This test measures the level of sodium in the blood. Sodium is an electrolyte that is vital to normal body processes, including nerve and muscle function. Sodium, along with other electrolytes, helps cells function normally and helps regulate the amount of fluid in the body. Sodium is present in all body fluids but is found in the highest concentration in the blood and in the fluid outside of the body’s cells. This extracellular sodium, as well as all body water, is regulated by the kidney. Sample: Blood (serum); urine Reference: For urine: between 40 mmol/24hrs and 200 mmol/24hrs Frequency: Daily