title: Functional magnesium deficiency in critically ill patients identified using a magnesium-loading test.
abstract: 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.
  
  
  METHODS
  Double-blind, randomized, controlled clinical investigation.
  
  
  METHODS
  Tertiary level intensive care unit.
  
  
  METHODS
  Forty-four consecutive critically ill patients without evidence of renal insufficiency.
  
  
  METHODS
  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.
  
  
  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.
authors.affiliation: Department of Pathology, Ottawa General Hospital, University of Ottawa, ON, Canada.
authors.forename: P
authors.initials: P
authors.lastname: Hébert
authors.forename: N
authors.initials: N
authors.lastname: Mehta
authors.forename: J
authors.initials: J
authors.lastname: Wang
authors.forename: T
authors.initials: T
authors.lastname: Hindmarsh
authors.forename: G
authors.initials: G
authors.lastname: Jones
authors.forename: P
authors.initials: P
authors.lastname: Cardinal
mesh_terms: D018806:APACHE; D000368:Aged; D003422:Critical Care / Q000379:methods*; D016638:Critical Illness; D004311:Double-Blind Method; D004573:Electrolytes / Q000097:blood; D005240:Feasibility Studies; D005260:Female; D017052:Hospital Mortality; D006801:Humans; D007362:Intensive Care Units; D007902:Length of Stay; D008275:Magnesium Deficiency / Q000175:diagnosis* / Q000378:metabolism*; D008278:Magnesium Sulfate / Q000008:administration & dosage* / Q000097:blood / Q000652:urine; D008297:Male; D008875:Middle Aged
chemical_list: D004573:Electrolytes; D008278:Magnesium Sulfate