C-Reactive Protein in
Coronary Artery Diseases:
New Roles for an Old Marker and a Very Old Protein

C-reactive protein (CRP) is a nonglycosylated polymeric protein consisting of five identical subunits. It is found in most vertebrate species, and is well conserved biologically throughout evolution.¹ It has been identified in the horseshoe crab, an arthropod that is some 500 million years old. CRP is one of the "acute phase reactants," i.e., proteins that are produced by the liver to combat the invasion of foreign antigens. CRP was discovered in 1930 when it was shown that it could bind to pneumococcal C polysaccharide. It is now known that CRP specifically recognizes phosphocholine, the hydrophilic portion of phosphatidylcholine, in cell membranes (Figure 1). Complexation of CRP to the cell wall activates complement via the classical pathway, and stimulates macrophages and other cells to undergo phagocytosis.

Figure 1.

Click for larger, annotated picture

The structure of CRP and its binding to the cell membrane. Reproduced with permission.

The reference range of CRP is 0 to 1.0 mg/dL. In patients with acute inflammation, the concentration can increase 1000-fold. In contrast, other acute phase reactants such as fibrinogen, haptoglobin, ceruloplasmin, and complement proteins C3 and C4 increase only two- to threefold or less, relative to baseline concentrations. Release of CRP is mediated by cytokines such as interleukin-6, which stimulate hepatocytes to preferentially produce positive acute phase proteins in lieu of other negative acute phase proteins (such as albumin and transferrin).² High concentrations are observed in patients with infections, malignancies, stress, arthritis, trauma, surgery, and acute myocardial infarction (AMI).

CRP in cardiovascular disease
Although CRP has been used for many years for diagnosis of tissue injury or inflammation, there is renewed interest in its use in cardiovascular diseases. This is due to a better understanding of the role of inflammation in the pathophysiology of atherosclerosis and acute coronary syndromes. Clinical studies have shown that CRP may be useful in the risk stratification of apparently healthy subjects, and in patients with unstable angina pectoris and AMI.

CRP for cardiovascular risk among healthy subjects
The Multiple Risk Factor Intervention Trial (MRFIT) and Physicians' Health Study (PHS) were the first to demonstrate the role of inflammation and C-reactive protein as a risk factor for cardiovascular disease.^3,4 The PHS study consisted of 543 apparently healthy men who reported AMI, stroke, or venous thrombosis over an 8-year follow-up period, and 543 cohorts who did not report any vascular disease. C-reactive protein was retrospectively measured in these subjects. As the concentration of CRP increased from greater than or equal to 0.055 to 0.211 mg/dL, the odds ratio (OR) for development of AMI significantly increased from 1.0 to 2.9 (Figure 2). The relative risk ratio was not lowered when the variable of smoking (which can increase acute phase proteins) was removed. CRP also predicted ischemic stroke, although the magnitude of increase was considerably less. No significant difference in CRP concentrations was observed for patients with venous thrombosis. The effect of aspirin therapy on cardiovascular risk was also assessed in the Physician Health Study. Subjects with high CRP (less than or equal to 0.2 mg/dL) who were given salicylates had a reduction in AMI incidence (OR: 1.80) versus those given placebo (OR: 4.16). Because aspirin has both antiinflammatory and antiplatelet activity, these results support the hypothesis that inflammation plays a role in the development of atherosclerosis, which can be modified pharmacologically.

Figure 2.

The correlation of CPR concentrations (in quartiles) to risk (odds ratios) for acute myocardial infarctions (AMI), stroke, and deep vein thrombosis (DVT).

Measurement of CRP for risk stratification in healthy subjects requires the individual to be free of any acute or chronic disease that might increase the baseline serum concentration of CRP. Moreover, ultrasensitive CRP assays must be used to distinguish minor increases from the upper limit of normal.

CRP in unstable angina
Cardiac troponin T and I are well-established markers for risk stratification for patients with acute coronary syndromes. Patients with abnormal concentrations of either troponin T or I during admission have a fivefold higher risk for AMI or cardiac death while hospitalized and within one month after presentation.⁵ Cardiac troponin is useful for risk stratification because it has high sensitivity for the detection of minor myocardial damage. Ischemic injury is caused by reduced coronary artery blood flow, a recognized event in the pathophysiology of acute coronary syndromes.

Inflammation has also been implicated as an important initiating step in the pathogenesis of unstable angina (UA) and AMI. Infiltration of monocytes, macrophages and T-lymphocytes produce metalloproteinases which degrade the fibrous cap that normally protects atherosclerotic lesions from rupturing. Thinning of the cap makes the plaque vulnerable to rupture. As such, investigators have examined the potential role of CRP for risk stratification. Two studies of 140 and 195 UA patients using in-hospital death and AMI as endpoints showed that CRP was, in fact, not predictive.^6,7 Cardiac troponin I was instead found to be useful (OR: 3.8) in one of these studies.⁷ However, in studies where the observation period was extended to a period of time after the patient was discharged from the hospital, CRP was shown to be predictive of untoward cardiac events. Morrow et al. reported that CRP was equivalent to cardiac troponin for predicting cardiovascular events at 14 days of follow-up, with ORs of 3.6 and 3.0, respectively.⁸ After 3 months of follow-up, the predictive value for CRP and cardiac troponin were even higher, as might be expected (OR: 7.6 and 10, respectively).⁹ Because CRP and cardiac troponin detect different steps of the pathophysiology of acute coronary syndromes, the combination of the two tests may produce more accurate predictions of future risks.

