Ionized calcium (Ca²⁺) (Ca²⁺ or iCa²⁺)

Ionized calcium (Ca²⁺) is the physiologically active form of calcium present in blood plasma at a concentration of approximately 1.25 mmol/L, representing about 40-50% of total circulating calcium.[1][2][3] Unlike protein-bound calcium (approximately 50%) and anion-complexed calcium (approximately 10%), ionized calcium is readily available for cellular use and is the only form that mediates physiological processes.[3] It functions as an essential cofactor in blood coagulation, serves as a second messenger in intracellular signaling pathways, and is critical for cardiac and skeletal muscle contraction, neuromuscular transmission, and nerve impulse conduction.[1][2] Ionized calcium levels are tightly regulated within the range of 1.10-1.35 mmol/L through integrated hormonal control involving parathyroid hormone (PTH) and calcitriol (active vitamin D metabolite), which act on the kidneys, gastrointestinal tract, and bone.[1][2] The body prioritizes maintaining plasma ionized calcium concentration over bone mineralization, sacrificing bone density if necessary to preserve calcium homeostasis.[2] Measurement of ionized calcium is more clinically relevant than total calcium measurement, particularly in patients with abnormal serum protein concentrations or acid-base disturbances, as pH significantly affects the proportion of calcium that remains in the ionized state.[1][3][4]

Other names

Free calciumCa²⁺iCa²⁺Unbound calciumNon-protein-bound calcium

Not applicable