Human procalcitonin (PCT) is the prohormone form of calcitonin which was first identified in 1981 [1].It is a glycoprotein composed of 116 amino acids without hormonal activity and is mainly produced by parafollicular cells (C cells) in the thyroid gland.PCT is enzymatically cleaved into lower molecular weight peptides,including the N-terminal with 57 amino acids,C-terminal with 21 amino acids,and calcitonin with 32 amino acids.The concentration of PCT is very low in normal human plasma,but is increased in the plasma of patients with sepsis and infection [2].Furthermore,PCT is mainly induced during severe systemic inflammation caused by bacterial infections,but not by non-bacterial infection.