This test measures the concentration of sodium in blood or urine. Sodium is an electrolyte present in all body fluids, and is vital to normal body function including nerve and muscle function. Your body tries to keep your blood sodium within a very small concentration range; it does so by:
- producing hormones that can increase (such as natriuretic peptides) or decrease (such as aldosterone) sodium losses in urine
- producing a hormone that prevents water losses (antidiuretic hormone [ADH], sometimes called vasopressin)
- controlling thirst (even a 1% increase in blood sodium concentration will make you thirsty and cause you to drink water, returning your sodium towards normal.)
These controls regulate the amount of water and sodium in the body and control blood pressure by keeping the amount of water available. An abnormal blood sodium concentration is usually due to some problem with one of these systems. When the concentration of sodium in the blood changes, the water content in your body changes. These changes can be associated with dehydration (too little fluid) or oedema (too much fluid, often resulting in swelling in the legs) and changes in blood pressure. In some people, too much sodium from salt in the diet can contribute to high blood pressure.
How is it used?
Blood sodium measurement is used to detect the cause and help monitor treatment in persons with dehydration, oedema, or with a variety of symptoms. Blood sodium concentration is abnormal in many diseases; your doctor may request this test if you have symptoms of illness involving the brain, lungs, liver, heart, kidney, thyroid, or adrenal glands.
Urine sodium concentrations are typically tested in patients who have abnormal blood sodium concentrations, to help determine whether an imbalance is due to taking in too much sodium or losing too much sodium. Urine sodium is also used to see if a person with high blood pressure is eating too much salt. It is often used in persons with abnormal kidney tests to help the doctor determine the cause of kidney disease, which can help guide treatment.
When is it requested?
This test is a part of the routine laboratory evaluation of most patients. It is one of the blood electrolytes, which are often requested as a group group (sodium, potassium, calcium, bicarbonate), when someone has non-specific health complaints. It is also tested when monitoring treatment involving intravenous (IV) fluids or when there is a possibility of developing dehydration. Electrolytes are also commonly used to monitor treatment of certain problems, including high blood pressure, heart failure, and liver and kidney disease.The osmolality of the blood or urine may be checked as well.
What does the test result mean?
A low concentration of blood sodium is called hyponatraemia, and is usually due to too much sodium loss, too much water intake or retention, or fluid accumulation in the body (oedema). If sodium falls quickly, you may feel weak and tired; in severe cases, you may experience confusion or even fall into a coma. When sodium falls slowly, however, there may be no symptoms. That is why sodium concentrations are often checked even if you don't have any symptoms.
Hyponatraemia is rarely due to decreased sodium intake (deficient dietary intake or deficient sodium in IV fluids). Most commonly, it is due to sodium loss (diarrhoea, vomiting, excessive sweating, diuretic administration, kidney disease or Addison's disease). In some cases, it is due to excess fluids in the body (drinking too much water, heart failure, cirrhosis, kidney diseases that cause protein loss (nephrotic syndrome) and malnutrition. In a number of diseases (particularly those involving the brain and the lungs, many kinds of cancer, and with some drugs), your body makes too much anti-diuretic hormone, causing you to keep too much water in your body.
A high blood sodium concentration is referred to as hypernatraemia, and is almost always due to excessive loss of water (dehydration) without enough water intake. Symptoms include dry mucous membranes (mouth, eyes etc.), thirst, agitation, restlessness, acting irrationally, and coma or convulsions if concentrations rise extremely high. In rare cases, hypernatraemia may be due to increased salt intake without enough water, Cushing's syndrome, or too little anti-diuretic hormone (called diabetes insipidus).
Is there anything else I should know?
Recent trauma, surgery, or shock may increase sodium concentration because blood flow to the kidneys is decreased.
Drugs such as lithium and anabolic steroids may increase sodium concentration. Corticosteroids, laxatives, cough medicines and oral contraceptives may cause increased concentrations of sodium in the blood.
Drugs such as diuretics, sulphonylureas (used to treat diabetes), ACE inhibitors (such as captopril), heparin, ibuprofen (NSAIDs), carbamazepine, tricyclic antidepressants, and vasopressin, among others, can decrease the sodium concentration within the blood.
Check with your doctor if you have any concerns about drugs or supplements you are taking and their effect on your body.
How much salt should I eat to maintain normal sodium?
Most sodium comes from table salt. In the UK, we take in an average of 3-4 grams (3000-4000 mg) of sodium per day. However, you need far less than this to meet the needs of the body. According to the Food Standards Agency, total daily sodium intake should not exceed 2500 mg.
Is anyone at particular risk for low or high sodium?
Should athletes pay particular attention to re-hydrating after playing sports to keep their sodium concentrations up?
Do men and women have the same sodium requirements?