CRP following acute myocardial infarction
The role of CRP in patients surviving AMI has been studied by a number of investigators. The hypothesis is that the CRP concentration is a measure of infarct size, with the higher values indicating more severe AMI and a worsening prognosis. It has been known for many years that CRP concentrations are higher in Q-wave AMI patients than in non-Q-wave patients, and higher in patients following successful thrombolysis than in those following unsuccessful thrombolysis or those not treated with thrombolytic agents.¹⁰ Because all AMIs are associated with release of CRP, higher cutoff values must be used to assess the degree of AMI severity. Appropriate cutoff concentrations should be determined by standard receiver operating characteristic (ROC) curve analysis.

Clinical studies have been conducted to determine a potential role of CRP following AMI. Tommasi et al. calculated an OR of 3.55 for cardiac events (death, new-onset angina, recurrent AMI) after 13 ± 4 months' follow-up of 64 AMI patients, using a cutoff of 2.55 mg/dL.¹¹ Using a cutoff of 20 mg/dL for CRP, Anzai et al. calculated respective ORs of 4.72, 2.11, and 3.44 for cardiac rupture, left ventricular aneurysmal formation, and one-year cardiac death among 220 patients with a first Q-wave AMI.¹² A summary of these and other findings is shown in Table 1.

Table 1. Use of CRP after acute myocardial infarction.

DPC IMMULITE® C-Reactive Protein assay
These studies collectively suggest that CRP should not be used just as a diagnostic marker for detection of inflammatory and infectious states, but also as a prognostic indicator for cardiovascular risk at all stages of the disease. Because of the diverse applications for CRP, the clinical laboratory needs a robust assay that is sensitive, specific, and has a wide, dynamic range. The DPC IMMULITE C-Reactive Protein assay is sensitive to 0.01 mg/dL, and has a calibration range of up to 50 mg/dL, considerably wider than the range for corresponding nephelometric assays. Moreover, this assay has no hook effect when tested up to 890 mg/dL. The within-run precision for this assay is typically less than 6 percent.

References

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2. Kushner I. C-reactive protein and the acute-phase response. Hosp Pract 1990;25(3A):13-28.

3. Kuller LH, Tracy RP, Shaten J, Meilahn EN for the MRFIT Research Group. Relation of C-reactive protein and coronary heart disease in the MRFIT nested case-control study. Am J Epidemiol 1996;114:537-47.

4. Ridker PM, Cushman C, Stampfer MJ, Tracy RP, Hennekens CH. Inflammation, aspirin, and the risk of cardiovascular disease in apparently healthy men. N Engl J Med 1997;336:973-9.

5. Olatidoye AG, Wu AHB, Feng Y-J, Waters D. Prognostic role of troponin T versus troponin I in unstable angina pectoris for cardiac events with meta-analysis comparing published studies. Am J Cardiol 1998;81:1405-10.

6. Oltrona L, Ardissino D, Merlini PA, Spinola A, Chiodo F, Pezzano A. C-reactive protein elevation and early outcome in patients with unstable angina pectoris. Am J Cardiol 1997;80:1002-6.

7. Benamer H, Steg PG, Benessiano J, Vicaut E, Gaultier CJ, Boccara A, et al. Comparison of the prognostic value of C-reactive protein and troponin I in patients with unstable angina pectoris. Am J Cardiol 1998;82:845-50.

8. Morrow DA, Rifai N, Antman EM, Weiner DL, McCabe CH, Cannon CP, et al. C-reactive protein is a potent predictor of mortality independently of and in combination with troponin T in acute coronary syndromes: a TIMI 11A substudy. J Am Coll Cardiol 1998;31:1460-5.

9. Rebuzzi AG, Quaranta G, Liuzzo G, Caligiuri G, Lanza GA, Gallimore JR, et al. Incremental prognostic value of serum levels of troponin T and C-reactive protein on admission in patients with unstable angina pectoris. Am J Cardiol 1998;82:715-9.

10. Pietilä K, Harmoinen A, Hermens W, Simoons ML, Van de Werf F, Verstraete M. Serum C-reactive protein and infarct size in myocardial infarct patients with a closed versus an open infarct-related coronary artery after thrombolytic therapy. Eur Heart J 1993;14:915-9.

11. Tommasi S, Carluccio E, Bentivoglio M, Buccolieri M, Mariotti M, Politano M, et al. C-reactive protein as a marker for cardiac ischemic events in the year after a first, uncomplicated myocardial infarction. Am J Cardiol 1999;83:1595-9.

12. Anzai T, Yoshikawa T, Shiraki H, Asakura Y, Akaishi M, Mitamura H, et al. C-reactive protein as a predictor of infarct expansion and cardiac rupture after a first Q-wave acute myocardial infarction. Circulation 1997;96:778-84.

13. Pietilä KO, Harmoinen AP, Jokiniitty J, Pasternack AI. Serum C-reactive protein concentration in acute myocardial infarction and its relationship to mortality during 24 months of follow-up in patients under thrombolytic treatment. Eur Heart J 1996;17;1345-9.