This test 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 the veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an airtight 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.

Because 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 one person may be more difficult than from another.

In infants or young children, the area is cleansed with antiseptic and punctured with a sharp needle or lancet. The blood may be collected in a small glass tube (pipette), on a slide, onto a test strip, or into a small container. If bleeding does not quickly stop, a cotton pad or bandage may be applied to the puncture site.

Your health care provider may advise you to withhold any medications you are taking that can affect the test (see Special Considerations.) DO NOT, however, stop or change your medications without your doctor's knowledge.

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.