Special Considerations

This test measures the concentration of sodium in the blood.

Serum sodium

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

Your health care provider will instruct you, if necessary, to discontinue drugs that may interfere with the test. Do not stop or change your medications without your doctor's knowledge.

Drugs that can increase blood sodium levels include:

• Anabolic steroids
• Birth control pills
• Certain antibiotics
• Clonidine
• Corticosteroids
• Laxatives
• Lithium
• Nonsteroidal anti-inflammatory drugs (NSAIDs)

Drugs that can reduce blood sodium levels include:

• Carbamazepine
• Diuretics
• Morphine
• Sulfonylureas
• Triamterene
• Vasopressin

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Your doctor may order this test if you have signs of a sodium imbalance or disorders associated with abnormal sodium levels.

Your blood sodium level represents a balance between the sodium and water in the food and drinks you consume and the amount in urine. A small percentage is lost through stool and sweat.

Many factors affect sodium levels, including:

• Recent trauma, surgery, or shock
• Consuming large or small amounts of salt or fluid
• Receiving intravenous (IV) fluids containing sodium
• Taking diuretics or certain other medications, including the hormone aldosterone

The normal range for blood sodium levels is 135 to 145 milliequivalents per liter (mEq/L).

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

Abnormal sodium levels can be due to many different conditions. To help determine the cause, your health care provider will consider the total amount of fluid in your body. This is done by looking at the turgor of your skin and swelling in the ankles, feet, and legs.

For greater than normal sodium levels (hypernatremia):

• If the amount of fluid in your body is low, you may have fluid loss due to excessive sweating, diarrhea, use of diuretics, or burns.
• If your total body water is normal, high sodium levels may be due diabetes insipidus (too little of the hormone vasopressin).
• If your total body water is high, this may indicate hyperaldosteronism, Cushing syndrome, or a diet that's too high in salt or sodium bicarbonate.

For lower than normal sodium levels (hyponatremia):

• Low total body water and sodium levels may be due to dehydration, vomiting, diarrhea, over diuresis, or ketonuria.
• Near-normal total body water may indicate SIADH, too much of the hormone vasopressin, hypothyroidism, or Addison's disease.
• An increase in total body water may indicate congestive heart failure, nephrotic syndrome or other kidney disease, or cirrhosis of the liver.

Additional conditions for which the test may be performed include:

• Acute adrenal crisis
• Diabetic hyperglycemic hyperosmolar coma
• Drug-induced hypothyroidism
• Hepatorenal syndrome
• Hypopituitarism

Possible risks from any blood test include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Shorecki K, Ausiello D. Disorders of sodium and water homeostasis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, PA: Saunders Elsevier; 2007:chap 117.