Serum chloride is the major extracellular anion and counter-balances the major cation, sodium, maintaining electrical neutrality of the body fluids. Two thirds of the total anion concentration in extracellular fluids is chloride and it is significantly involved in maintaining proper hydration and osmotic pressure. Movement of chloride ions across the red blood cell membrane is essential for the transport of biocarbonate ions in response to changing concentrations of carbon dioxide. Chloride measurements are used in the diagnosis and treatment of electrolyte and metabolic disorders such as cystic fibrosis and diabetic acidosis.