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What is a C-Reactive Protein Test?

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  • Written By: J.M. Willhite
  • Edited By: Heather Bailey
  • Last Modified Date: 18 November 2016
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A C-reactive protein test (CRP) is a diagnostic procedure used to evaluate inflammation in the body. Utilized as a generalized tool, a CRP test may be administered to detect and evaluate a variety of conditions. Though minimal, a C-reactive protein test does carry some risks and these should be discussed with a qualified health care provider prior to scheduling the test.

C-reactive protein is naturally produced by the liver and, under normal circumstances, remains undetectable in the blood. In the presence of inflammation, levels of CRP increase and serve as a marker or indicator that there is a problem within the body system. Since it is considered a generalized diagnostic tool, a CRP test is not specific in indicating the origin or location of the inflammation.

A relatively common procedure, a C-reactive protein test involves the drawing of blood, generally from a vein located in the elbow. Samples collected may be sent for laboratory analysis where they are mixed with a solution known as antiserum. During analysis, substances contained in the antiserum locate CRP and serve to mark and measure elevated levels in the blood.

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The administration of the test is usually conducted in the same fashion as any other blood draw. Once the area is sterilized with an antiseptic, an elastic band may be wrapped around the upper arm and the needle is inserted into the vein. After the blood is collected in a vial attached to the needle, the elastic band is removed and the needle withdrawn.

A CRP test is usually conducted to check for signs of disease-induced inflammation or to evaluate treatment progress in alleviating existing inflammation. Individuals with conditions such as lupus or vasculitis may undergo routine CRP tests to monitor flare-ups or changes in their condition. Sometimes, when used as a screening tool, levels of CRP reflected in test results may be inconclusive if the individual does not present increased levels in the presence of inflammation. In cases where the test results are uncertain, a more responsive CRP test known as a high-sensitivity C-reactive protein (hs-CRP) assay may be conducted to further evaluate inflammatory markers indicative of risk factors for certain conditions, such as cardiovascular disease.

Normal C-reactive protein test results may vary depending on lab analysis procedures. When results from an hs-CRP are interpreted diagnostically to determine one's risk for developing heart disease, individuals who present a hs-CRP level of 1.0 mg/L (milligrams per liter) (1 ppm or parts per million) are considered to be at a low risk for disease development. If the results demonstrate a hs-CRP level of 3.0 mg/L (3 ppm) or higher, the individual is at the greatest risk for heart disease.

Positive CRP test results are considered to be abnormal. Manifesting C-reactive protein levels are dependent on the degree of inflammation present. Diagnoses of conditions such as cancer, inflammatory bowel disease (IBD), and rheumatic fever may all be confirmed using positive C-reactive protein test results. Abnormal results may also be used to confirm the presence of tuberculosis, cardiovascular disease, and systemic diseases such as lupus and rheumatoid arthritis.

A C-reactive protein test does carry some risk for complications. Women who are pregnant or taking oral contraceptives may demonstrate a false positive result to a CRP test. Though the risks associated with a routine blood draw are minimal, some individuals may still develop complications. Certain individuals may experience a gathering of blood just beneath the injection site, known as a hematoma, or excessive bleeding following a CRP test. Additional risks may include light-headedness, fainting, and infection.

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