C-Reactive Protein (CRP)
A blood sample taken from a vein in your arm
C-reactive protein (CRP) is an acute phase reactant, a protein made by the liver and released into the blood within a few hours after tissue injury, the start of an infection, or other cause of inflammation. Markedly increased levels are observed, for example, after trauma or a heart attack, with active or uncontrolled autoimmune disorders, and with serious bacterial infections like sepsis. The level of CRP can jump as much as a thousand-fold in response to inflammatory conditions, and its rise in the blood can precede pain, fever, or other clinical indicators. The test measures the amount of CRP in the blood and can be valuable in detecting inflammation due to acute conditions or in monitoring disease activity in chronic conditions.
The CRP test is not diagnostic, but it provides information to a health practitioner as to whether inflammation is present. This information can be used in conjunction with other factors such as signs and symptoms, physical exam, and other tests to determine if someone has an acute inflammatory condition or is experiencing a flare-up of a chronic inflammatory disease. The health practitioner may then follow up with further testing and treatment.
This standard CRP test is not to be confused with an hs-CRP test. These are two different tests that measure CRP and each test measures a different range of CRP level in the blood for different purposes:
- The standard CRP test measures markedly high levels of the protein to detect diseases that cause significant inflammation. It measures CRP in the range from 10 to 1000 mg/L.
- The hs-CRP test accurately detects lower levels of the protein than the standard CRP test and is used to evaluate individuals for risk of cardiovascular disease. It measures CRP in the range from 0.5 to 10 mg/L. (See the article on hs-CRP.)
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
The C-reactive protein (CRP) test is used by a health practitioner to detect inflammation. CRP is an acute phase reactant, a protein made by the liver and released into the blood within a few hours after tissue injury, the start of an infection, or other cause of inflammation. The CRP test is not diagnostic of any condition, but it can be used together with signs and symptoms and other tests to evaluate an individual for an acute or chronic inflammatory condition.
For example, CRP may be used to detect or monitor significant inflammation in an individual who is suspected of having an acute condition, such as:
The CRP test is useful in monitoring people with chronic inflammatory conditions to detect flare-ups and/or to determine if treatment is effective. Some examples include:
CRP may sometimes be ordered along with an erythrocyte sedimentation rate (ESR), another test that detects inflammation. While the CRP test is not specific enough to diagnose a particular disease, it does serve as a general marker for infection and inflammation, thus alerting health practitioners that further testing and treatment may be necessary. Depending on the suspected cause, a number of other tests may be performed to identify the source of inflammation.
When is it ordered?
The CRP test may be ordered when an individual is suspected of having a serious bacterial infection based on the person's medical history and signs and symptoms. It may be ordered, for example, when a newborn shows signs of infection or when an individual has symptoms of sepsis, such as fever, chills, and rapid breathing and heart rate.
It may also be ordered on a regular basis to monitor conditions such as rheumatoid arthritis and lupus and is often repeated at intervals to determine whether treatment is effective. This is particularly useful for inflammation problems since CRP levels drop as inflammation subsides.
What does the test result mean?
The level of CRP in the blood is normally low.
A high or increasing amount of CRP in the blood suggests the presence of inflammation but will not identify its location or the cause. In individuals suspected of having a serious bacterial infection, a high CRP can be confirmatory. In people with chronic inflammatory conditions, high levels of CRP suggest a flare-up or that treatment has not been effective.
If the CRP level is initially elevated and drops, it means that the inflammation or infection is subsiding and/or responding to treatment.
Is there anything else I should know?
CRP levels can be elevated in the later stages of pregnancy as well as with use of birth control pills or hormone replacement therapy (i.e., estrogen). Higher levels of CRP have also been observed in people who are obese.
The erythrocyte sedimentation rate (ESR) test will also be increased in the presence of inflammation; however, CRP increases sooner and then decreases more rapidly than the ESR.
What are chronic inflammatory diseases?
"Chronic inflammatory diseases" is a non-specific term used to characterize long-lasting or frequently recurring bouts of inflammation associated with a more specific disease. Chronic inflammation can be caused by a number of different pathological conditions such as arthritis, lupus, or inflammatory bowel disease (Crohn disease or ulcerative colitis).
What is the difference between CRP and hs-CRP tests?
Both tests are essentially the same, measuring the same substance in the blood. However, the high sensitivity CRP (hs-CRP) test measures very small amounts of CRP in the blood and is ordered most frequently for seemingly healthy people to assess their potential risk for heart problems. It typically measures CRP in the range from 0.5 to 10 mg/L. The regular CRP test is ordered for those at risk for infections or chronic inflammatory diseases (see above). It measures CRP in the range from 10 to 1000 mg/L.
Should everyone have a CRP test?
The standard CRP test is not intended to be a general screening test and many people will never have one done. It is specifically used to detect or confirm inflammation and significant bacterial infections.
Can a CRP test be performed in my doctor's office?
It may be performed in a larger clinic, but most CRP tests will be performed in a laboratory.
On This Site
Tests: ESR, Complement, Procalcitonin, ANA, Rheumatoid Factor
Conditions: Arthritis, Autoimmune Disorders, Inflammatory Bowel Disease, Pelvic Inflammatory Disease, Systemic Lupus Erythematosus, Sepsis, Vasculitis, Meningitis and Encephalitis
Elsewhere On The Web
National Institute of Arthritis and Muscoskeletal and Skin Diseases: Understanding Autoimmune Diseases
Mayo Clinic: C-Reactive Protein Test
Womenshealth.gov: Autoimmune diseases factsheet
KidsHealth.com: Blood test - C-Reactive Protein