Hypocalcemia refers to an abnormal decrease in serum ionized calcium concentration. However, since only total calcium is generally measured clinically, a total calcium level lower than normal is also called hypocalcemia. A decrease in total blood calcium may occur in hypoproteinemia and does not necessarily reflect a decrease in ionized calcium. Therefore, a decrease in blood calcium is not necessarily consistent with a decrease in ionized calcium, but in general the two are consistent. When the serum protein concentration is normal, blood calcium is lower than 2.2mmol/L, which is called hypocalcemia. Hypocalcemia generally refers to free calcium lower than normal. In acidosis or hypoproteinemia, only protein-bound calcium is reduced; conversely, in alkalosis or hyperproteinemia, although free calcium is reduced, protein-bound calcium is increased, so serum calcium can still be normal.