Magnesium (Mg²⁺) ranks as the fourth most abundant cation in the human body, playing a crucial role in various cellular and biological processes (1). Hypomagnesemia, characterized by depletion of Mg²⁺ in the blood, is mainly due to conditions that promote renal or gastrointestinal Mg²⁺ wasting and is responsible for cardiac, neurological, and neuromuscular disorders. Mg²⁺ deficit, often overlooked compared to other electrolyte disorders, raises substantial health concerns and is frequently encountered in clinical settings, affecting as much as 30% of the elderly population (2). Hypomagnesemia has been linked to cardiovascular risk and is commonly associated with unfavorable clinical outcomes and higher mortality (3–7).

Dysmagnesemia has also been linked to unfavorable outcomes in individuals undergoing dialysis and those with chronic kidney disease (CKD), acting as a potential predictor for both mortality and the deterioration of renal function in CKD patients (1, 7). In kidney transplant (KT) patients, hypomagnesemia is widespread, often encountered in the first weeks after KT, and frequently promoted by calcineurin inhibitors (CNIs), other concomitant medications (i.e., thiazide-type and loop diuretics and pump proton inhibitors), and tubular leakage (1). In more than 20% of KT patients, hypomagnesemia persists for several years after KT (6). Several studies have demonstrated that pre- and post-KT hypomagnesemia is independently associated with immune dysfunction, thus likely promoting opportunistic infections (8, 9). KT patients experiencing hypomagnesemia may also be prone to impaired glucose metabolism, increasing their susceptibility to the development of diabetes mellitus (6, 10). Moreover, associations between low Mg²⁺ levels and a more rapid decline in renal function, increased risk of graft loss (1, 6, 11), and major cardiovascular outcomes have been reported (7). This underscores the importance of paying more attention to abnormal Mg²⁺ values in clinical practice within this patient population.

Clinical laboratories typically measure total Mg²⁺ (tMg²⁺), which represents the sum of both the protein-bound (approximately 20%–30%) and the free and biologically active form of Mg²⁺ (approximately 60%–70%), also referred to as ionized magnesium (iMg²⁺). Even though protein-bound and free Mg²⁺ fractions are usually in a steady state changing in similar directions, they may also change independently from each other with complex buffering and compartment-shift mechanisms involved. Dysproteinemias may change bound Mg²⁺ fractions without necessary changes in free Mg²⁺. Acid–base disorders may affect Mg²⁺ binding affinities, resulting in altered iMg²⁺ fraction, while tMg²⁺ level remains unchanged (12, 13). Evidence suggests that tMg²⁺ may not adequately reflect a patient’s Mg²⁺ state, underestimating the prevalence of clinically relevant hypomagnesemia (14–16). Earlier studies have shown discrepancy and poor correlation between tMg²⁺ and iMg²⁺. Favoring iMg²⁺ over tMg²⁺ could enhance the detection of hypomagnesemia, enabling more accurate identification of patients in need of supplementation and mitigating the risk of toxicity (14–18). However, there is still a lack of comprehensive understanding regarding the clinical utility and the subsequent impact on therapeutic interventions. Consequently, the measurement of iMg²⁺ remains uncommon as a routine test in most clinical settings.

The study aims to prospectively investigate plasma tMg²⁺ and iMg²⁺ levels in both healthy subjects and a cohort of KT patients and to define 1) the cross-sectional prevalence of hypomagnesemia and 2) correlate this state with clinical, demographic, and anthropometric parameters.

In this study, we investigated the add-on value of assessing serum iMg²⁺ compared to tMg²⁺ in detecting KT patients with pathological Mg²⁺ levels. The ultimate goal was to establish the cross-sectional prevalence of hypomagnesemia and examine its correlation with various clinical, demographic, and anthropometric parameters.

