C-Reactive Protein (CRP) and High-Sensitivity CRP

C-reactive protein is a pentameric plasma protein and a prototypical positive acute-phase reactant: its concentration can rise rapidly and by orders of magnitude during inflammation, making it one of the most useful and most frequently measured markers of the acute phase response. High-sensitivity CRP assays extend measurement into the low concentration range used in cardiovascular risk research.

Definition

C-reactive protein is a member of the pentraxin family of plasma proteins, synthesized mainly by hepatocytes under cytokine stimulation, whose circulating concentration rises during inflammation and serves as a quantitative marker of the acute phase response.

Scope

This entry covers what CRP is, how its synthesis is driven, its rapid kinetics, the distinction between conventional and high-sensitivity (hs-CRP) assays, and the general ways CRP is used as a marker of inflammation. It is a reference description of the analyte, not a source of diagnostic cut-offs or treatment guidance.

Core questions

• What stimulates hepatic CRP synthesis?
• How fast does CRP rise and fall after an inflammatory stimulus?
• What distinguishes a high-sensitivity CRP assay from a conventional one?
• Why is CRP a sensitive but non-specific marker of inflammation?

Key concepts

• Pentraxin family protein
• Positive acute-phase reactant
• Interleukin-6 driven hepatic synthesis
• Rapid kinetics and short half-life
• Conventional versus high-sensitivity (hs-CRP) assay
• Phosphocholine binding and opsonization
• Non-specificity for cause of inflammation

Mechanisms

CRP is produced predominantly by the liver in response to interleukin-6, with interleukin-1 acting synergistically. After an acute stimulus its plasma concentration can begin rising within hours and increase several-hundred-fold, then fall promptly once the stimulus resolves because its plasma half-life is short and clearance is relatively constant; circulating level therefore tracks synthesis and reflects ongoing inflammation. Functionally, CRP binds phosphocholine and related ligands on damaged cells and microorganisms and can activate the classical complement pathway and promote opsonization, linking it to innate host defence.

Clinical relevance

CRP is used broadly to detect inflammation and to follow its intensity over time, and because its level changes quickly it is convenient for tracking the course of an inflammatory process. High-sensitivity CRP has been studied as a marker of cardiovascular risk in population research. CRP is a non-specific signal that reflects the presence and degree of inflammation rather than its cause; this entry describes the analyte and does not provide thresholds or management advice.

Epidemiology

CRP is one of the most commonly ordered laboratory tests for inflammation. In large cohort studies, high-sensitivity CRP has been examined as a population marker associated with future cardiovascular events (Ridker et al., 2002), illustrating its use beyond acute infection.

Evidence & guidelines

The biology and laboratory behaviour of CRP are summarized in widely cited reviews (Pepys & Hirschfield, 2003; Gabay & Kushner, 1999), and its role as a cardiovascular risk marker derives from prospective cohort evidence (Ridker et al., 2002). This entry summarizes that literature at a reference level rather than as guideline recommendations.

History

CRP was discovered in 1930 by Tillett and Francis, who found that serum from patients with acute pneumococcal pneumonia precipitated the C-polysaccharide of the pneumococcus, giving the protein its name. Through subsequent decades it was characterized as an acute-phase reactant and a pentraxin, and from the 1990s high-sensitivity assays opened its use in cardiovascular epidemiology.

Debates

Is CRP a cause or only a marker of cardiovascular disease?

High-sensitivity CRP is associated with cardiovascular risk, but whether it contributes causally to atherosclerosis or simply marks underlying inflammation has been debated, and the prevailing reading treats it primarily as a risk marker.

Key figures

• Mark Pepys
• Paul Ridker
• William Tillett
• Thomas Francis

Seminal works

• tillett-francis-1930
• pepys-hirschfield-2003
• ridker-2002

Frequently asked questions

What is the difference between CRP and high-sensitivity CRP?

They measure the same protein; a high-sensitivity (hs-CRP) assay is calibrated to quantify low concentrations accurately, which is needed for cardiovascular risk research, whereas a conventional assay is used for higher levels seen in overt inflammation.

Why does CRP rise and fall faster than the sedimentation rate?

CRP has a short plasma half-life, so its level closely tracks current synthesis and changes within hours, while the erythrocyte sedimentation rate depends on slower-changing proteins such as fibrinogen and therefore lags.