The incidence of postoperative hypomagnesemia in patients undergoing spinal surgery has been reported to be 70%. Ionized magnesium is considered to be the biologically active form, but until the early 1990s, only the total magnesium concentration could be measured. Currently, the ionized magnesium concentration as well as total magnesium concentration can be assessed due in part to the development of a selective electrode. The aim of this study was to more fully characterize the changes in the total and ionized magnesium concentrations in patients undergoing elective spinal fusion surgery.

The total and ionized magnesium, creatinine, albumin, urinary magnesium concentration, hematocrit, total amount of fluid administration, transfusion, blood loss, and urine output were evaluated both preoperatively and postoperatively in each patient.

The total and ionized magnesium concentrations decreased from 0.783 mM/L and 0.529 mM/L preoperatively to 0.717 mM/L and 0.511 mM/L during the postoperative period, respectively.

The incidence of total hypomagnesemia during spinal surgery was 15% but the incidence of ionized hypomagnesemia was only 3%.