C-reactive protein (Blood)

Diagnostic Use

CRP is a protein produced by the liver. Its production is regulated by cytokines. The plasma CRP level also is partly genetically determined.

CRP as a marker of inflammation

In unwell patients CRP is useful as a marker of severity of inflammation (for example infections, autoimmune diseases, trauma and malignancies).

In almost all clinical settings it is a more sensitive and specific marker than ESR. It also follows the time course of inflammation more closely, with rise within 4-8 hr of tissue damage and more rapid fall on resolution (half-life about 19 hr). It is also not affected by other factors such as high fibrinogen (pregnancy), haematocrit, or monoclonal bands.

Monitoring of the acute phase response should be done at an interval of at least 24 hours between CRP tests, to observe a significant change in results.

ESR, however, is preferred over CRP in the following conditions:

• SLE and variants
• Rheumatoid arthritis
• Other connective tissue diseases including ankylosing spondylitis, reactive arthritis, Sjogren’s syndrome, systemic sclerosis, antiphospholipid syndrome
• Acute rheumatic fever and endocarditis monitoring

Usually only ESR or CRP is required. There is no need to request both. The only known evidence-based exception is for assessment of periprosthetic joint infections of the hip and knee (Guideline published by the American Academy of Orthopaedic Surgeons in 2019, which recommended both ESR and CRP).

CRP as a cardiovascular risk marker

In well patients CRP has some utility as a CVD risk marker. In this context it is called high sensitivity or hsCRP as t he levels measured are usually below 5mg/L.

At LabPlus the CRP assay can measure down to 0.6 mg/L: results lower than this are reported as “less than 0.6 mg/L”.

Recent guideline documents recommend use of hsCRP currently for cardiovascular risk assessment only in patients where it will influence evidence-based treatment decisions , e.g. those who fall near the threshold for drug treatment.

Measurement is not recommended for:

• routine CVD risk screening of the general population
• for those already known to be at high risk, or
• for monitoring

As an example, the Canadian Cardiovascular Society guidelines suggest it may be helpful in certain patients at intermediate (10-19.9% five year) risk, specifically men ≥50yr and women ≥60yr with LDL <3.5 mmol/L. In this group, patients with a CRP level ≥2mg/L have been shown to benefit from statin therapy.

Reference Intervals

Units: mg/L

Reference ranges: