C-Reactive Protein (CRP)
A blood sample taken from a vein
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.
The CRP test is not diagnostic, but it provides information to your healthcare practitioner as to whether inflammation is present, without identifying the source of the inflammation. This information can be used in conjunction with other factors such as signs and symptoms, physical exam, and other tests to determine if you have an acute inflammatory condition or are experiencing a flare-up of a chronic inflammatory disease. Your healthcare 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 high levels of the protein observed in diseases that cause significant inflammation. It measures CRP in the range from 8 to 1000 mg/L (or 0.8 to 100 mg/dL).
- The hs-CRP test precisely detects lower levels of the protein than that measured by the standard CRP test and is used to evaluate individuals for risk of cardiovascular disease. It measures CRP in the range from 0.3 to 10 mg/L. (See the article on hs-CRP.)
How is the test used?
The C-reactive protein (CRP) test is used to detect inflammation.
For example, CRP may be used to detect or monitor significant inflammation in acute conditions, such as:
- A serious bacterial infection of the lung, urinary tract, digestive tract, skin, or other sites, with or without sepsis
- A fungal or viral infection
- Pelvic inflammatory disease (PID)
The CRP test is useful in monitoring chronic inflammatory conditions to detect flare-ups and/or to determine if treatment is effective. Some examples include:
- Inflammatory bowel disease
- Some forms of arthritis, including rheumatoid arthritis
- Autoimmune diseases, such as lupus or vasculitis
Examples of other uses:
- The CRP test is widely used to detect sepsis in newborn babies.
- The CRP test may be used to monitor patients after surgery. Generally, CRP levels increase after surgery and drop down to normal unless post-surgery infection is present.
- CRP can be a good predictor of rejection in kidney transplant recipients.
CRP may sometimes be ordered along with an erythrocyte sedimentation rate (ESR), another test that detects inflammation, or procalcitonin, a test that indicates sepsis. 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 healthcare 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 it is suspected that you have a serious bacterial infection based on your medical history and signs and symptoms. It may be ordered, for example, when a newborn shows signs of infection or when you have symptoms of sepsis, such as:
- Fever, chills
- Rapid breathing
- Rapid 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.
It is also ordered and monitored after surgery to ensure that you are free of post-surgery infection.
What does the test result mean?
The level of CRP in the blood is normally low.
Increased CRP level:
- A high or increasing amount of CRP in the blood suggests the presence of inflammation but will not identify its location or the cause.
- Suspected bacterial infection—a high CRP level can provide confirmation that you have a serious bacterial infection.
- Chronic inflammatory disease—high levels of CRP suggest a flare-up if you have a chronic inflammatory disease 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.
What are chronic inflammatory diseases?
"Chronic inflammatory diseases" is a general term used to describe long-lasting or frequently recurring bouts of inflammation associated with a more specific disease. Chronic inflammation can be caused by a number of different 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 0.3 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 8 to 1000 mg/L (or 0.8 to 100 mg/dL).
What can I do to lower my CRP level?
Can a CRP test be performed in my doctor's office?
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. CRP can also be increased in people who have cancer.
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.