C-Reactive Protein (CRP) Test
The C-Reactive Protein (CRP) test measures an acute-phase protein produced by the liver in response to inflammatory cytokines, providing a sensitive, rapidly responding marker of inflammation and infection. The standard CRP assay quantifies the higher concentrations of overt inflammation, in contrast to the high-sensitivity (hs-CRP) assay used for cardiovascular risk.
1
Biomarkers
Blood (serum)
Sample
1–2 business days
Turnaround
No
Fasting
What is the CRP test?
The C-Reactive Protein Test measures CRP, an acute-phase protein produced by the liver in response to inflammatory cytokines, providing a sensitive and rapidly responding marker of inflammation and infection. Because CRP rises within hours of an inflammatory stimulus and falls quickly once it resolves, it closely reflects current inflammatory activity and is well suited to detecting acute processes and monitoring their course. In the diagnostic landscape, the standard CRP assay is a first-line, widely used inflammatory marker for investigating suspected infection, monitoring inflammatory and autoimmune disease activity, and assessing tissue injury or post-operative complications. It differs from the high-sensitivity (hs-CRP) assay only in the concentration range measured: standard CRP quantifies the higher levels of overt inflammation, while hs-CRP resolves the low concentrations relevant to cardiovascular risk. CRP responds faster and more directly than ESR, and the two are often used together in chronic inflammatory disease. Practitioners order standard CRP to detect and follow inflammation, interpreting it as a quantitative but non-specific indicator alongside the clinical context.
What does the CRP test measure?
C-reactive protein is an acute-phase pentraxin synthesised by hepatocytes in response to interleukin-6 and other inflammatory cytokines. It rises within hours of an inflammatory or infective stimulus and falls rapidly once the stimulus resolves, with a plasma half-life of around 19 hours, so its concentration tracks current inflammatory activity closely. The standard CRP assay quantifies the higher concentrations seen in overt inflammation and infection.
Standard CRP is a sensitive, rapidly responding marker of acute and chronic inflammation, used to detect and monitor infection, inflammatory disease, and tissue injury. Its rapid kinetics make it well suited to tracking response to treatment over hours to days. It is non-specific as to cause, so it is interpreted with the clinical picture and, in chronic inflammatory disease, alongside ESR.
Why is the CRP test ordered?
- Investigation of suspected acute infection, where a raised CRP supports an inflammatory or infective process
- Monitoring of inflammatory and autoimmune disease activity, such as rheumatoid arthritis or inflammatory bowel disease, over time
- Assessment of response to treatment for infection or inflammation, using CRP's rapid kinetics to track change
- Evaluation of suspected post-operative or post-procedural infection and complications
- Investigation of unexplained fever, constitutional symptoms, or systemic illness where inflammation is considered
- Differentiation of inflammatory from non-inflammatory causes of symptoms, alongside the clinical picture
- Assessment of disease flares in chronic inflammatory conditions, often together with ESR
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
1–2 business days
Collection notes
No special preparation required. Document recent infection, injury, surgery, or vigorous exercise, and any anti-inflammatory or immunosuppressive therapy, as these affect interpretation. For cardiovascular risk assessment, order the high-sensitivity (hs-CRP) assay instead.
Specimen requirements
Minimum 0.5 mL serum. Separate serum from cells within 2 hours of collection.
What can affect CRP results?
Any acute inflammatory, infective, or traumatic process rapidly raises CRP, which is the intended signal but means recent illness or surgery must be accounted for when interpreting a result.
Active chronic inflammatory conditions sustain CRP elevation, so a raised value may reflect known underlying disease rather than a new process.
Adipose tissue secretes interleukin-6, modestly raising baseline CRP through low-grade inflammation independent of acute illness.
Oral oestrogen can modestly raise CRP, which should be considered in interpretation. # REVIEWER FLAG: Confirm desired handling of this caveat.
Strenuous exertion and tissue injury can transiently raise CRP, so collection should avoid the period immediately after such events where a baseline is sought.
Corticosteroids, biologics, and other anti-inflammatory treatments lower CRP, so values in treated patients reflect therapy as well as underlying activity.
What do CRP results mean?
Elevated
A raised CRP indicates active inflammation, infection, or tissue injury, with the magnitude broadly reflecting intensity — modest elevation in mild or chronic inflammation and high levels (often into the hundreds of mg/L) in significant bacterial infection or major inflammation. It is non-specific as to cause and is interpreted with the clinical picture.
Normal
A normal CRP suggests the absence of significant systemic inflammation at the time of sampling and is interpreted with symptoms and any trend. In chronic disease monitoring, a normal CRP supports quiescent disease activity, often alongside ESR.
Reduced
A low or normal CRP makes significant active inflammation or infection less likely at the time of testing, though early in an acute process the level may not yet have risen. A normal value does not exclude all pathology, particularly low-grade or localised disease.
Note: Standard CRP is non-specific and reflects current inflammatory activity, so it identifies and quantifies inflammation but not its cause. It should not be confused with the high-sensitivity (hs-CRP) assay, which is required for cardiovascular risk stratification; the standard assay is not validated for that purpose. CRP responds faster than ESR, and discordance between the two is itself informative. Anti-inflammatory therapy can mask elevation.
Frequently asked questions
It measures C-reactive protein, an acute-phase protein the liver produces in response to inflammatory cytokines. Because it rises within hours and falls quickly, CRP is a sensitive, rapidly responding marker of inflammation and infection.
How to access the CRP test
The CRP reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: June 3, 2026
Last reviewed: June 3, 2026
- Pepys MB, Hirschfield GM. C-reactive protein: a critical update. J Clin Invest. (2003)
- Bray C, et al. Erythrocyte Sedimentation Rate and C-reactive Protein Measurements and Their Relevance in Clinical Medicine. WMJ. (2016)
- Markanday A. Acute Phase Reactants in Infections: Evidence-Based Review and a Guide for Clinicians. Open Forum Infect Dis. (2015)
- Harrison M. Erythrocyte sedimentation rate and C-reactive protein. Aust Prescr. (2015)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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