Introduction: Medical practice rests on the foundation of science. Clinicians are constantly making practical decisions and dealing with situations that demand quick and efficient solutions. The extra ordinary complex functions of the human being depend on the narrow range of the volume and composition of body fluids and electrolytes. These electrolytes run the life with help of meticulously regulated control mechanisms that strictly maintain their levels both extracellularly and intracellularly.1 Materials and methods: This prospective study was conducted over a period of one year and included critically ill children aged 1 month to 12 years admitted to a paediatric tertiary care hospital. Informed consent was obtained from the parents of all the children enrolled in the study. At admission, detailed history was taken and systemic examination was recorded. Venous blood samples were obtained and serum electrolytes were estimated by using Combi line Ion-Selective Electrode (ISE). An Ion-Selective Electrode (ISE), also known as a specific ion electrode (SIE), is a transducer (or sensor) that converts the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter.