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Introduction ⬇

Magnesium (Mg2+) Level

A magnesium (Mg2+) level measures the amount of magnesium in the body and is used to evaluate kidney function, electrolyte status, and magnesium metabolism. Magnesium in the body is concentrated (40%–60%) in the bone, 20% in muscle, 30% within the cell itself, and 1% in the serum and is required for the use of adenosine triphosphate as a source of energy. It is therefore necessary for the action of numerous enzyme systems such as carbohydrate metabolism, protein synthesis, nucleic acid synthesis, and contraction of muscular tissue. Along with sodium, potassium, and calcium ions, magnesium also regulates neuromuscular irritability and the clotting mechanism.

Magnesium and calcium are intimately linked in their body functions, and deficiency of either one has a significant effect on the metabolism of the other because of magnesium’s importance in the absorption of calcium from the intestines and in calcium metabolism. Magnesium deficiency will result in the drift of calcium out of the bones, possibly resulting in abnormal calcification in the aorta and the kidney. This condition responds to administration of magnesium salts. Normally, 95% of the magnesium that is filtered through the glomerulus is reabsorbed in the tubule. When there is decreased kidney function, greater amounts of magnesium are retained, resulting in increased blood serum levels.

Procedure ⬆ ⬇

  1. Obtain a fasting (4 hours), 5-mL in an SST or red-topped tube, venous blood sample (see Chapter 2, for venous blood collection).

  2. Avoid hemolysis and separate serum from cells as soon as possible. Heparinized blood may be used.

Procedural Alert

Blood sample should be drawn while the patient is in a prone position because an upright position increases the magnesium level by 4%

Clinical Implications ⬆ ⬇

  1. Hypomagnesemia occurs with the following conditions:

    1. Hypercalcemia of any cause

    2. Diabetic acidosis

    3. Hemodialysis

    4. Chronic kidney disease (glomerulonephritis)

    5. Chronic pancreatitis

    6. Hyperaldosteronism

    7. Pregnancy (second and third trimester)

    8. Hypoparathyroidism

    9. Excessive loss of body fluids (due to sweating, lactation, diuretic abuse, chronic diarrhea)

    10. Malabsorption syndromes

    11. Chronic alcoholism (hepatic cirrhosis)

    12. Long-term hyperalimentation

    13. SIADH

  2. Hypermagnesemia occurs in the following conditions:

    1. Acute kidney injury or reduced kidney function

    2. Dehydration

    3. Hypothyroidism

    4. Addison disease

    5. Adrenalectomy (adrenocortical insufficiency)

    6. Diabetic acidosis (severe)

    7. Use of antacids containing magnesium (e.g., milk of magnesia), administration of magnesium salts

    8. Oliguria

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and blood-drawing procedure.

  2. Ensure that patient is fasting for at least 4 hours if possible and is in a prone position when blood is drawn.

  3. Follow guidelines in Chapter 1 regarding safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Treatment of diabetic coma often results in low plasma magnesium levels. This change occurs because magnesium moves with potassium into the cells after insulin administration.

  2. Measure serum magnesium in persons receiving aminoglycosides and cyclosporine. There is a known association between these therapies and hypermagnesemia. Treatment of hypermagnesemia involves withholding source of magnesium excess, promoting excretion, giving calcium salts, and performing hemodialysis.

  3. Magnesium deficiency may cause apparently unexplained hypocalcemia and hypokalemia. In these instances, patients may have neurologic or GI symptoms. Observe for the following signs and symptoms:

    1. Muscle tremors, twitching, tetany

    2. Hypocalcemia

    3. Hyperactive deep tendon reflexes

    4. ECG: Prolonged PR and QT intervals; broad, flat T waves; ventricular tachycardia (VT) and ventricular fibrillation (VF)

    5. Anorexia, nausea, vomiting

    6. Insomnia, delirium, convulsions

  4. Observe for signs of hypermagnesemia (which acts as a sedative):

    1. Lethargy, flushing, nausea, vomiting, slurred speech

    2. Weak or absent deep tendon reflexes

    3. ECG: Prolonged PR and QT intervals; widened QRS; bradycardia

    4. Hypotension, drowsiness, respiratory depression

  5. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆ ⬇

  1. Prolonged salicylate therapy, lithium, and magnesium products (e.g., antacids, laxatives) will cause falsely increased magnesium levels, particularly if there is kidney damage.

  2. Calcium gluconate, as well as a number of other drugs, can interfere with testing methods and cause falsely decreased results.

  3. Hemolysis will invalidate results because about three fourths of the magnesium in the blood is found intracellularly in the red blood cells.

Reference Values ⬆

Normal

  • Adults: 1.8–2.6 mg/dL (0.74–1.07 mmol/L)

  • Children: 1.7–2.1 mg/dL (0.70–0.86 mmol/L)

  • Newborns: 1.5–2.2 mg/dL (0.62–0.91 mmol/L)

Clinical Alert

Critical Values

  1. Hypomagnesemia (low Mg2+ level): <1.2 mg/dL (<0.49 mmol/L), tetany occurs

  2. Hypermagnesemia (high Mg2+ level): >5.0 mg/dL (>2.1 mmol/L)

    1. 5.0–10.0 mg/dL (2.1–4.1 mmol/L): Central nervous system depression, nausea, vomiting, fatigue

    2. 10–15 mg/dL (4.1–6.2 mmol/L): Coma, ECG changes, respiratory paralysis

    3. 30 mg/dL (12.3 mmol/L): Complete heart block

    4. 34–40 mg/dL (14.0–16.0 mmol/L): Cardiac arrest