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.

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

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

Sodium (Na+) is the major positive ion in the fluids outside of cells. The concentration of sodium inside cells is only about 5 mEq/L compared with 140 mEq/L outside. The sodium content of the blood is a result of a balance between the amount in the food and beverages 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 steroid hormone aldosterone, which decreases loss of sodium 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 integral relationship between sodium and water, the body regulates them independant of each other if necessary.

Greater-than-normal sodium levels may indicate many different conditions. It is helpful to your physician to also consider the amount of fluid in your body that is outside cells (extracellular volume):

• If the extracellular volume is low, you may have lost water due to burns, excessive sweating, diarrhea, or osmotic/loop diuretics (usually not thiazides).
• If the extracellular volume is normal, this may indicate diabetes insipidus
• If the extracellular volume is high, this may indicate hyperaldosteronism, Cushing's Syndrome, or salt or sodium bicarbonate ingestion

Lower-than-normal sodium levels (hyponatremia) may be similarly classified according to total body water:

• A decrease in total body water (hypovolemic hyponatremia) indicates dehydration, over diuresis, ketonuria (excretion of ketone bodies in urine), vomiting, or diarrhea
• Near-normal total body water (normovolemic hyponatremia) indicates syndrome of inappropriate antidiuretic hormone secretion (SIADH), hypothyroidism, or Addison's Disease
• An increase in total body water (hypervolemic hyponatremia) indicates kidney failure, congestive heart failure, nephrotic syndrome, or cirrhosis of the liver

• acute adrenal crisis
• diabetic hyperglycemic hyperosmolar coma
• drug-induced hypothyroidism
• hepatorenal syndrome
• hypopituitarism
• hypothyroidism
• hypothyroidism - primary
• hypothyroidism - secondary

• 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

• recent trauma, surgery, or shock
• ingestion of large or small amounts of salt or fluid
• intravenous fluids containing sodium
• use of diuretics or certain other medications