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Magnesium

Magnesium is the fourth most abundant cation in the body, behind sodium, potassium, and calcium. It is the second most prevalent intracellular cation after potassium. The normal body magnesium content is approximately 1000 mmol or 25 g, of which about half is in bone and the other half is intracellular in soft tissue and muscle. Less than 1% of the total body magnesium is present in blood. Magnesium is essential for the function of many important enzymes, including reactions involving ATP synthesis and DNA replication and transcription. Magnesium is also required for cellular energy metabolism, membrane stabilization, nerve conduction, calcium channel activity and ion transport. Magnesium deficiency results in a variety of metabolic abnormalities and clinical consequences.

GI absorption and renal excretion regulate total body magnesium levels. The average daily dietary intake is about 325 mg and intestinal absorption is inversely proportional to the amount ingested. Most magnesium is absorbed in the ileum and colon. Cereal, grains, nuts legumes, and chocolate are relatively rich in magnesium. Vegetables, fruits, meats and fish have intermediate amounts and dairy products are low in magnesium. The kidney is the major excretory organ for magnesium. Approximately 70% of plasma magnesium is filtered through the glomerular membrane. Only about 6% of filtered magnesium (120 mg) is excreted daily into the urine, because of reabsorption in the Loop of Henle. The major regulator of tubular reabsorption is the plasma magnesium concentration. Hypermagnesemia inhibits and hypomagnesemia stimulates renal transport.

Serum magnesium exists in three states: approximately 60% is ionized (free), 33% is protein bound, and 7% is complexed to phosphate, citrate, and other anions. Approximately 75% of the protein bound fraction is bound to albumin and 25% to globulins. Plasma magnesium concentration does not correlate very well with tissue magnesium levels. Plasma levels are useful for assessing acute changes in magnesium states, especially in patients with cardiac arrhythmias, acute onset of seizures, and diabetic ketoacidosis.

Hypomagnesemia occurs in up to 12 percent of hospitalized patients Risk factors for hypomagnesemia include chronic diarrhea, proton pump inhibitor therapy, alcoholism, and diuretic use. Clinical manifestations of hypomagnesemia include unexplained hypocalcemia, refractory hypokalemia, neuromuscular disturbances, and ventricular arrhythmias. Plasma magnesium levels below 1.0 mg/dL can cause tetany, arrhythmias, or seizures.

Hypocalcemia is common in patients with severe hypomagnesemia, usually appearing when the serum magnesium level is less than 1.0 mEq/L. PTH levels are usually low and rise rapidly following magnesium replacement. Hyokalemia also frequently accompanies hypomagnesemia. It does not respond to potassium replacement until the magnesium deficit is corrected.

Hypermagnesemia usually occurs in two clinical settings: impaired renal function and/or after administration of a large magnesium load. The incidence of hypermagnesemia in hospitalized patients is estimated to be 10 to 15 percent. Other causes of severe hypermagnesemia are accidental poisoning with Epsom salts in children and overuse of magnesium-containing cathartics.

Clinical symptoms may be seen when the plasma magnesium concentration exceeds 4.8 mg/dL. Plasma magnesium concentration between 4.8 and 7.2 mg/dL is associated with nausea, flushing, headache, lethargy, drowsiness, and diminished deep tendon reflexes. Levels above 7.2 mg/dL cause somnolence, hypocalcemia, absent deep tendon reflexes, hypotension, bradycardia, and ECG changes. Magnesium levels above 12 mg/dL cause muscle paralysis leading to flaccid quadriplegia, respiratory failure, complete heart block, and cardiac arrest.

Parenteral magnesium is commonly used to decrease neuromuscular excitability in pregnant women with severe preeclampsia or eclampsia. The usual plasma concentration achieved is 6 to 8.4 mg/dL, but much higher levels can occur. Maternal complications include hypocalcemia and hyperkalemia.   Neonatal complications include hypocalcemia, hypotonia, osteopenia, and an increased rate of admissions to the neonatal intensive care unit.

Reference range is 1.6 - 2.6 mg/dL or 1.3 - 2.2 mEq/L. Levels below 1.0 mg/dL are considered critical values. 

Specimen requirement is one green top tube or one red top tube of blood.

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