C-reactive protein (CRP) as a lab test !

CRP is a liver protein that rises within hours with inflammation or infection; it does not say where the problem is.

Updated 2026-11-01

Key facts

A pentameric acute-phase protein made by the liver, mainly under interleukin 6.
It binds phosphocholine on bacteria and damaged cells and activates complement (classical pathway): it aids opsonisation.
It rises in 6–8 hours, peaks at about 48 h and falls quickly (half-life about 19 h) once the stimulus ends: better than ESR for following progress.
It is non-specific: bacterial infection, trauma, surgery, infarction, autoimmune disease and cancer raise it.
High-sensitivity CRP measures low values and has been used as a cardiovascular risk marker; cut-offs depend on the laboratory.

What it measures: serum CRP concentration by immunoassay.
Where it comes from: hepatocytes raise its synthesis hundreds of times under IL-6, IL-1 and TNF-α during the acute-phase response.
Why it rises: bacterial infections (more than viral ones, in general), sepsis, pneumonia, complicated appendicitis, pancreatitis, active rheumatoid arthritis, vasculitis, active Crohn disease, trauma, burns, recent surgery.
Pitfalls: in active lupus CRP usually rises little unless there is infection or serositis; severe liver failure and some biologics (anti-IL-6) block its rise; obesity and smoking raise baseline levels.
It belongs to the acute-phase response with fibrinogen, ferritin and serum amyloid A (positive) and albumin and transferrin (negative).

A normal CRP does not exclude infection in the first hours; a CRP falling on treatment supports a good response.
Ferritin rises as an acute-phase reactant: with a high CRP, a normal ferritin does not exclude iron deficiency.
In acute pancreatitis a very high CRP at 48 hours is linked to greater severity.

Think of the answer, then tap the question to see it.

Which cytokine mainly drives hepatic CRP synthesis?

Interleukin 6, together with IL-1 and TNF-α.

Why is CRP better than ESR for following an infection?

It rises and falls within hours (half-life about 19 h), while ESR changes slowly over days or weeks.

What does CRP do in immune defence?

It binds phosphocholine on microbes and damaged cells and activates complement by the classical pathway, helping phagocytosis.

Why does a normal ferritin not rule out iron deficiency when CRP is high?

Ferritin is also an acute-phase reactant: inflammation raises it and can hide the lack of iron.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: C-Reactive Protein (CRP) Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); C-reactive protein: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.