• Serum sodium
• Na⁺ (Serum)

Red, green or tiger top tube.

5 mL of venous blood.

Sodium (Na) is the primary extracellular cation (balanced with chloride and bicarbonate anions). Na serves an important role in maintenance of osmotic pressure, blood pressure, renal retention/secretion of water and conduction within nerve axons.

This test measures serum sodium.

Hyponatremia:

Hyponatremia is an abnormally low level of Na < 135 mEq/L.

Mild hyponatremia is only occasionally clinically significant and is extremely common. Significant hyponatremia is 120 mEq/L and severe hyponatremia is 110 mEq/L.

Typical symptoms include confusion, poor appetite, failure to thrive, confusion, coma, agitation, seizures, weakness, or muscular weakness.

It is important for the clinician to assess whether the hyponatremia is acute or chronic. Rapid onset of hyponatremia will result in cerebral edema, herniation and seizures and requires rapid therapy to avoid herniation and death. This rapid correction for acute hyponatremia differs substantially from the more common situation of chronic hyponatremia where a gradual correction is recommended and rapid correction can result in cerebral pontine myelinolysis (highest risk when initial Na 110 mEq/L).

Pseudohyponatremia occurs due to laboratory error with some laboratory methods when a patient has coexisting extreme hyperlipidemia or hyperproteinemia.

Hyperglycemia requires an adjustment to the reported serum Na. For each 100 mg/dL above the normal level of 100 mg/dL a patient's blood sugar is, the reported serum Na should be "corrected" by having 1.6 mEq/L added to it. For example, a patient with a blood sugar of 600 mg/dL with a reported serum Na of 128 mEq/L would be adjusted to 128 + 1.6 × 5 = 136 mEq/L.

Hypernatremia:

Hypernatremia is an abnormally high level of Na > 145 mEq/L.

Typical symptoms relate to dehydration of the brain. These include altered level of consciousness, coma, lethargy, ataxia, irritability, twitching, spasticity and/or hyperreflexia.

Just as with hyponatremia, acute hypernatremia may be corrected rapidly, but chronic hypernatremia requires gradual replacement of free water as too rapid a correction can result in cerebral edema.

Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below.

Hypernatremia is an abnormally high level of Na > 145 mEq/L.

Conditions that cause hypernatremia include:

• Inadequate fluid intake as compared to losses (especially in ill elderly who stop consuming fluids)
• Diabetes Insipidus (defect in action/secretion of ADH)
• Primary Hypodipsia (loss of the hypothalamic thirst center; suspect in the non-thirsty hypernatremic patient)
• Hypertonic saline gain (Ingestion of hypertonic solutions, salt water drowning, etc)
• Primary hyperaldosteronism
• Cushing's syndrome

Drugs which may cause hypernatremia include:

• Alpha blockers (Clonidine/Methyldopa)
• Cisplatin
• Estrogens/OCP's
• Gamma globulin
• Mannitol
• Penicillins
• Steroids (Both anabolic and corticosteroids)

Hyponatremia is an abnormally low level of Na < 135 mEq/L.

Conditions that cause hyponatremia include:

• Medication related (see below)
• Increased free water intake (Ecstasy use with secondary water intoxication, Beer in large quantities, psychogenic polydipsia)
• Adrenal insufficiency (Addison's disease)
• Hypothyroidism
• SIADH
• Lung problems (Pneumonia, TB, lung abscess or cancer)
• Cerebral salt wasting syndrome seen with meningitis, head trauma, brain tumor, intracranial hemorrhage or brain surgery
• Losses in diarrhea/vomiting
• Chronic renal insufficiency
• Congestive heart failure
• Cirrhosis/Ascites

Drugs which may cause hyponatremia include:

• ACE inhibitors
• Aminoglycosides (Gentamicin/Tobramycin)
• Amphotericin B
• Angiotensin II Receptor blockers
• Azoles (Ketoconazole, Itraconazole)
• Carbamazepine
• Cathartics/Laxative abuse
• Diuretics
• Haloperidol
• Heparin
• Lithium
• Morphine
• NSAIDS
• SSRI's
• Sulfonylureas
• Tricyclic antidepressants
• Vasopressin

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