Scientists' understanding of coronary heart disease, or CHD, has come a long way since the Framingham Heart Study was launched in 1948. The initial discovery of cholesterol's role in CHD led to a more intensive search for other mechanisms that might contribute to the development of heart disease. Inflammation, a relative newcomer to the list of risk factors for CHD, can be measured with a blood test called C-reactive protein.
Inflammation in CHD
The hallmark of coronary heart disease is the atherosclerotic plaque, a conglomeration of white blood cells, cholesterol and connective tissue that forms along the inner wall of a coronary artery. The initial stimulus for plaque formation is white cell invasion of the arterial wall, accompanied by inflammation within the wall's layers. As the plaque thickens, it gradually blocks the artery and interferes with blood flow. Alternatively -- and more commonly -- the plaque ruptures suddenly and creates a clot, which can occlude the artery and lead to a heart attack.
A Marker for Inflammation
C-reactive protein, or CRP, is a protein produced by your liver and released into your bloodstream in response to infections, pancreatitis, heart attacks, cancer, arthritis and other inflammatory conditions. One of CRP's primary functions is to attach to the outer surface of bacteria circulating in your bloodstream and initiate their destruction. According to "The Merck Manual of Diagnosis and Therapy," CRP is an "acute phase reactant," meaning its levels increase whenever tissue damage occurs. Therefore, while an elevated CRP indicates the presence of inflammation, it is not a specific test for CHD.
High-Sensitivity CRP
Because your CRP rises in response to any inflammatory process in your body, it is difficult to attribute a high CRP to a specific disorder. However, in the 1990s scientists began performing highly sensitive CRP tests, or hs-CRP, in individuals who were otherwise at risk for CHD, and they discovered that hs-CRP levels above 2.5 mg/L increased the likelihood of a coronary "event," such as a heart attack, in these patients. In contrast, the usefulness of hs-CRP in asymptomatic people without risk factors for CHD has not been established. Therefore, in 2009 the U.S. Preventive Services Task Force concluded that there was insufficient evidence to support the use of hs-CRP to screen for CHD in healthy people at low risk for heart disease.
C-Reactive Protein a Culprit?
According to experts at the Royal Free and University College Medical School in London, there is some evidence, although it is controversial, that CRP itself plays a role in the process that leads to plaque formation. By binding to LDL cholesterol in atherosclerotic plaques, decreasing nitric oxide synthesis and stimulating the production of "adhesion molecules" and clot-promoting proteins, CRP could be an active participant in the development of coronary heart disease.
Considerations
C-reactive protein is a nonspecific measure of inflammation in your body. Highly sensitive CRP, or hs-CRP, can be used to gauge the risk of a coronary event in people with known CHD or who have risk factors for CHD, such as high blood pressure, high LDL, low HDL, obesity or diabetes. However, there is insufficient evidence to determine whether hs-CRP is a useful screening tool for healthy people who are at low risk for CHD. Ask your doctor if you should have your CRP measured.
References
- Framingham Heart Study: Research Milestones
- "The Merck Manual of Diagnosis and Therapy, 18th Edition: Acute Phase Reactants"; Mark H. Beers, M.D., Editor-in-Chief; 2006
- "Journal of Clinical Investigation"; C-Reactive Protein: A Critical Update; M.B. Pepys, G.M. Hirschfield; July 2003
- "Annals of Internal Medicine"; Using Nontraditional Risk Factors in Coronary Heart Disease Risk Assessment: U.S. Preventive Services Task Force Recommendation Statement; N. Calonge, et al.; October 2009
