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Magnesium

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Contents

 

 

Physiology, Hypomagnesaemia, Table, Hypermagnaesaemia, Measurement

Physiology

Magnesium is an intracellular cation.   It is essential for enzyme activity, for the synthesis of nucleic acids and proteins, and has an important physiological role in the neuromuscular and cardiovascular systems.  Total body magnesium is approximately 1000 mmols, of which 60% is in bone, 20% in skeletal muscle, and less than 1% in the extracellular fluid.  In the circulation, 65% of serum magnesium is free (ionised), about 20% is protein bound, and the rest is complexed with various anions (eg: phosphate and citrate).   The body magnesium balance is regulated by intestinal absorption (predominantly in the ileum and colon), and renal reabsorption (65-75% by the thick ascending loop of Henle, 15-20% in the proximal tubules).   The most commonly used method for assessing magnesium status is serum magnesium concentration.  Spot or 24-hour urine magnesium may be useful in determining renal or intestinal wasting especially in the setting of hypomagnaesemia.

Hypomagnesaemia              top of page

Hypomagnesaemia is common (7-11%) in hospital patients, and the prevalence is higher in critically ill patients.  It often coexists with other electrolyte disturbances, especially in the setting of hypokalaemia, hypophosphataemia, hyponatraemia, or hypocalcaemia. Low serum magnesium may be a cause of hypocalcaemia due reduced PTH secretion and activity.

 

Table: Causes of Hypomagnaesemia            top of page

Redistribution of magnesium

 

  • Refeeding

  • Insulin therapy (eg Rx of DKA)

  • Correction of acidosis

  • Catecholamine excess

  • Massive blood transfusion

Gastrointestinal Causes

  • Reduced intake

  • Reduced absorption (eg: malabsorption, chronic diarrhoea, intestinal resection)

Renal Loss

 

  • Drugs: 
        Diuretics (eg thiazides, frusemide)
        Aminoglycoside (eg: Gentamicin)
        Cytotoxics (eg: cisplatin, carboplatin)
       Amphoterocin

  • Renal Disease
        Post obstructive nephropathy
        Post renal transplantation
        Dialysis
        Diuretic phase of acute renal failure
        Inherited disorders (eg: Bartter’s and
                   Gitelman’s syndromes)

Alcoholism

 

 Causes of Hypermagnesaemia                              top of page

Hypermagnesaemia most commonly occurs due to excessive administration of magnesium salts or magnesium-containing drugs (eg: antacids), especially in patients with renal impairment. Mild elevations of serum magnesium are usually asymptomatic. Hypotension and depressed reflexes seen with concentrations over 3 mmol/L. Very high serum magnesium concentrations are usually iatrogenic with serum levels > 4mmol/L associated with paralysis, hypotension, bradycardia, cardiac arrest.

Measurement

Magnesium can be measured in serum (gold or red top tubes) or heparin plasma (lime top tubes). Full blood count tubes (EDTA, purple top) and coagulation tubes (Citrate, light blue top) are NOT acceptable as magnesium is chelated by these anticoagulants.
The assay is available 7 days, 24 hours in the main chemistry laboratory.

References Intervals are 0.7 - 1.05 mmol/L

 

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For further information please contact Dr Graham Jones on 8382-9100

gjones@stvincents.com.au
Last updated 27/5/03