Functional magnesium deficiency in critically ill patients identified using a magnesium-loading test

Crit Care Med. 1997 May;25(5):749-55. doi: 10.1097/00003246-199705000-00007.


Objective: To determine the feasibility of the magnesium-loading test in the critically ill and to validate serum ionized magnesium assay using the magnesium-loading test as a reference in this same patient population.

Design: Double-blind, randomized, controlled clinical investigation.

Setting: Tertiary level intensive care unit.

Patients: Forty-four consecutive critically ill patients without evidence of renal insufficiency.

Intervention: Patients were randomly allocated to receive 30 mmol (7.5 g) of magnesium sulfate daily for 3 days, or an equivalent amount of normal saline.

Measurements and main results: We recorded baseline characteristics, and serial serum biochemical measurements included creatinine, glucose, sodium, potassium, phosphate, total calcium, ionized calcium, total magnesium, and ionized magnesium. Serum assays were accompanied by 24-hr urine collections of creatinine and magnesium over the 3-day period. Baseline characteristics were comparable in both groups. In patients receiving magnesium, serum ionized magnesium and total magnesium concentrations were increased by 43% (p = .0001) and 59% (p = .0002), respectively, on day 1 as compared with the control group. Magnesium excretion in the control group averaged 4.8 +/- 2.3 mmol/day during the 3-day study period, while the magnesium excretion in the magnesium-loaded group was significantly increased to 22.7 +/- 10.9 mmol/day (p < .0001). Following day 1 magnesium loading, patients who excreted < 70% of the total magnesium (30 mmol infused magnesium plus 4.8 mmol basal excretion) were termed as functionally magnesium-deficient retainers (n = 12), and patients who excreted > 70% of the total magnesium were termed as nonretainers (n = 7). In addition, magnesium retainers on day 2 (nine of ten patients) and day 3 (five of six patients) excreted > 70% of the total magnesium, indicating a replenishment of body magnesium stores. In contrast, nonretainers on day 2 (four of five patients), and day 3 (four of four patients) continued to excrete excess amounts of magnesium. In the retainer group, only two patients had a low serum ionized magnesium concentration, while two other patients had low total serum magnesium values. In addition, magnesium retention was associated with low ionized calcium and high phosphate values.

Conclusions: The magnesium-loading test is feasible and appears to be valid based on its performance during the 3-day evaluation. Using the magnesium-loading test as a reference, serum ionized magnesium appears to be an insensitive biochemical marker of functional hypomagnesemia. Larger cohort studies using the magnesium-loading test will help establish the true prevalence of magnesium deficiency and its associated risk factors in critically ill patients.

Publication types

  • Clinical Trial
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aged
  • Critical Care / methods*
  • Critical Illness
  • Double-Blind Method
  • Electrolytes / blood
  • Feasibility Studies
  • Female
  • Hospital Mortality
  • Humans
  • Intensive Care Units
  • Length of Stay
  • Magnesium Deficiency / diagnosis*
  • Magnesium Deficiency / metabolism*
  • Magnesium Sulfate / administration & dosage*
  • Magnesium Sulfate / blood
  • Magnesium Sulfate / urine
  • Male
  • Middle Aged


  • Electrolytes
  • Magnesium Sulfate