During an inflammatory process, a specific abnormal protein named C-reactive protein (CRP) appears in the blood in response to inflammatory cytokines such as interleukin 6 (IL-6). This protein is virtually absent from the blood serum of healthy persons. CRP is one of the most sensitive acute-phase reactants. Levels of CRP can increase dramatically (100-fold or more) after severe trauma, bacterial infection, inflammation, surgery, or neoplastic proliferation. Measurement of CRP has been used historically to assess activity of inflammatory disease, to detect infections after surgery, to detect transplant rejection, and to monitor these inflammatory processes.
There are two types of CRP assays. One measures a wide range of CRP levels to include those found in patients with acute infections. The reportable range is typically 0.3–20 mg/dL (3–200 mg/L). The second is a high-sensitivity CRP (hs-CRP) assay. The latter can detect a lower level of CRP to include those that may be of value in measuring the risk for a cardiac event. The sensitivity is to 0.01 mg/dL (0.10 mg/L). The hs-CRP is useful, therefore, for assessment of risk for developing myocardial infarction (MI) in patients presenting with acute coronary syndromes.
Negative or <1.0 mg/dL or <10 mg/L by rate nephelometry for CRP
<0.1 mg/dL or <1 mg/L by immunoturbidimetric assay for hs-CRP
Collect a 7-mL blood serum sample in a red-topped tube. Observe standard precautions.
Label the specimen with the patient’s name, date, and test(s) ordered and place in a biohazard bag for transport to the laboratory.
The traditional test for CRP has added significance over the elevated erythrocyte sedimentation rate (ESR), which may be influenced by altered physiologic states. CRP tends to increase before rises in antibody titers and ESR levels occur. CRP levels also tend to decrease sooner than ESR levels.
The traditional test for CRP is elevated in rheumatic fever, RA, MI, malignancy, bacterial and viral infections, and postoperatively (declines after fourth postoperative day).
A single test for hs-CRP may not reflect an individual patient’s basal hs-CRP level; therefore, follow-up tests or serial measurements may be required in patients presenting with increased hs-CRP levels.
CRP levels may predict future cardiovascular events and can be used as a screening tool.
CRP Levels
<0.1 mg/dL or <1 mg/L: low risk
0.1–0.3 mg/dL or 1–3 mg/L: average risk
>0.3 mg/dL or >3 mg/L: high risk
>1.0 mg/dL or >10 mg/L: noncardiovascular cause should be considered
Pretest Patient Care
Explain the test purpose and procedure. A fasting sample is preferred. Water may be taken.
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
Posttest Patient Care
Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment. Repeat testing is often necessary to establish an individual’s basal hs-CRP concentration. A positive test indicates active inflammation but not its cause. CRP is an excellent tool for monitoring disease activity. hs-CRP is a tool for assessing cardiovascular risk.
With RA, the traditional test for CRP becomes negative with successful treatment and indicates that the inflammation has subsided.
Follow guidelines in Chapter 1 for safe, effective, informed posttest care.