Procalcitonin, or PCT, is a precursor protein of the hormone calcitonin. Under normal conditions, the level of PCT in the blood is very low.
 
 During systemic bacterial infection, PCT levels can increase significantly. This is why PCT testing is used to help assess the possibility of systemic bacterial infection, especially sepsis.
 
The test may be considered in suspected sepsis, severe infections such as severe pneumonia and bacteremia, and to help distinguish possible bacterial infection from viral infection or non-infectious inflammation. PCT may also be used to monitor response to antibiotic treatment and in ICU patients with suspected or diagnosed sepsis.
 
One commonly used interpretation notes that PCT levels above 10 ng/mL are often found in severe sepsis or septic shock. However, this level is not a diagnostic criterion for sepsis.
 
PCT can also rise after severe trauma, burns, major surgery, and certain inflammatory conditions. A low PCT level does not completely exclude bacterial infection, especially in the early stage or in localized infection.
 
For this reason, PCT results should always be interpreted together with the patient's condition. Serial PCT measurements are often more useful than a single result.