The biochemical and physiologic bases for clinical use of ionized calcium, magnesium, or lactate measurements are described, with emphasis on the critical care setting. Ionized calcium or magnesium are usually measured to prevent hypocalcemia or hypomagnesemia, conditions that most affect cardiovascular function. Blood lactate measurements have been used more frequently in the last 5 years, despite decreases in overall laboratory test volume. Lactate measurements appear to have clinical value, particularly for children having major cardiac surgery or extracorporeal membrane oxygenation. The changes in test volumes in the past few years due to the changing economic climate in medical practice are also described.