Sodium (Na) in Blood

A sodium test checks how much sodium (an electrolyte and a mineral) is in the blood. Sodium is both an electrolyte and mineral. It helps keep the water (the amount of fluid inside and outside the body's cells) and electrolyte balance of the body. Sodium is also important in how nerves and muscles work.

Most of the sodium in the body (about 85%) is found in blood and lymph fluid. Sodium levels in the body are partly controlled by a hormone called aldosterone, which is made by the adrenal glands. Aldosterone levels tell the kidneys when to hold sodium in the body instead of passing it in the urine. (See an illustration of the adrenal glands or the kidneys.) Small amounts of sodium are also lost through the skin when you sweat.

Most foods have sodium naturally in them or as an ingredient in cooking. Sodium is found in table salt as sodium chloride or in baking soda as sodium bicarbonate. Many medicines and other products also have sodium in them, including laxatives, aspirin, mouthwash, and toothpaste.

Too much sodium in the diet may raise blood pressure in some people. For those who have high blood pressure, eating foods with a lot of sodium makes their chance of heart disease, stroke, and kidney damage higher. Heart failure gets worse when too much sodium is eaten. It increases the amount of water the body holds in and this causes swelling of the legs and hands. Some people have problems when they eat more than 4,000 milligrams (mg) of sodium per day.

Low sodium levels are uncommon and most often occur as a side effect of taking medicines that make you urinate more, such as diuretics. Severe diarrhea or vomiting or heavy sweating may also cause low sodium levels.

Other electrolytes, such as potassium, calcium, chloride, magnesium, and phosphate, may be checked in a blood sample at the same time as a blood test for sodium.

A blood test to check sodium levels is done to:

• Check the water and electrolyte balance of the body.
• Find the cause of symptoms from low or high levels of sodium.
• Check the progress of diseases of the kidneys or adrenal glands.

You do not need to do anything before having this test.

Talk to your health professional about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

Blood test

The health professional drawing blood will

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then a bandage.

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

There is very little chance of a problem from having blood sample taken from a vein.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

A sodium test checks how much sodium (an electrolyte and a mineral) is in the blood. Normal values may vary from lab to lab. Results are ready in 1 day.

Many conditions can affect sodium levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and medical history.

High values

• High sodium levels (hypernatremia) can be caused by a high-sodium diet or by not drinking enough water and being dehydrated. Dehydration may also be caused by severe vomiting or diarrhea, Cushing's syndrome, kidney disease or injury, diabetic ketoacidosis, or a condition called diabetes insipidus that makes it hard to balance the water level in the body.
• High sodium levels can also be caused by high levels of the hormone aldosterone (hyperaldosteronism).

Low values

• Low sodium levels (hyponatremia) can be caused by a lot of sweating, burns, severe vomiting or diarrhea, drinking too much water (psychogenic polydipsia), or poor nutrition.
• Low sodium levels can also be caused by underactive adrenal glands or thyroid gland, heart failure, kidney disease, cirrhosis, cystic fibrosis, or SIADH (syndrome of inappropriate antidiuretic hormone secretion).

Reasons you may not be able to have the test or why the results may not be helpful include:

• Taking medicines, such as birth control pills, corticosteroids, antibiotics, estrogens, tricyclic antidepressants, heparin, nonsteroidal anti-inflammatory drugs (NSAIDs), diuretics, lithium, and many medicines used to treat high blood pressure.
• Having high levels of glucose, triglycerides, or protein.
• Getting sodium in intravenous (IV) fluids given during a recent surgery or hospitalization.

• When the sodium level changes quickly, you are likely to have more symptoms than when the level changes slowly. Symptoms of an abnormal sodium level include confusion, lack of energy (lethargy), or seizures.
• To see whether the body is passing too little or too much sodium in the urine, a value called the fractional excretion of sodium (FENa) can be found by looking at the amounts of sodium and creatinine in blood and urine. In a person with kidney failure, a low FENa may mean less blood flow to the kidneys is causing the kidney failure. A urine test for sodium may be done. For more information, see the medical test Sodium (Na) in Urine.
• Other electrolytes, such as calcium, chloride, magnesium, potassium, phosphate, blood urea nitrogen (BUN), and creatinine, may be checked in a blood sample at the same time as a blood test for sodium. For more information, see the medical tests Calcium (Ca) in Blood, Chloride (Cl), Magnesium (Mg), Potassium (K) in Blood, Phosphate, Blood Urea Nitrogen, and Creatinine and Creatinine Clearance.

Other Works Consulted

• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.