Substances, for example, potassium or sodium chloride or bicarbonate, whose molecules split into their constituent electrically charged particles, known as ions, when dissolved in fluid. In medicine the term is customarily used to describe the ion itself. The description ‘serum electrolyte concentration’ means the amounts of separate ions – for example, sodium and chloride in the case of salt – present in the serum of the circulating blood. Various diseases alter the amounts of electrolytes in the blood, either because more than normal are lost through vomiting or diarrhoea, or because electrolytes may be retained as the kidney fails to excrete them properly. Measurements of electrolytes are valuable clues to the type of disease, and provide a means of monitoring a course of treatment. Electrolyte imbalances can be corrected by administering appropriate substances orally or intravenously, or by DIALYSIS. (See APPENDIX 6: MEASUREMENTS IN MEDICINE.)