C-reactive protein (CRP) is a non-specific test. It is used by a doctor to detect inflammation if there is a high suspicion of tissue injury or infection somewhere in the body, but the test cannot tell where the inflammation is or what condition is causing it. CRP 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.
C-reactive protein is produced by the liver. The level of CRP rises when there is inflammation throughout the body.
This article discusses the blood test done to measure the amount of CRP in your blood.
How the Test is Performed
A blood sample is needed. This is usually taken from a vein. The procedure is called a venipuncture.
How to Prepare for the Test
No preparation is necessary for this test.
How the Test Will Feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the Test is Performed
The CRP test is a general test to check for inflammation in the body. It is not a specific test. That means it can reveal that you have inflammation somewhere in your body, but it cannot pinpoint the exact location.
Your doctor may order this test to:
- Check for flare-ups of inflammatory diseases such as rheumatoid arthritis, lupus, or vasculitis
- Determine if anti-inflammatory medicine is working to treat a disease or condition
However, a low CRP level does not always mean that there is no inflammation present. Levels of CRP may not be increased in people with rheumatoid arthritis and lupus. The reason for this is unknown.
A more sensitive CRP test, called a high-sensitivity C-reactive protein (hs-CRP) assay, is available to determine a person’s risk for heart disease. Many consider a high CRP level to be a risk factor for heart disease. However, it is not known whether CRP is merely a sign of cardiovascular disease or if it actually plays a role in causing heart problems.
Normal CRP values vary from lab to lab. Generally, there is no CRP detectable in the blood.
Your doctor may also use a highly sensitive test called hs-CRP to help determine your risk of heart disease. According to the American Heart Association:
- You are at low risk of developing cardiovascular disease if your hs-CRP level is lower than 1.0mg/L
- You are at average risk of developing cardiovascular disease if your levels are between 1.0 and 3.0 mg/L
- You are at high risk for cardiovascular disease if your hs-CRP level is higher than 3.0 mg/L
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.
What Abnormal Results Mean
A positive test means you have inflammation in the body. This may be due to a variety of different conditions, including:
- Connective tissue disease
- Heart attack
- Inflammatory bowel disease (IBD)
- Pneumococcal pneumonia
- Rheumatoid arthritis
- Rheumatic fever
- This list is not all inclusive.
C-reactive protein “hawks” think the time has come when everyone should get the test and that it may eventually supplant cholesterol testing. “Doves” say that there just isn’t enough evidence about how doctors should treat high levels, even if there is a link to cardiovascular risk.
As we went to press (2003), neither the American Heart Association (AHA) nor the American College of Cardiology had made a formal recommendation on C-reactive protein. Published comments from AHA leaders hint that it might stake out a middle ground and suggest testing people who fall into an intermediate-risk group because of their age (60 and older), weight, or blood pressure. AHA and other guidelines will influence how doctors will use the CRP test during the next year or so. Ultimately, though, the fate of the test rests with prospective, randomized studies of CRP-lowering interventions still in progress.
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
No test, no matter how good it is, changes anyone’s health. It’s what we do in response that matters. Statins have put a pharmaceutical face on heart disease prevention, but the old truths about getting exercise, eating right, and not smoking still hold. If the CRP test gets more people to follow that advice, then it may be worthwhile.