Clinical

Ionized (free) calcium is the fraction of total calcium in the body that is physiologically active. Ionized calcium is in equilibrium with protein bound calcium, and calcium complexed with small diffusible ligands such as citrate, lactate, phosphate, and bicarbonate. The equilibrium is kept in balance by pH, protein concentration, and chelating agents. Calcium plays a vital role in cell physiology, neuromuscular function, and coagulation. McLean and Hastings described an equation for the precise calculation of ionized calcium based on levels of total calcium and total protein. Studies have shown that this method compares well to the direct measurement of ionized calcium in most normal and some diseased states.