This test measures the concentration of sodium in the blood.

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

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.