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The misuse of C-reactive protein
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I saw a 52-year-old man in consult for type 2 diabetes. In addition to oral antihyperglycemic agents, he was on an angiotensin-converting enzyme inhibitor for hypertension. However, his LDL cholesterol was 130 mg/dL and he was not on antihyperlipidemic therapy.
Further review of the records revealed a highly sensitive C-reactive protein which was not elevated. His primary care physician commented on this in his office notes. He stated that because the patient did not have an elevated CRP, lipid-lowering therapy was not necessary.
CRP is an acute-phase protein produced by the liver due to infection or inflammation. All other factors being equal, an elevated CRP suggests a higher risk of future cardiovascular events. However, CRP is not specific; there are innumerable reasons unrelated to vascular disease for CRP to be increased. Also, CRP is variable. Before modifying therapy based on CRP, it must be confirmed to be high more than once. There are newer assays, such as lipoprotein-associated phospholipase A2, which may be more specific markers of rupture-prone plaque and cardiovascular risk. A discussion of this will be the subject of another post.
As an endocrinologist/lipidologist, I frequently order advanced lipid testing and other biomarkers, including CRP. However, I do so only when such information would change my management.
An example might be an intermediate-risk patient with additional risk factors, such as worrisome family history. Waiting until atherosclerosis makes itself obvious before beginning aggressive therapy is not acceptable; on the other hand, I am hesitant to initiate decades of medication without additional information telling me that I should.
A patient with type 2 diabetes and hypertension and dyslipidemia is already at high risk and should be treated aggressively no matter what the CRP is. A non-elevated CRP does not make other risk factors disappear, it only means that we do not need to be any more aggressive than the current guidelines tell us we should. This is why I do not order CRP or other studies in high-risk patients, unless I have a specific indication to do so. Most of the time, it would not change my management nor would it provide me with information I do not already know.
Advanced lipid testing and biomarkers certainly have their place but we must be rational about it. Ignoring well-validated traditional risk factors because an advanced lipid profile or emerging biomarker is normal does not make sense.
When I am asked by primary care physicians whether they should order CRP and other markers of cardiovascular risk, I answer: “It depends ... if you are unsure of when/where you would use these, the limitations of such studies or whether such information would change your management ... then do not order them!”