C-reactive protein (CRP) is a protein made by the liver. CRP levels in the blood increase when there is a condition causing inflammation somewhere in the body. A CRP test measures the amount of CRP in the blood to detect inflammation due to acute conditions or to monitor the severity of disease in chronic conditions.

CRP is a non-specific indicator of inflammation and one of the most sensitive acute phase reactants. That means that it is released into the blood within a few hours after an injury, the start of an infection, or other cause of inflammation. Markedly increased levels can occur, for example, after trauma or a heart attack, with active or untreated autoimmune disorders, and with serious bacterial infections, such as in sepsis. The level of CRP can jump as much as a thousand-fold in response to bacterial infection, and its rise in the blood can precede pain, fever, or other signs and symptoms.