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Ask the Lab: C-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR).

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Welcome to the ADVANCE for Nurses column designed to address all your questions about lab tests, specimen collection, interpreting results and more.

Question No. 1:

I work for a home health infusion company and we do a lot of IV antibiotics. Lately, the doctors have been ordering CRP along with ESRs for weekly testing.

What are the clinical basics of these two tests?

Answer:

Both CRP and the ESR are, when increased, indicators of systemic inflammation, many infectious processes, as well as tissue damage, including MI. Neither test is specific to a particular disease nor employed as a diagnostic test.

The primary value of these tests is to monitor the patient's condition, including the effectiveness of treatment, such as an antibiotic regimen.

Of these two tests, the ESR has been around the longest and is ordered more frequently by clinicians. In the lab, we simply determine the rate at which erythrocytes (red blood cells) fall in a graduated tube.

Elevated ESRs occur due to the presence of inflammatory proteins and increased antibody levels in the blood. In addition to monitoring systemic inflammatory processes and recovery from bacterial infections, an elevated ESR can also be an indicator of malignancy.

Changes in the ESR may also reflect abnormalities in the number of red cells present, such as polycythemia, when the ESR is decreased, or in conditions where the shape of the red cells is altered, such as spherocytosis, which will cause an elevated ESR.

Elevations of the ESR are also seen during pregnancy and are common in the elderly population where minor increases are generally not an area of concern. A normal ESR can rule out a diagnosis of temporal arteritis or polymyalgia rheumatica.

CRP is an "acute phase reactant", and becomes elevated in an infectious process, often prior to the onset of fever. Marked elevations occur when there is trauma, as in acute MI and as a consequence of surgery. Therefore, this test is clinically useful as an indicator of recovery, as it returns to normal.

Continued elevation or spiking of the CRP can be indicative of postop complications.

Like the ESR, CRP can be used to monitor the progress of systemic inflammatory conditions such as rheumatic disease, as well as serve as an indicator of the efficacy of treatment in infectious disease. CRP may also be elevated in certain cancers.  

From the above descriptions, it is apparent that both tests are similar in their clinical usefulness.

Of the two tests, the CRP changes more rapidly, and more accurately mirrors a change in the patient's condition. Significant changes may be noted in less than a 6 hour time frame, and an elevated CRP will return to normal prior to the ESR.

Question No. 2

If someone has a CRP of 12.3 mg/dl, what is it indicative of, if not cardiovascular disease? The cardiologist is not concerned about this level.

Answer

There is a "new" CRP test available, referred to as "hsCRP" or "cardioCRP." The (hs) stands for "highly sensitive."

The hsCRP detects the very same protein as the CRP discussed above, but is able to detect small changes at much lower levels.

Patients who have inflammatory conditions with high CRPs, as noted in Question No.1, cannot benefit from the hsCRP as the elevated CRP masks the smaller changes, which are a newly recognized indicator of cardiovascular risk.

Because CRP is a very sensitive acute phase reactant, small elevations have been found to correlate with endothelial injury and inflammation, components of the atherogenic process.

Physiologically, CRP has a role in plaque formation and subsequent destabilization. The hsCRP is an indicator of these small changes associated with atherosclerosis. The clinical usefulness of hsCRP is as a predictor of first coronary events. Additionally, in patients with unstable angina, an elevated hsCRP is associated with poor outcome.

Resources:

Woodhouse, S. (2002). C-reactive protein: from acute phase reactant to cardiovascular risk factor, Medical Laboratory Observer, 34(3), 12-21. 

Sally Perry Ball is a Medical Laboratory Science faculty member at Northeastern University in Boston and a freelance writer.


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