C-reactive protein (CRP) is a protein produced by the liver and not normally found in the blood, except for people who smoke, are obese, or have an inflammatory condition somewhere in the body. CRP is measured in samples of blood and reported as ultra-sensitive CRP, US-CRP, or high sensitive CRP, HS-CRP.
C-reactive protein appears in response to inflammation, infection, and illnesses such as heart disease, cancer, high blood pressure, and connective tissue diseases like rheumatoid arthritis and lupus erythematosis.
Individuals vary in the amount of CRP their livers produce. Some people with serious diseases, like rheumatoid arthritis and lupus, may not have elevated blood levels of CRP.
CRP is a sensitive but nonspecific test that signifies inflammation somewhere in the body; it does not help determine where or how serious it is. CRP levels begin rising within a few hours following an injury, surgery, or heart attack. Very high CRP levels are found after surgery, acute heart attack, or serious injury. There is a current theory that there is an inflammatory component factor in developing atherosclerotic plaques in the coronary and other major arteries, leading to hardening of the arteries. But, for people at low risk of heart disease, without cardiac symptoms, CRP levels are not useful as screening tests for heart-disease risk.
By taking multiple tests over time, the level of activity of some chronic conditions, like arthritis and other connective tissue diseases, can be followed.
CRP levels are useful in predicting the risk of complications in people with heart or vascular disease who have had a stroke, heart attack, or coronary treatment procedure like angioplasty. But therapy decisions are made on the basis of all clinical findings, not only on CRP levels.
Normal CRP test results are zero to one milligram (mg) per liter of blood.