Low serum magnesium levels are frequently observed in patients undergoing surgery and commonly necessitate intravenous (IV) magnesium supplementation. Given the kidneys' role in magnesium regulation, extending the infusion time of IV magnesium has been suggested to enhance retention by minimizing urinary loss. Nevertheless, supporting data for this approach remain scarce. To compare the rise in serum magnesium concentration from baseline following IV magnesium administration between extended infusions (rate < 0.5 g/h) and rapid infusions (rate ≥ 0.5 g/h) among hospitalized patients who have undergone surgery. We conducted a retrospective review of medical records from surgical patients admitted to a university hospital between 2012 and 2022 who had hypomagnesemia, received IV magnesium supplementation for three consecutive days. Patients were divided into two groups based on infusion rate: extended infusion and rapid infusion. The main outcome measure was the increase in serum magnesium level per gram of magnesium given relative to baseline. A secondary outcome was the proportion of patients reaching a target serum magnesium concentration following supplementation. A total of 114 patients were included. The rapid infusion group demonstrated a significantly larger increase in serum magnesium per gram administered (0.07 mg/dL/g) compared with the extended infusion group (0.05 mg/dL/g) (p = 0.04), with a between-group difference of 0.013 mg/dL/g. No significant difference was found in the proportion of patients achieving target serum magnesium levels (extended infusion: 66.7% vs. rapid infusion: 70.2%; p = 0.84). Factors influencing the change in serum magnesium included kidney function and the interval between completion of infusion and magnesium level assessment. In surgical inpatients, slowing the IV magnesium infusion to below 0.5 g/h offered no added advantage in elevating serum magnesium concentrations compared with faster infusion rates.
