Treatment of Magnesium Deficiency

NHS Greater Glasgow and Clyde

Clinical Biochemistry Service Test Database

Index

Treatment of Magnesium Deficiency in Adults

Guideline prepared by GRI formulary.

(Refer to BNF for recommendations for magnesium use in arrhythmias, myocardial infarction and eclampsia.)

The reference range for serum magnesium is 0.7-1.0 mmol/L. However, since magnesium is mainly an intracellular ion, serum concentrations are not an exact measurement of total body stores. Serum concentrations should be used in conjunction with presenting signs and symptoms to diagnose hypomagnesaemia. Twenty-four hour urinary magnesium excretion (reference range 2.3-10.7 mmol/day) together with serum magnesium concentration, is required for an accurate diagnosis of hypomagnesaemia.

Hypomagnesaemia should be corrected over about 5 days, since magnesium equilibrates slowly within intracellular compartments. In addition, this helps to avoid toxic concentrations of magnesium.

Adults with normal renal function

Magnesium levels should be monitored daily, and dose adjusted as necessary.

Oral administration: is used for chronic magnesium loss or moderately severe hypomagnesaemia i.e. where there is a serum magnesium level of 0.4-0.7mmol/L and no symptoms of hypomagnesaemia.

Preparations and dosage: Magnesium hydroxide mixture (Milk of Magnesia) 5ml should be given twice daily. 5ml of magnesium hydroxide mixture will provide 7 mmol magnesium. The dose can be increased as tolerated to approximately 50 mmol daily in 4 divided doses i.e. 10 ml magnesium hydroxide mixture four times daily. Diarrhoea tends to limit the amount of magnesium that can be given orally. If diarrhoea develops reduce the dose.

Intravenous administration: is used if the patient is symptomatic and serum magnesium is less than 0.4 mmol/L. A continuous infusion of magnesium sulphate is more effective in correcting hypomagnesaemia than intermittent infusions.

Preparations and dosage: 5 ml ampoules of magnesium sulphate 50% contain 500 mg/ml of magnesium sulphate (equivalent to 2 mmol/ml of magnesium). Up to 160 mmol of magnesium over 5 days may be necessary. The following regimen is suggested:

Day 1: 25 mmol over 3 hours followed by 20 mmol over 21 hours.

Day 2-5: 25 mmol over 24 hours.

25 mmol of magnesium, i.e. 12.5 ml magnesium sulphate 50% injection, should be administered in at least 500 ml dextrose 5% or normal saline. There may be problems with administering this volume of fluid to fluid-restricted patients. This should be discussed with your pharmacist.

Adults with impaired renal function

The above dosage recommendations should be halved in patients with impaired renal function.

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