Magnesium infusion for the investigation and treatment of magnesium deficiency
Magnesium deficiency is often considered the forgotten anion in view of the magnitude of unrecognised deficiency in hospital in-patients (approximately 10%). Oral replacement therapy for the majority of patients does not need to be assessed. This test will probably only rarely be used for those unusual occasions when signs of magnesium deficiency occur with borderline low plasma magnesium concentrations.
This loading dose of magnesium should not be given in subjects with renal failure, cardiac dysrhythmias or respiratory failure. It should not be performed unless causes of renal loss have been excluded (alcoholism, salt-losing nephropathy, hyperaldosteronism, glycosuria etc) nor in subjects who are taking drugs which affect tubular function (diuretics, cisplatin, cyclosporin etc)
Magnesium is predominantly an intra-cellular ion with less than 1% in the extracellular compartment. Plasma or erythrocyte magnesium concentrations are therefore relatively poor indicators of body magnesium status. This loading test measures the retention of magnesium and therefore reflects the degree of deficiency.
Tiredness, sensation of heat and occasionally mild generalised redness of the skin.
No specific patient preparation is required
- 30 mmol magnesium in 500 mL 5% dextrose for intravenous administration
- 24 h urine bottle
Administer the magnesium intravenously over 12h. Collect all urine passed over the 24h period which begins with onset of magnesium infusion.
Magnesium depletion unlikely if more than 24 mmol magnesium are excreted in the 24 h collection. Normal subjects retain less than 10% of infused load.
Patients with acute MI, with chronic IHD or on diuretics have been demonstrated to have mild magnesium deficiency as they retain more magnesium. These patients will excrete less than 20 mmol in the 24 h urine collection.
- Dyckner T, Wester PO. Magnesium deficiency - guidelines for diagnosis and substitution therapy. Acta Med Scand 1982;suppl 661:37-41.
- Gullestad L, Midtvedt K, Dolva LO, Norseth J, Kjekshus J. The magnesium loading test: reference values in healthy subjects. Scand J Clin Lab Invest 1994;54:23-31.
- Rasmussen HS, McNair P, Goransson L, Belslor S, Larsen OG, Aurup P. Magnesium deficiency in patients with ischaemic heart disease with and without acute myocardial infarction uncovered by an intravenous magnesium loading test. Arch Int Med 1988;148:329-32.