Sodium is an essential mineral and electrolyte, a positively charged molecule that performs key roles along with other electrolytes such as calcium, potassium, bicarbonate and chloride. The body maintains sodium blood levels in a narrow range; urine is the primary way in which the body balances the amount of sodium and water ingested with the amount eliminated. Urine sodium levels are tested in patients who have abnormal blood sodium levels in order to determine whether the imbalance is from inadequate water intake or excessive loss. The sodium test is done to determine electrolyte balance in the body as well as kidney function. A test for sodium, along with other electrolytes, is routinely included in the basic metabolic panel (BMP).