-KT hypomagnesemia and allograft dysfunction. The concentration of the active form, the ionized Mg2+ (iMg2+), is not measured clinically, and total Mg2+ (tMg2+) and iMg2+ correlations are conflicting. We assess the cross-sectional prevalence of hypomagnesemia in KT patients. The correlation of demographic and anthropometric parameters was also studied.

Methods

A prospective, single-center analysis of KT patients was conducted at the University Hospital of Bern, Switzerland (March 2023-August 2023). Blood samples were collected at least twice for the majority of patients. tMg2+ has been quantified from a plasma sample at the Clinical Chemistry Department of the University Hospital of Bern. The PRIME® ES analyzer (Nova Biomedical, USA) provided results for iMg2+. The following co-variables were considered: age, comorbidities, kidney disease, KT history, estimated glomerular filtration rate (eGFR), and treatment (including Mg2+ supplementation and immunosuppression).

Results

A total of 208 measurements in 104 patients were performed [once in 9/104 patients (8.7%), twice in 86/104 (82.7%), and three times in 9/104 (8.7%)]. Compared to that in healthy volunteers (51 measurements in 51 participants), mean iMg2+ was significantly lower in KT patients {KT: 0.46 mmol/L [interquartile range (IQR): 0.40-0.50], volunteers: 0.57 mmol/L (IQR 0.54-0.61), p < 0.01}. Overall, iMg2+ and tMg2+ showed strong category agreement (r2 = 0.93, p < 0.01). In linear regression, low iMg2+ correlated with CNI exposure. For 110/208 measurements (52.9%), a reduced iMg2+ (cutoff: 0.42 mmol/L) was shown. In 58/208 (27.9%), both values were reduced, and 52/208 (25%) had isolated reduced iMg2+. In principal component analysis, patients with isolated low iMg2+ clustered with patients with low iMg2+ and tMg2+.

Conclusion

iMg2+ and tMg2+ were strongly correlated. A substantial proportion of patients show isolated low iMg2+. Currently, it is unclear if these patients suffer from Mg2+ deficiency.

Ionized Magnesium Correlates With Total Blood Magnesium in Pediatric Patients Following Kidney Transplant

Background

Abnormal serum magnesium (Mg) concentrations are common and associated with worse mortality in kidney-transplant recipients. Many kidney and transplant-related factors affect Mg homeostasis. The concentration of the active form, ionized Mg (iMg), is not measured clinically, and total Mg (tMg) and iMg correlations have conflicted. We hypothesized that iMg and tMg concentrations show poor categorical agreement (i.e., low, normal, and high) in kidney-transplant recipients but that ionized calcium (iCa) correlates with iMg.

Methods

We retrospectively evaluated hypomagnesemia in kidney-transplant recipients over a 2-yr period. We prospectively collected blood at 0-28 days post-transplant to measure correlations between iMg and iCa/tMg. iMg and iCa concentrations in the reference ranges of 0.44-0.65 and 1.0-1.3 mmol/L, respectively, were considered normal. Fisher's exact test and unweighted kappa statistics revealed category agreements. Pearson's correlation coefficients and linear regression measured correlations.

Results

Among 58 retrospective kidney-transplant recipients, 54 (93%) had tMg<0.66 mmol/L, 28/58 (48%) received Mg supplementation, and 20/28 (71%) had tacrolimus dose adjustments during supplementation. In 13 prospective transplant recipients (N=43 samples), iMg and tMg showed strong category agreement (P=0.0003) and correlation (r=0.71, P<0.001), whereas iMg and iCa did not (P=0.7; r=-0.25, P=0.103, respectively).

Conclusions

tMg and iMg exhibited strong correlation following kidney transplantation. However, iCa may not be an accurate surrogate for iMg. Determining the effect of Mg supplementation and the Mg concentration where supplementation is clinically necessary are important next steps.

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).

Diurnal variation of magnesium and the mineral metabolism in patients with chronic kidney disease

Increasing levels of magnesium in blood are associated with reduced risk of cardiovascular disease in chronic kidney disease (CKD). Magnesium supplementation may reduce the progression of vascular calcification in CKD. The diurnal pattern and effect of fasting on magnesium in blood and urine in CKD is unknown, and knowledge of this may influence management of magnesium supplementation. We included ten patients with CKD stage four without diabetes mellitus and ten healthy controls. Participants were admitted to our hospital ward for a 24-h study period. Blood and urine samples were collected in a non-fasting state at 8 o'clock in the morning and every third hour hereafter until the final samples in a fasting state at 8 o'clock the following morning. We found no diurnal variation in plasma magnesium (p = 0.097) in either group, but a significant diurnal variation in urinary excretion of magnesium (p = 0.044) in both CKD and healthy controls with no significant interaction between the two groups, and thus no suggestion that CKD affects diurnal variation of plasma magnesium or urinary magnesium excretion. The levels of plasma magnesium were not significantly different in fasting and non-fasting conditions. Magnesium in plasma does not display a significant diurnal variation and can be measured at any time of day and in both fasting and non-fasting conditions. Urinary magnesium excretion displays diurnal variation, which is likely related to increased uptake of magnesium during meals and helps maintain a stable concentration of magnesium in blood.