Association between Serum Magnesium Levels and Risk of Acute Kidney Injury in Patients with Traumatic Brain Injury: A Retrospective Cohort Study from the MIMIC-IV Database

<b><i>Introduction:</i></b> The occurrence of acute kidney injury (AKI) is associated with a higher risk of mortality in patients with traumatic brain injury (TBI). This study aimed to explore the relationship between serum magnesium levels and the risk of AKI in patients with TBI. <b><i>Methods:</i></b> Patients with TBI were identified from the Medical Information Mart Intensive Care IV (MIMIC-IV) 2008–2019. The relationship between serum magnesium levels at admission and magnesium coefficient of variation (CV) during hospitalization and the risk of AKI was analyzed using multivariable logistic regression analysis and expressed as odds ratio (OR) and 95% confidence interval (CI). Subgroup analyses were performed according to Glasgow Coma Scale (GCS) score (&lt;14, ≥14), sepsis (no, yes), and estimated glomerular filtration rate (eGFR; &lt;60, ≥60). <b><i>Results:</i></b> Of the 991 patients included, 140 (14.13%) developed AKI during hospitalization. Patients with magnesium levels ≤1.7 mg/dL (tertile 1) (OR = 1.68, 95% CI: 1.01–2.81) were associated with a higher risk of AKI compared to those with magnesium levels of 1.7–2.0 mg/dL (tertile 2), but no association was found in those with magnesium levels &gt;2.0 mg/dL (tertile 3) (<i>p</i> = 0.479). For magnesium CV, patients with magnesium CV &gt;10% (tertile 3) (OR = 2.26, 95% CI: 1.16–4.41) were linked to an increased risk of AKI compared to those with magnesium CV ≤4% (tertile 1), but there may be a slight association between magnesium CV of 4%–10% (tertile 2) and AKI risk (OR = 1.86, 95% CI: 0.99–3.48; <i>p</i> = 0.053). Subgroup analyses showed that lower magnesium levels (≤1.7 mg/dL) or greater magnesium CV (&gt;10%) were associated with a higher risk of AKI only in patients with a GCS score ≥14, non-sepsis, or eGFR ≥60 mL/min/1.73 m² (<i>p</i> &lt; 0.05). <b><i>Conclusion:</i></b> Lower serum magnesium levels at admission or greater magnesium CV during hospitalization were associated with a higher risk of AKI in patients with TBI.