Alternative names
Sodium - serum; Serum sodium

This tests measures the amount of sodium in the blood.

How the test is performed

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.

How to prepare for the test
The health care provider may advise you to withhold drugs that may affect the test. (See special considerations.)

In infants and children, the preparation you can provide for this test depends on your child’s age, previous experience, and level of trust. For general information regarding how you can prepare your child, see the following:

How the test will feel
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.

Why the test is performed

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. 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.

Normal Values

The normal range is 135 to 145 mEq/L.

Note: mEq/L = milliequivalents per liter

What abnormal results mean

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

Additional conditions under which the test may be performed:

What the risks are

  • 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

Special considerations

The following factors can interfere with the test:

  • 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

Drugs that can increase sodium measurements include anabolic steroids, antibiotics, clonidine, corticosteroids, cough medications, laxatives, methyldopa, nonsteroidal anti-inflammatory analgesics, and oral contraceptives.

Drugs that can decrease 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.

Johns Hopkins patient information

Last revised: December 8, 2012
by Armen E. Martirosyan, M.D.

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.