Procalcitonin (PCT) is the precursor of the hormone calcitonin, which in normal metabolic conditions is mainly produced by the C‐cells of the thyroid medulla and to a lesser extent by other neuroendocrine cells. The blood of healthy individuals contains only very low levels of PCT. It elevates in plasma when severe bacterial, fungal, parasitic infections, sepsis and multiple organ failure occur. CRP is a typical acute phase protein. It is synthesized by the liver and the epithelial cell in response to infections or tissue damages, and the synthesis is triggered by interleukin-6 (IL-6) and other cytokines produced by macrophages and other white blood cells which are activated in these conditions.