This tests measures the amount of sodium in the blood.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. 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, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

The health care provider may advise you to withhold drugs that can affect the test. (See special considerations.) DO NOT, however, stop or change your medication without the knowledge and direction of your doctor.

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.

This test is performed when you have symptoms of a sodium imbalance, or when you develop disorders associated with abnormal sodium levels.

Sodium is the major positive ion in the fluids outside of cells. The sodium level of your blood is the balance between the amount of sodium in the food and drinks you consume, and the amount your kidneys excrete. (In addition, a small percent is lost through the stool and sweat.)

Many factors affect sodium levels, including the hormone aldosterone, which reduces sodium loss in the urine. (For more information, see the aldosterone test.) ANP (atrial natriuretic protein) is a hormone secreted from the heart that increases sodium loss from the body.

Despite the tight relationship between sodium and water, the body regulates them independant of each other, if necessary.

The normal range is 135 to 145 mEq/L.

Note: mEq/L = milliequivalents per liter

Greater-than-normal sodium levels may indicate many different conditions. To help determine the cause, your doctor will also consider the amount of fluid in your body, or total body water. The doctor does this during the physical exam by looking at things like the turgor of your skin and abnormal fluid buildup in the ankles, feet, and legs.

• If the amount of fluid in your body (total body water) is low, you may have lost water due to burns, excessive sweating, diarrhea, or use of diuretics (for example, furosemide, bumetanide, or torsemide).
• If your total body water is normal, having a high sodium may indicate diabetes insipidus.
• If your total body water is high, this may indicate hyperaldosteronism (too much of the hormone aldosterone), Cushing's Syndrome (too much of a hormone called cortisol), or salt or sodium bicarbonate ingestion.

Lower-than-normal sodium levels also may be classified according to total body water:

• Low total body water may indicate dehydration, vomiting, diarrhea, over diuresis, or ketonuria (excretion or loss of ketone bodies in urine).
• Near-normal total body water may indicate SIADH, hypothyroidism (an underactive thyroid), or Addison's Disease (low production of cortisol and other hormones).
• An increase in total body water may indicate congestive heart failure, nephrotic syndrome and other causes of kidney disease, or cirrhosis of the liver.

Additional conditions for which the test may be performed include:

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

Possible risks from any blood test include:

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

The following factors can interfere with the test:

• Recent trauma, surgery, or shock
• Consuming large or small amounts of salt or fluid
• Intravenous fluids containing sodium
• Diuretics or certain other medications

Drugs that can increase sodium measurements include anabolic steroids, antibiotics, clonidine, corticosteroids, cough medications, laxatives, methyldopa, nonsteroidal anti-inflammatory drugs (NSAIDs), and birth control pills.

Drugs that can reduce sodium measurements include carbamazepine, diuretics, sulfonylureas, triamterene, and vasopressin.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.