A sodium blood test is used to detect abnormal concentrations of sodium, including low sodium (hyponatremia) and high sodium (hypernatremia). It is often used as part of an electrolyte panel or basic metabolic panel for a routine health checkup.

Sodium is an electrolyte present in all body fluids and is vital to normal body function, including nerve and muscle function. It helps cells function normally and helps regulate the amount of fluid in the body.

A blood sodium test may also be used to detect the cause and help monitor treatment in people with dehydration, excess fluid (edema), or with a variety of symptoms (e.g., weakness, confusion, thirst and/or dry mucous membranes). Blood sodium can be abnormal in many diseases. A health practitioner may order this test, along with other electrolytes, to identify an electrolyte imbalance or if there are symptoms of illness involving the brain, lungs, liver, heart, kidney, thyroid, or adrenal glands.

In people with a known electrolyte imbalance, a blood sodium test may be used to monitor the effectiveness of treatment or to monitor people taking medications that can affect sodium levels, such as diuretics.

Urine sodium levels may be tested in people who have abnormal blood sodium levels to help determine whether an imbalance is due to, for example, consuming too much sodium or losing too much sodium. Urine sodium testing is also used for people with abnormal kidney tests to help the healthcare practitioner determine the cause of kidney disease and to help guide treatment.

Sodium testing is a part of the routine lab evaluation of most people as part of an electrolyte panel or a basic metabolic panel. These may be ordered during an annual physical or when someone has non-specific health complaints.

A blood sodium test may be ordered when a person has symptoms of low sodium, such as weakness, confusion, and lethargy. If the sodium level falls quickly, the person may feel weak and fatigued; in severe cases, the person may experience confusion or even fall into a coma. When the sodium level falls slowly, however, there may be no symptoms. That is why sodium levels are often checked even if someone has no symptoms.

Sodium blood testing may be ordered when a person has symptoms of high sodium, such as thirst, dry mucous membranes (e.g., mouth, eyes), less frequent urination, muscle twitching, and/or agitation. If the sodium level rises to extremely high concentrations, symptoms can include restlessness, acting irrationally, and coma or convulsions.

Electrolytes may be measured when monitoring treatment involving intravenous (IV) fluids or when there is a possibility of developing dehydration. Electrolyte panels and basic metabolic panels are also commonly ordered on a regular basis when monitoring treatment of certain conditions, including high blood pressure, heart failure, and liver and kidney disease.

A urine sodium test may be ordered when a blood sodium test result is abnormal to help determine the cause of the imbalance or to monitor treatment.

A low level of blood sodium (hyponatremia) may be due to:

• Losing too much sodium, most commonly from conditions such as diarrhea, vomiting, excessive sweating, use of diuretics, kidney disease or low levels of cortisol, aldosterone and sex hormones (Addison disease)
• Drinking too much water as might occur during exercise
• Excess fluid accumulation in the body (edema) caused by heart failure, cirrhosis, and kidney diseases that cause protein loss (nephrotic syndrome) or malnutrition. In a number of diseases, particularly those involving the brain and the lungs, many kinds of cancer, and with some drugs, the body makes too much anti-diuretic hormone (ADH), causing a person to keep too much water in the body.

Low blood sodium is rarely due to decreased sodium intake (deficient dietary intake or deficient sodium in IV fluids).

A high blood sodium level (hypernatremia) is almost always caused by losing too much water (dehydration) without drinking enough water. In rare cases, it may be due to increased salt intake without enough water, Cushing syndrome, or a condition caused by too little ADH called diabetes insipidus.

Sodium urine concentrations must be evaluated in association with blood levels. The body normally elimiates excess sodium, so the concentration in the urine may be elevated because it is elevated in the blood. It may also be elevated in the urine when the body is losing too much sodium; in this case, the blood level would be normal to low. If blood sodium levels are low due to insufficient intake, then urine concentrations will also be low.

• Decreased urinary sodium levels may indicate dehydration, congestive heart failure, liver disease, or nephrotic syndrome.
• Increased urinary sodium levels may indicate diuretic use or Addison disease.

Sodium levels are often evaluated in relation to other electrolytes and can be used to calculate a quantity termed anion gap. The anion gap is useful in identifying the presence of unknown substances such as toxins in the blood.

Certain drugs such as anabolic steroids, antibiotics, corticosteroids, laxatives, cough medicines, and oral contraceptives may cause increased levels of sodium. Other drugs such as ACE inhibitors, diuretics, carbamazepine, heparin, and tricyclic antidepressants may cause decreased levels of sodium.