"Clinical Use

Assess relative risk of cardiovascular disease (CVD)
Assess risk of a recurrent cardiovascular event in patients with coronary heart disease (CHD)
Reclassify intermediate CVD risk patients into a high-risk category
Help guide therapeutic decisions for patients with borderline or intermediate CVD risk

Clinical Background

C-reactive protein (CRP) is a non-specific acute-phase protein produced by the liver in response to tissue injury, infection, and inflammation. CRP levels rise as much as 1,000-fold after an acute event; these high levels can be used to diagnose and monitor acute inflammatory states. Levels within the normal, non-acute-phase range (≥10 mg/L) can help assess cardiovascular risk. The high-sensitivity CRP (hs-CRP) test is used for this purpose because it can accurately determine CRP levels in the low range of 1 to 10 mg/L.

Mildly elevated CRP levels have been linked to increased risk for various cardiovascular-related disorders, including coronary heart disease (CHD), peripheral artery disease (PAD), stroke, and sudden cardiac death.1 The predictive value of hs-CRP for cardiovascular events is independent of other established risk factors, including LDL-cholesterol.2 Mildly elevated hs-CRP levels also predict recurrent CHD events and poor prognosis in some patients, including those who have PAD.3 Furthermore, in patients receiving statin or ezetimibe therapy, measurement of hs-CRP levels can improve the prediction of myocardial infarction.4

Because hs-CRP levels are associated with cardiovascular risk, they can contribute to risk stratification. Recommendations on the clinical utility of hs-CRP test results are divided, however. The American Association of Clinical Endocrinologists supports using hs-CRP as part of the Reynold's Risk Score for predicting CHD events.5 The US Preventive Services Task Force similarly found that adding hs-CRP to traditional risk factors provides more accurate risk assessment but found that more evidence was required to support that measuring hs-CRP improves clinical outcomes.6

The American College of Cardiology/American Heart Association Task Force recommends using hs-CRP test results as a possible ""risk enhancer"" for risk-based treatment decisions.7 After a quantitative risk assessment based on the pooled cohort equations for evaluating the 10-year risk of a CVD event, hs-CRP results may inform that moderate intensity statin treatment is appropriate for patients with ""borderline"" or ""intermediate"" CVD risk.7

Note that this test is not intended for the diagnosis or monitoring of acute inflammatory states. A standard (conventional) CRP test is available for those purposes (test code 4420).

Individuals Suitable for Testing

Individuals without a previous history of CHD, especially those with intermediate CHD risk
Patients with stable or acute CHD

Method

Nephelometric method utilizing latex particles coated with CRP monoclonal antibodies
Standardized against the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC)/ Bureau Communautaire de Référence (BCR)/College of American Pathologists (CAP) CRP reference preparation
Analytical sensitivity: 0.2 mg/L
Results are reported in mg/L
Interpretive Information

Ideally, the average of 2 hs-CRP measurements, done 2 weeks apart, should be used when interpreting hs-CRP values. hs-CRP values in the range of 3.1 to 10 mg/L indicate an approximate 2-fold increased risk of CVD compared with values <1.0 mg/L. Levels persistently above 10 mg/L may indicate an acute inflammatory process; sources of infection or inflammation should be sought and the test repeated at least 2 weeks after the inflammatory response has resolved.8

In a patient with borderline 10-year CVD risk (5% to <7.5%), hs-CRP levels ≥2 mg/L support considering moderate intensity statin treatment.7

In a patient with intermediate 10-year CVD risk (7.5% to 20%), hs-CRP levels ≥2 mg/L support initiating moderate intensity statin treatment.7

Increased CRP levels are associated with elevated blood pressure, elevated body mass index, cigarette smoking, metabolic syndrome, diabetes, low HDL levels, high triglyceride levels, use of estrogen or progesterone, and chronic infections or inflammation. Decreased levels are associated with moderate alcohol intake, physical activity, weight loss, and medications including statins, fibrates, and niacin.8"

Methodology
Immunoturbidimetric Assay

Reference Range(s)
≤17 years    Not established
>17 years    Optimal <1.0 mg/LJellinger PS et al. Endocr Pract. 2017;23(Suppl 2):1-87.

For ages >17 years
hs-CRP
 (mg/L)    Risk According to AHA/CDC Guidelines
<1.0
Lower relative cardiovascular risk.
1.0-3.0
Average relative cardiovascular risk.
3.1-10.0

Higher relative cardiovascular risk.
Consider retesting in 1 to 2 weeks to exclude a benign transient elevation in the baseline CRP value secondary to infection or inflammation.
>10.0
Persistent elevation, upon retesting, may be associated with infection and inflammation.

Alternative Name(s)
hs-CRP, Cardio IQ®,CardioIQ®