CRP, high-sensitivity

What is being analysed?
C-reactive protein is a protein produced by the liver and released into the bloodstream. Since the plasma concentration of CRP rises within a few hours of the start of an infection, CRP is particularly useful for monitoring its course. CRP is used as a marker of systemic inflammation. Inflammation plays a key role in the development of atherosclerotic disease, and recent studies have shown that CRP may also be an indicator of cardiovascular risk in apparently healthy people. In these people, however, plasma CRP is usually so low that a particularly sensitive test is needed to measure it; this test is called high-sensitivity C-reactive protein, or hs-CRP.

How is the sample collected for testing?
The sample is taken from a vein in the arm.

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
High-sensitivity C-reactive protein is used to predict the risk of cardiovascular disease in healthy people, together with other markers such as the lipid profile.

A rise in plasma hs-CRP simply reflects ongoing inflammation; moreover, the rise is not specific to the site of inflammation. Since an inflammatory response is triggered in the artery walls while atherosclerotic plaque is forming, small rises in plasma CRP are thought to indicate an ongoing atherosclerotic process. It should be noted, however, that particularly high hs-CRP levels can result from any inflammatory condition, even one that is not clinically apparent.

When is it requested?
There is currently no full consensus on when to request this test, who should have it or what the reference values are; nevertheless, many doctors consider high-sensitivity C-reactive protein a useful aid in assessing cardiovascular risk, and it is requested together with other biochemical cardiovascular risk markers, such as total cholesterol, HDL, LDL and triglycerides.

Based on the conclusions reached in 2004 by the American Heart Association/Centers for Disease Control and Prevention (AHA/CDC), hs-CRP should not be used for mass screening of the general adult population; the major risk factors, such as high blood pressure, high total and LDL cholesterol, smoking and diabetes mellitus, are more appropriate for this. A rise in plasma hs-CRP can simply add value to the assessment based on the major risk factors. In addition, there is currently no experimental evidence supporting the use of high-sensitivity C-reactive protein for monitoring treatment.

What does the test result mean?
Some laboratories currently use the risk categories provided by the American Heart Association/Centers for Disease Control and Prevention (AHA/CDC), based on the distribution of plasma CRP concentrations in the US population; according to these categories,
patients with plasma CRP < 1 mg/L have a low risk of cardiovascular disease; patients with plasma CRP between 1 and 3 mg/L have a moderate risk; the risk becomes high if CRP is above 3 mg/L. For values >10 mg/L the test is not considered meaningful for assessing cardiovascular risk, as it is probably affected by inflammation from another cause.

Is there anything else I should know?
Plasma CRP levels can fall after treatment with NSAIDs, salicylates or statins.
Since CRP is a marker of systemic inflammation, it is important that anyone having this test does not have an infection or inflammation at the time, so that the high-sensitivity test is meaningful for assessing cardiovascular risk. Any infection or systemic inflammation, even if not clinically apparent, can raise CRP and so give a false estimate of risk.
For the same reason, patients with chronic inflammatory diseases, such as rheumatoid arthritis, should not have the hs-CRP test; in these people plasma CRP may even be too high to be measured with the high-sensitivity test.

1. Is the hs-CRP test specific for assessing cardiovascular risk?
2. The standard CRP test is much more widely used than the high-sensitivity one: why?
3. What is the difference between the CRP test and the hs-CRP test?

1. Is the hs-CRP test specific for assessing cardiovascular risk?
Any rise in plasma CRP, even a slight one that can only be detected with the high-sensitivity test, reflects systemic inflammation that can have different causes. However, the rise is not specific to the site of inflammation. Many studies have focused on cardiovascular disease, but recent research has shown that CRP values close to the upper limit of the reference range may also be associated with diseases such as bowel cancer, complications of diabetes and obesity.

2. The standard CRP test is much more widely used than the high-sensitivity one: why?
The high-sensitivity CRP test is the result of the evolution of analytical methods, which have become increasingly sensitive over time, and it is therefore more recent than the test performed with traditional methods. In addition, there is still no full agreement on when to request the hs-CRP test.

3. What is the difference between the CRP test and the hs-CRP test?
Both tests measure the same molecule in the blood, but the sensitivity of the methods used differs. The hs-CRP test is generally used to assess cardiovascular risk in apparently healthy people and measures the protein in a concentration range of 0.5 to 15 mg/L. The CRP test, on the other hand, is requested for patients at risk of bacterial or viral infections (such as after surgery) or with chronic inflammatory diseases (such as rheumatoid arthritis) and measures the protein in a concentration range of 10 to 100 mg/L.