C-reactive protein (CRP) is a protein that is found in blood and indicates an inflammatory response when the body is fighting off an infection or when there has been some trauma or tissue damage, so it is nonspecific and appears to increase in a range of disorders associated with inflammation.
Background
The C-reactive protein is known as an early defense system against infections of the innate immunity present in blood. Thus, it is used commonly as a marker of inflammation. C-reactive protein (CRP) is produced by the liver. It may also be synthesized by cells in the vascular wall, internal lining of the blood vessels (endothelium), and muscle cells. Adipose tissues (fat) are also recognized as a source of CRP. CRP levels in blood increase when there is damage to the tissues in the body, for example, due to trauma or bacterial infection that leads to a process known as inflammation. This is why it belongs to a group of proteins called “acute phase reactants”.
When the cells are damaged, this protein binds to the surface of these cells and also some bacteria, thus activating one of the important parts of the immunity called the complement system, promoting entrapment of bacteria by immune cells and therefore clearing of dead and injured cells and bacteria. The acute phase response causes the release of cytokines that in turn trigger the synthesis of CRP and fibrinogen by the liver.
Why this test is used?
CRP rises within six to twelve hours of the onset of the underlying condition and reaches its peak at 48 hours. The half-life of the protein is 18 hours. Its level is determined by the rate of production and the severity of the underlying cause. This means that CRP is a marker for inflammation that can be used to screen for inflammation and provide an assessment of the prescribed anti-inflammatory therapy. This fact is probably connected with the chronic vessel lining inflammation associated with atherosclerosis.
When is the test indicated?
The test is performed when:
- The doctor suspects inflammation (vasculitis, autoimmune disorders, bacterial infection, etc.) and, for example, such symptoms as fever, chills, rapid, shallow breathing, and fast heart rate are present;
- CRP may sometimes be ordered along with erythrocyte sedimentation rate (ESR). CRP is a more sensitive indicator of the acute phase response than ESR. CRP returns to normal sooner than ESR in response to therapy. It is supposed that ESR and CRP levels in serum usually correlate, but in case of viral infection or aseptic lesion SRP rises while ESR remains normal and in case of severe intoxication and chronic arthritis CRP remains normal while ESR grows;
- CRP may also be ordered to monitor chronic inflammatory conditions (rheumatoid arthritis and lupus) and to estimate the treatment effectiveness;
Liver failure and interferon alpha interfere with CRP production. This results (due to interferon production) in the relatively low levels of CRP found during viral infections compared to bacterial infections.
Interpretation of test results
| Level of CRP | Condition |
| 0-10mg/L | Normal |
| 10–40 mg/L | Late pregnant women (3rd trimester), mild inflammation and viral infections |
| 40–200 mg/L | Active inflammation, myocardial infarction, bacterial infection |
| >200 mg/L | Severe bacterial infections |
CRP and cardiovascular diseases
High-sensitivity CRP (hs-CRP) is more sensitive compared to the standard test and it is able to detect even minor changes, therefore, the reference value for this test is lower of < 3 mg/L. hs-CRP is performed for patients with some known risk factors of coronary heart disease and patients with acute coronary syndromes.
The risk of developing cardiovascular disease correlates with the level of CRP:
- hs-CRP level under 1.0 mg/L corresponds to a low risk of cardiovascular disease;
- when CRP values are between 1.0 and 3.0 mg/L risk of heart disease is considered average;
- levels of hs-CRP above 3.0 mg/L are suggestive of a high risk of developing cardiovascular disease;