Serum Sodium

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Why is the Test 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. (For more information, see the aldosterone test.) 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.