U-Shaped Relationship Between Preoperative Serum Magnesium and Postoperative Acute Kidney Injury in Elderly Patients Undergoing Noncardiac Surgery: A Retrospective Cohort Study.

Abstract

Objectives: To evaluate the association between preoperative serum magnesium (sMg) and postoperative acute kidney injury (PO-AKI) risk in elderly noncardiac surgery patients, and to explore the potential mediating role of systemic inflammation measured by C-reactive protein (CRP).

Methods: We conducted a retrospective cohort study including patients aged ≥65 years undergoing elective noncardiac surgery at a tertiary center from 2014 to 2021. Preoperative sMg and CRP were measured within 30 days before surgery. PO-AKI within 7 days postoperatively was defined using Kidney Disease: Improving Global Outcomes criteria. Confounders were selected using a directed acyclic graph. Logistic regression and restricted cubic spline models assessed the association between sMg and PO-AKI, and CRP-mediated effects were evaluated via formal mediation analysis.

Results: Among 13,245 patients (median age, 71 years; 45.6% male), 884 (6.7%) developed PO-AKI. A U-shaped relationship was observed between preoperative sMg and PO-AKI risk. Compared with patients in the fourth quintile (0.90-0.95 mmol/L), those in the lowest (≤0.83 mmol/L) and highest (>0.95 mmol/L) quintiles had 50% and 25% higher odds of PO-AKI, respectively. Mediation analysis indicated that CRP partially explained this association, accounting for 14.35% of the total effect, with higher contributions in cancer patients and younger elderly individuals.

Conclusions: Deviations from the optimal preoperative sMg range, particularly lower levels, were associated with an increased risk of PO-AKI in elderly surgical patients, whereas high-normal levels conferred a modest relative risk elevation. Systemic inflammation partially mediated these associations, suggesting that preoperative sMg assessment may help identify vulnerable patients and inform perioperative monitoring strategies.