Blood sodium testing is used to detect abnormal concentrations of sodium, termed hyponatremia (low sodium) and hypernatremia (high sodium). A doctor may order this test, along with other electrolytes, to identify an electrolyte imbalance. It may be ordered to determine if a disease or condition involving the brain, lungs, liver, heart, kidney, thyroid, or adrenal glands is causing or being exacerbated by a sodium deficiency or excess. In patients with a known electrolyte imbalance, a blood sodium test may be ordered at regular intervals to monitor the effectiveness of treatment. It may also be ordered to monitor patients taking medications that can affect sodium levels, such as diuretics.
Urine sodium levels are typically tested in patients who have abnormal blood sodium levels to help determine whether an imbalance is from, for example, taking in too much sodium or losing too much sodium. Urine sodium testing 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 damage, which can help guide treatment.
Sodium testing is a part of the routine lab evaluation of most patients. It is one of the blood electrolytes, which are often ordered as a group. The most common group of blood electrolytes is sodium, potassium, chloride, and bicarbonate (total CO2). These electrolytes are also included in the basic metabolic panel, a larger group of tests widely used when someone has non-specific health complaints. Electrolytes are also measured when monitoring treatment involving IV fluids or when there is a possibility of developing dehydration. Electrolyte panels and basic metabolic panels are also commonly used to monitor treatment of certain conditions, including high blood pressure, heart failure, and liver and kidney disease.
A blood sodium test may be ordered when a person has symptoms of hyponatremia, such as weakness, confusion, and lethargy, or symptoms of hypernatremia, such as thirst, decreased urinary output, muscle twitching, and/or agitation.
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 is usually due to loss of too much sodium, too much water intake or retention, or to excess fluid accumulation in the body (edema). If the sodium level falls quickly, the person may feel weak and fatigued; in severe cases, he 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.
Hyponatremia is rarely due to decreased sodium intake (deficient dietary intake or deficient sodium in IV fluids). Most commonly, it is due to sodium loss from conditions such as Addison's disease, diarrhea, diuretic administration, or kidney disease. In some cases, it may be due to excessive water consumption as might occur during exercise or excessive fluid accumulation as might occur in heart failure, cirrhosis, and kidney diseases that cause protein loss (nephrotic syndrome). In other cases (particularly diseases involving the brain and the lungs, many kinds of cancer, and in response to some drugs), the body makes too much anti-diuretic hormone (ADH), causing a person to keep too much water in their body.
A high blood sodium level is almost always due to inadequate water intake and dehydration. Symptoms include dry mucous membranes, thirst, agitation, restlessness, acting irrationally, and coma or convulsions if the sodium level rises to extremely high concentrations. In rare cases, hypernatremia may be due to 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 excretes 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's 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.
This article was last reviewed on September 2, 2011. | This article was last modified on October 15, 2013.
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