What is CRP?

C-reactive protein (CRP) is a substance found in human blood. The body produces this protein in response to infection, making it a substance which can be targeted in blood tests which look for signs of systemic infection. A CRP test is a quick and relatively painless procedure which can be performed on an outpatient basis at a hospital or medical clinic, and it can provide rapid information about a patient's condition.

Under normal conditions, low levels of CRP are present in the blood. When an infection occurs, the liver and fat cells start to produce CRP, at levels which can vary, depending on the nature of the infection. Specific diseases can sometimes attach particular sugars to this protein, leaving tell-tale fingerprints which have potential diagnostic uses. Once the infection is resolved, the protein breaks down, returning to negligible or low levels.

If a doctor suspects that a patient may have an infection, a blood test may be ordered to check for levels of CRP and other substances in the blood. The test can also be used to monitor the progress of a chronic condition such as cancer or arthritis, and to see how the body is responding to a particular medication. If protein levels drop after a medication change, it suggests that the medication may be working, causing the infection to die down. Normal ranges for CRP vary, depending on the patient and his or her medical history.

In addition to being useful in the evaluation of particular medical conditions, levels of this protein can also be used as a yardstick for general health. Using what is known as an highly sensitive CRP (hs-CRP) test, a laboratory can detect the very low levels of CRP present in the blood of people without active infections. Higher levels of ambient CRP appear to be linked with an increased risk of cardiovascular disease, hypertension, and diabetes.

In addition to infection, several other things appear to be able to impact the production of this protein. A high amount of dietary fat can cause an increase, especially if the fat comes from transfats. Pregnancy also appears to elevate CRP levels, as does the use of hormonal birth control products. Liver disease can also alter the level of these proteins in the blood, since the liver is involved in the production of C-reactive protein. If a hs-CRP test comes back with a somewhat high level, a doctor may ask a few questions to rule out these potential causes.