Approach to magnesium repletion in adults with hypomagnesemia and normal kidney function

IV: intravenous.

* Hypomagnesemia is typically defined as a plasma magnesium concentration <1.9 mg/dL. However, the threshold plasma magnesium level that defines hypomagnesemia may vary among clinical laboratories.

¶ In addition to magnesium repletion, the underlying cause for hypomagnesemia should be addressed. Refer to UpToDate content on the causes of hypomagnesemia for further details.

Δ In general, great caution should be exercised in treating patients who have acute or chronic kidney injury with magnesium-containing medications. However, patients with reduced kidney function may require magnesium repletion if they have severe hypomagnesemia (ie, <1 mg/dL [0.4 mmol/L or 0.8 mEq/L]). Refer to UpToDate content on the treatment of hypomagnesemia in patients with impaired kidney function for more details.

◊ Intravenous magnesium supplementation is inefficient. When an intravenous magnesium infusion is given, an abrupt but temporary elevation in plasma magnesium concentration will partially inhibit the stimulus to magnesium reabsorption in the loop of Henle. Thus, up to 50% of the infused magnesium will be excreted in the urine. For these reasons, intravenous magnesium should be administered slowly in nonemergent situations.

§ The magnesium concentration should be measured 6 to 12 hours after each dose of IV magnesium. Repeat doses are given based upon the follow-up measurement. Of note, plasma magnesium levels do not correlate well with total body stores, as the majority of magnesium is intracellular; plasma concentrations may be transiently elevated for a few hours after administration of an IV dose.

¥ A number of oral magnesium salts are available. Each differs in the content of elemental magnesium, but all suffer from limited bioavailability. Sustained-release preparations (eg, magnesium chloride, magnesium L-lactate) have the advantage that they are slowly absorbed and thereby minimize kidney excretion of the administered magnesium. If a sustained-release preparation is not available, magnesium oxide may be used, but diarrhea frequently occurs with magnesium oxide therapy.