The tests for blood electrolytes measure certain essential elements and chemical substances that are required for basic body functions. Typically the test measures the levels of sodium, potassium, chloride, and bicarbonate in the body. In addition, certain other related substances in the blood are often measured along with the electrolytes, such as blood urea nitrogen (BUN), creatinine, and glucose. Sometimes this entire group of tests is referred to as blood chemistries, electrolytes, or just lytes. Taken all together, these substances determine how the liquid parts of the blood behave as a chemical solution.  

Like most blood tests, the skin is cleaned with alcohol first, then a needle is inserted into a vein and blood is drawn off into specific tubes. These are sent to a laboratory and processed by analytic machines.  

Depending on the situation, the results of the electrolyte tests can be known within minutes. For example, in an emergency, the blood sample is processed right away. When done as a routine test, however, the samples are evaluated in batches so results may not be available for 3 to 6 hours or, if the samples are shipped to a central location for processing, the next day.

Sodium is the major mineral element in the blood and body fluids, and it plays a major role in regulating the amount of water throughout the body. Its passage in and out of cells through pores called ion channels is essential for many body functions, including the transmission of electrical signals in the brain and muscles. Low sodium levels in children may be caused by severe diarrhea or vomiting that remove sodium-rich body fluid. High sodium levels may occur with dehydration in which water loss is disporportionately greater than sodium loss. Low sodium levels in the blood can result in brain swelling as water moves into the brain cells; high blood sodium levels can result in the brain shrinking as water moves out of brain cells into the blood. Because of these changes, very high or very low blood sodium levels can make kids confused, weak, and lethargic and may result in seizures. 

Chloride is an element in the blood and body fluid that functions like sodium to maintain fluid balance. Table salt is composed of a combination of sodium and chloride. Disporportionate loss of chloride can lead to the body's environment becoming more acidic.

Potassium is the major electrolyte inside cells. Its passage in and out of cells is essential to regulate heart contractions. High potassium levels can be dangerous and may result in abnormal heart rhythms. Low potassium levels also increase the risk of dangerous abnormal heartbeat and are associated with muscle weakness.

Bicarbonate is a measure of the alkalinity of of the body's tissues. The bicarbonate content of the body is delicately balanced by the kidneys and also by the lungs. It's job is to prevent the body's environment from getting too acidic or too basic (that is, having too little acid or too much alkali).