Calcium has numerous functions within the body, not only as a structural factor in bones and teeth, but also in normal neuromuscular function and the clotting of blood. The calcium content of an adult is somewhat over 1.0 kg i.e. about 2% of the body weight. Of this, 99% is present as calcium hydroxyapatite in bones and less than 1% is present in the extra-osseous ICS (intracellular space) or ECS (extracellular space). The calcium level in the ECS is in dynamic equilibrium with the rapidly exchangeable fraction of bone calcium. Calcium ions affect the contractility of the heart and the skeletal musculature and are essential for the function of the nervous system.
In addition, calcium ions play an important role in blood clotting and bone mineralization. In plasma, calcium is bound to considerable extent to proteins (40%), 10% is in the form of inorganic complexes and 50% is present as free (ionized) calcium. The body’s calcium balance is regulated by parathyroid hormone (PTH), calcitriol (CT) and calcitonin. The test is used for the diagnosis and monitoring of hypocalcemia (calcium deficiency) and hypercalcemia (excess calcium) in serum. The characteristic symptoms of hypocalcemia are latent or manifest tetany and osteomalacia.