In a study sample including 208 measurements, we found that hypomagnesemia was frequent, with a prevalence of 52.9% (reduced iMg²⁺ and tMg²⁺). The notable high percentage contrasts with findings in other studies (20, 21), likely attributable to variations in methodology and the accepted Mg²⁺ cutoff criteria. This highlights the importance of determining the best measurement method and universal reference values. We also proved a strong correlation between iMg²⁺ and tMg²⁺ concentrations. These results have also been found in previous studies (16, 19). Most of our KT patients had CKD with moderate renal failure, and normo-magnesemia was expected. The high prevalence of hypomagnesemia in our study can also be explained by the use of CNI (76% of KT patients received cyclosporine or tacrolimus therapy). Indeed, after analysis of various factors, only the use of CNI correlated statistically significantly with hypomagnesemia. CNI induces hypomagnesemia through urinary Mg²⁺ wasting by downregulating the renal expression of the epidermal growth factor (EGF) and the Mg²⁺ channel Transient Receptor Potential Pelastatin 6 (TRMP6) in the distal collecting tubule (1, 22). The incidence of tacrolimus- or cyclosporine-induced hypomagnesemia has been studied in-depth, and these common side effects are now well known in the medical world (20, 21, 23).

Studies demonstrated that hypomagnesemia is associated with unfavorable outcomes and mortality (21, 22). Favoring iMg²⁺ over tMg²⁺ could enhance the detection of hypomagnesemia, enabling more accurate identification of patients in need of supplementation. Previous studies have investigated the sensitivity and correlations of measurements of iMg²⁺ compared to tMg²⁺ in detecting hypomagnesemia (24, 25). Our study suggests that tMg²⁺ could represent the active form of Mg²⁺ and be sufficient to detect hypomagnesemia (strong category agreement and correlation between iMg²⁺ and tMg²⁺). In contrast, the PCA illustrated that a significant proportion of patients otherwise considered normo-magnesemic on the basis of tMg²⁺ can be reclassified as hypomagnesemic if considering only iMg²⁺. This seems particularly true for patients with short KT follow-up ( Supplementary Figure 2 ). The misclassification could have clinical and therapeutic implications. Non-treated hypomagnesemia is associated with poor outcomes, including allograft function and mortality (26, 27). On the contrary, the administration of Mg²⁺ supplements is not without side effects. Furthermore, there is a lack of well-established knowledge regarding the influence of substitution therapy notably, on iMg²⁺ levels, and the appropriate timing for its initiation. While some studies (28) indicate that Mg²⁺ supplementation leads to an elevation in tMg²⁺, Rooney et al. demonstrated that Mg²⁺ substitution did not produce a statistically significant alteration in iMg²⁺ concentrations (29). These inquiries merit additional investigation.

Our study also has some limitations. First, although extensive in terms of parameters evaluated, it is a prospective and single-center study. Moreover, the study sample was small, limiting the generalizability of our results. In addition, the lack of standardized iMg²⁺ reference thresholds, inter-personal variability, and relation to the device used is a further obstacle to the generalization of the results. However, the inclusion of a control group allowed us to better define the reference value of Mg²⁺ and not base it solely on the already known literature, which we further specify is variable and not well defined. Second, although correlations with drugs have been studied, only analyses with immunosuppressant and Mg²⁺ supplements have been carried out. Exploring additional variables would have been valuable, particularly investigating the usage of other commonly prescribed drugs within the KT population. This could include diuretics and pump proton inhibitors, both known to contribute to urinary Mg²⁺ losses. Additionally, concerning the use of CNI, we were unable to investigate whether there was a therapy-dependent dose effect on the occurrence of hypomagnesemia. Finally, we could not capture the untoward clinical consequences of the reported ionized hypomagnesemia, such as allograft or cardiac outcomes. The percentage of participants on Mg²⁺ replacement therapy was small, and we did not study the potential beneficial or damaging effect of supplementation.

In conclusion, the determination of iMg²⁺ as a standard of care could be useful in recognizing false normo-magnesemic patients, particularly in high-risk populations such as KT patients. Currently, it is unclear if these patients suffer from Mg²⁺ deficiency and if supplementation is indicated. Further prospective interventional trials are needed.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding author.

The studies involving humans were approved by the Medical Ethics Committee of the study centers (Project-ID 2022-01953). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

FB: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Software, Supervision, Validation, Writing – original draft, Writing – review & editing. SS: Conceptualization, Investigation, Validation, Writing – review & editing. UH: Conceptualization, Investigation, Supervision, Validation, Writing – review & editing. BV: Conceptualization, Investigation, Supervision, Validation, Writing – review & editing. DS: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Software, Supervision, Validation, Writing – review & editing.