Procalcitonin (PCT) is a 116 amino acid peptide precursor of calcitonin. In the absence of infection, it is produced at very low levels in a few highly selected tissues. However, at the onset of infection bacterial factors and host inflammatory cytokines induce the expression of PCT in many different tissues.  This results in a rapid rise in PCT levels from virtually undetectable (< 0.05ng/L) to detectable levels within 2-4 hours. Peak levels occur at 24-48 hours and the degree of the rise correlates with the severity of the systemic inflammatory response.

CRP or C-reactive protein is one of the acute phase proteins1, produced in the liver in response to release of cytokines from inflammatory cells. CRP acts as a pattern recognition molecule assisting the innate immune system in various pathways.  CRP is useful in that an increase generally reflects the presence and intensity of an inflammatory process. However, it is not specific for infection and can’t distinguish non-infectious causes from infectious causes, nor bacterial from non-bacterial causes.

Minimum repeat intervals for procalcitonin (PCT) and C-reactive protein (CRP)