Ionized calcium

Ionized calcium (iCa²⁺), also called free calcium, is the biologically active fraction of total serum calcium, comprising approximately 45–50% of the total and not bound to albumin or complexed with anions such as phosphate and citrate. It is maintained within a narrow range (1.15–1.35 mmol/L) by a tightly coupled hormonal axis involving PTH, calcitriol, and calcitonin acting on bone, kidney, and intestine. Ionized calcium measurement is preferred over total calcium in settings where albumin is abnormal — as in critical illness, liver disease, or hypoalbuminaemia — because total calcium corrected by simple albumin formulas can be unreliable. Hypercalcaemia raises suspicion for primary hyperparathyroidism or malignancy, while hypocalcaemia presents with neuromuscular irritability and, if severe, arrhythmia.