Which value should lead the nurse to conclude that the clients serum magnesium level is therapeutic?

Topic Resources

Hypomagnesemia is serum magnesium concentration < 1.8 mg/dL (< 0.70 mmol/L). Causes include inadequate magnesium intake and absorption or increased excretion due to hypercalcemia or drugs such as furosemide. Clinical features are often due to accompanying hypokalemia and hypocalcemia and include lethargy, tremor, tetany, seizures, and arrhythmias. Treatment is with magnesium replacement.

Serum magnesium concentration, even when free magnesium ion is measured, may be normal even with decreased intracellular or bone magnesium stores.

Cisplatin can cause increased magnesium losses by the kidneys as well as generalized decrease in kidney function. Magnesium loses can be severe and persist despite discontinuation of cisplatin. Discontinuation of cisplatin is still recommended if signs of renal toxicity occur during therapy.

• Serum magnesium concentration < 1.8 mg/dL (< 0.70 mmol/L)

Magnesium deficiency should be suspected even when serum magnesium concentration is normal in patients with unexplained hypocalcemia or refractory hypokalemia. Magnesium deficiency should also be suspected in patients with unexplained neurologic symptoms and alcohol use disorder, with chronic diarrhea, or after cyclosporine use, cisplatin-based chemotherapy, or prolonged therapy with amphotericin B or aminoglycosides.

• Oral magnesium salts

• IV or IM magnesium sulfate for severe hypomagnesemia or inability to tolerate or adhere to oral therapy

Treatment with magnesium salts is indicated when magnesium deficiency is symptomatic or the magnesium concentration is persistently < 1.25 mg/dL (< 0.50 mmol/L). Patients with alcohol use disorder are treated empirically. In such patients, deficits approaching 12 to 24 mg/kg are possible.

About twice the amount of the estimated deficit should be given in patients with intact renal function because about 50% of the administered magnesium is excreted in urine. Oral magnesium salts (eg, magnesium gluconate 500 to 1000 mg orally 3 times a day) are given for 3 to 4 days. Oral treatment is limited by the onset of diarrhea.

Parenteral administration is reserved for patients with severe, symptomatic hypomagnesemia who cannot tolerate oral drugs. Sometimes a single injection is given in patients with alcohol use disorder who are unlikely to adhere to ongoing oral therapy. When magnesium must be replaced parenterally, a 10% magnesium sulfate solution (1 g/10 mL) is available for IV use and a 50% solution (1 g/2 mL) is available for IM use. The serum magnesium concentration should be monitored frequently during magnesium therapy, particularly when magnesium is given to patients with renal insufficiency or in repeated parenteral doses. In these patients, treatment is continued until a normal serum magnesium concentration is achieved.

In severe, symptomatic hypomagnesemia (eg, magnesium < 1.25 mg/dL [< 0.5 mmol/L] with seizures or other severe symptoms), 2 to 4 g of magnesium sulfate IV is given over 5 to 10 minutes. When seizures persist, the dose may be repeated up to a total of 10 g over the next 6 hours. In patients in whom seizures stop, 10 g in 1 L of 5% D/W (dextrose in water) can be infused over 24 hours, followed by up to 2.5 g every 12 hours to replace the deficit in total magnesium stores and prevent further drops in serum magnesium.

When serum magnesium is ≤ 1.25 mg/dL (< 0.5 mmol/L) but symptoms are less severe, magnesium sulfate may be given IV in 5% D/W at a rate of 1 g/hour as slow infusion for up to 10 hours. In less severe cases of hypomagnesemia, gradual repletion may be achieved by administration of smaller parenteral doses over 3 to 5 days until the serum magnesium concentration is normal.

• Hypomagnesemia may occur in patients with alcohol use disorder, uncontrolled diabetes, or hypercalcemia or patients taking loop diuretics.

• Symptoms include anorexia, nausea, vomiting, lethargy, weakness, personality change, tetany (eg, positive Trousseau or Chvostek sign, spontaneous carpopedal spasm, hyperreflexia), tremor, and muscle fasciculations.

• Treat with magnesium salts when magnesium deficiency is symptomatic or persistently < 1.25 mg/dL (< 0.50 mmol/L).

• Give oral magnesium salts unless patients have seizures or other severe symptoms, in which case, give 2 to 4 g of magnesium sulfate IV over 5 to 10 minutes.

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