What Is the CRP Test (C-Reactive Protein)?

Blood Test Detects Acute Inflammation

95466873.jpg
Tetra Images/Getty Images

The CRP (C-Reactive Protein) test is a blood test that measures the concentration of a special type of protein produced in the liver. The protein is present during episodes of acute inflammation or infection. In the body, CRP interacts with the complement system, an immunologic defense mechanism.

An elevated CRP test result is an indication of acute inflammation. In cases of inflammatory rheumatic diseases, such as rheumatoid arthritis and lupus, doctors can utilize the CRP test to assess the effectiveness of a specific arthritis treatment and monitor periods of disease flare-up.

Its value is as a general indicator, though, not specific. By "not specific", it is meant that the CRP test does not reveal what is causing inflammation in the body. It simply indicates that there is inflammation.

With inflammatory disease, a low CRP level is possible. While that seems counterintuitive, a low CRP level does not necessarily indicate that there is no inflammation present. While it is true that CRP levels may not be increased in some people diagnosed with rheumatoid arthritis or lupus, the reason is unknown.

From Lab Tests Online, "A high or increasing amount of CRP in the blood suggests the presence of inflammation but will not identify its location or the condition causing it. In individuals suspected of having a serious bacterial infection, a high CRP suggests the presence of one. 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, then it means that the inflammation or infection is subsiding and/or responding to treatment."

Aside from inflammatory rheumatic diseases, a positive CRP may occur with:

A positive CRP also can be detected during the last half of pregnancy or in people using oral contraception.

Sedimentation Rate Is Another Test for Inflammation

Another blood test that is often ordered together with CRP is known as the erythrocyte sedimentation rate (ESR or sed rate). Both CRP and ESR yield non-specific information about inflammation. One notable difference between the two tests is that changes are reflected more quickly with CRP compared to ESR. For example, your CRP level may drop to normal following successful treatment more quickly, while ESR remains elevated for a longer period.

CRP and Heart Disease

Studies have indicated that CRP may be elevated with heart attacks. It is yet to be determined if CRP serves as a marker of heart disease or whether it plays a part in causing atherosclerotic disease (hardening of the arteries).

There is also a high-sensitivity CRP test (hs-CRP), in addition to the regular CRP test. The hs-CRP measures very low amounts of CRP in the blood and is typically used to assess risk for heart problems.

CRP Results

With the regular CRP test, the normal reference range may vary from laboratory to laboratory.

As a general rule, there is no CRP detectable in normal blood.

With the hs-CRP test, a result of lower than 1.0 mg/L is associated with a low risk of developing cardiovascular disease; average risk of developing cardiovascular disease is associated with a level between 1.0 and 3.0 mg/L; and high risk for cardiovascular disease is linked to hs-CRP above 3.0 mg/L.

Source:

C-Reactive Protein. The Test. Lab Tests Online. Last modified October 29, 2015.

C-Reactive Protein. Godron A. Starkebaum, MD. MedlinePlus. Reviewed 1/20/2015.

Continue Reading