CRP (C-Reactive Protein)

What is CRP?

C-Reactive Protein (CRP) is an acute-phase protein produced by the liver in response to inflammation, infection, or tissue damage. It is part of the innate immune system — one of the body's first-line defenders against pathogens — and its levels can rise from very low baseline values to several hundred milligrams per litre within just 6–12 hours of an acute insult.

CRP has two clinical applications that use different measurement scales. Standard CRP (typically measuring from 5 mg/L upwards) is used to detect and monitor acute infection, post-operative complications, inflammatory disease, and disease flares. High-sensitivity CRP (hs-CRP), which measures at lower levels (0.5–10 mg/L), is used to assess chronic low-grade inflammation as a risk factor for cardiovascular disease.

Because CRP rises so rapidly and falls quickly when inflammation resolves, it is an excellent marker both for identifying acute problems and for tracking how well treatment is working. Serial CRP measurements after surgery or during antibiotic treatment provide objective evidence of recovery or ongoing infection.

Unlike the erythrocyte sedimentation rate (ESR) — another inflammation marker — CRP is not affected by anaemia, pregnancy, or most medications, making it more specific as a pure inflammation signal.

Normal reference range

• Standard CRP (normal): below 5 mg/L
• hs-CRP low cardiovascular risk: below 1 mg/L
• hs-CRP intermediate cardiovascular risk: 1–3 mg/L
• hs-CRP high cardiovascular risk: above 3 mg/L

Important: Reference ranges vary between laboratories and depend on individual factors including age, sex, pregnancy status, and medication. Always interpret your result in the context of a clinician review. The Vesey reviews every result before release.

What a high CRP may indicate

The level of CRP elevation broadly indicates the nature of the underlying process. Possible causes include:

• Bacterial infection — CRP typically rises above 100 mg/L (sometimes much higher) in severe bacterial infection such as pneumonia, sepsis, or pyelonephritis
• Viral infection — usually causes a more modest rise (10–50 mg/L); useful in distinguishing viral from bacterial infection alongside clinical features
• Autoimmune and inflammatory conditions — rheumatoid arthritis, inflammatory bowel disease, vasculitis, and lupus all cause CRP elevation during active disease
• Tissue injury and surgery — trauma, burns, and surgical procedures produce CRP elevations proportional to the extent of tissue damage
• Malignancy — some cancers, particularly lymphoma and colorectal cancer, are associated with elevated CRP
• Obesity and metabolic syndrome — chronic low-grade inflammation driven by adipose tissue raises hs-CRP
• Cardiovascular disease — mildly elevated hs-CRP is an independent risk factor for heart attack and stroke

What a normal or low CRP indicates

A CRP within the normal range provides useful reassurance in several contexts:

• Makes significant bacterial infection much less likely in an unwell patient (high negative predictive value)
• Suggests an inflammatory condition such as rheumatoid arthritis is currently well controlled
• Indicates post-operative recovery is progressing without infection
• A very low hs-CRP in the context of a cardiovascular risk assessment is reassuring

Note that CRP can be normal even in the very early hours of acute infection (before it has had time to rise) and in some conditions such as SLE (lupus), where CRP may remain paradoxically low despite significant disease activity.

Get tested at The Vesey, Sutton Coldfield

CRP is available within the following panels at The Vesey Private Hospital:

• Inflammation profile — £95 (CRP plus ESR and full blood count)
• Lifestyle Screen — broad health check including CRP
• Most comprehensive screen panels — CRP is a standard component

Results are reviewed by a clinician before secure release, typically within 24 hours. Appointments available Monday–Saturday.

Frequently asked questions

What is a normal CRP level?

A CRP below 5 mg/L is generally considered normal. CRP rises very rapidly — within hours — in response to infection or tissue injury, making it one of the most useful acute markers of inflammation available.

What does a high CRP mean?

A high CRP indicates inflammation or infection. Very high levels (above 100 mg/L) are typically associated with bacterial infection. Moderate elevation (5–50 mg/L) may reflect viral infection, autoimmune flare, or inflammatory disease. Mildly elevated hs-CRP (1–3 mg/L) is associated with cardiovascular risk.

What is the difference between standard CRP and hs-CRP?

Standard CRP detects acute inflammation and infection (useful range 5–500 mg/L). High-sensitivity CRP detects chronic low-grade inflammation at levels below 5 mg/L associated with cardiovascular risk. Different assay sensitivities are used; your clinician will advise which is appropriate.

Can lifestyle factors affect CRP?

Yes. Smoking, obesity, inactivity, and a pro-inflammatory diet raise CRP. Regular exercise, a Mediterranean-style diet, adequate sleep, and not smoking are associated with lower CRP. Reducing chronic inflammation is one benefit of a healthier lifestyle that a CRP test can help you track.

How quickly does CRP fall when inflammation resolves?

CRP has a half-life of approximately 19 hours, so it falls quickly once the underlying cause is treated. In a straightforward bacterial infection responding to antibiotics, CRP typically halves every day. A persistently elevated CRP despite treatment suggests the infection has not been fully controlled or a complication has developed.