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What is a C-reactive Protein (CRP) test?

A CRP test is an analytical technique used in laboratories to determine the concentration of C-reactive protein in blood. For this test, blood samples are subjected to techniques, such as nephelometry and turbidometry. CRP is synthesised by hepatocytes in liver. The synthesis of this protein increases several folds during inflammatory diseases, bacterial and viral infections and cardiovascular diseases. There are two types of CRP assays:

  • A wide-range assay performed in case of acute infections and inflammatory diseases
  • A high-sensitive CRP assay for cardiovascular diseases, also known as hs-CRP or cardio-CRP.

hs-CRP is highly sensitive and can determine very low concentrations of this protein in blood. CRP levels rise immediately within hours of inflammation and fall rapidly once the treatment is initiated. CRP test is more significant than erythrocyte sedimentation rate (ESR) test for dignosing inflammatory conditions, as it is not affected by physiological states.

  1. Why is a CRP test performed?
  2. How do you prepare for a CRP test?
  3. How is a CRP test performed?
  4. What do CRP test results indicate?

CRP is almost absent in healthy individuals. It is a very sensitive reactant to acute-phase infections and inflammatory processes. Inflammatory substances, such as interleukin (IL)-1 and IL-6, are responsible for the increased secretion of CRP by macrophages involved in the inflammatory processes. Higher CRP levels accurately point at the presence of infection and inflammation. Presence of hs-CRP is an indicator of myocardial infarction, and it can be a predictor of paediatric coronary heart disease. Following are the conditions in which a CRP test is prescribed by a doctor:

  • Autoimmune inflammatory diseases, such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE)
  • Acute infections, such as meningitis, cerebral abscess and viral encephalitis
  • To check for prognosis in patients with atherosclerotic disease, congestive heart failure, atrial fibrillation, myocarditis, aortic valve disease and heart transplant
  • Neonatal sepsis and conditions, such as appendicitis, pancreatitis and other inflammatory conditions
  • Monitoring dosage regimen for disease-modifying anti-rheumatic drugs (DMARDs) and antibiotics
  • Myalgia and osteomyelitis
  • Cancers of the lung, breast and kidney
  • Trauma

CRP test is a simple blood test and does not require any special preparation. The test can be carried out anytime, and fasting is usually not necessary. Fasting may be preferred in some cases, though the patient may be allowed to drink water.

The test requires only a few millilitres of blood sample. A skilled laboratory technician will clean your skin with an antiseptic, insert a needle in the arm, draw a blood sample in a pre-sterilised tube or vial and send it for analysis. The procedure is simple and painless, although patients may occasionally experience pain at blood withdrawal site.

Normal results: In healthy individuals, there is less or almost no CRP. The normal levels of CRP in the serum are 0-6 milligrams per litre (mg/L). These values indicate an absence of acute infection or inflammation.

However, for an hs-CRP test, a concentration of 3 mg/L may also be considered high risk for cardiovascular diseases.

Abnormal results: A higher concentration of CRP of >4 mg/L to 100 mg/L suggest conditions, such as active SLE, RA, viral infections, tumours and myeloma.

Moderate concentrations between 100-200 mg/L indicate active RA, polymyalgia, lymphoma and bacterial infection.

Highly elevated concentrations of CRP of >200 mg/L suggest bacterial sepsis and acute vasculitis.

A concentration of more than 400 mg/L is an indication of deep tissue abscess, and death may occur in such cases.

Hs-CRP is an independent marker of cardiovascular diseases. For the hs-CRP marker, patients can be grouped into various risk groups depending on their hs-CRP levels:

  • Low-risk group: less than 1 mg/L,
  • Moderate-risk group: 1-3 mg/L
  • High-risk group: more than 3 mg/L.

CRP concentrations are not only indicators of certain diseases but can also be used to differentiate certain symptoms. Determination of CRP level can help in the following ways:

  • Detect chronic and systemic inflammation
  • Monitor antibiotic treatment for bacterial infections
  • Differentiate concurrent infections, such as those due to SLE or ulcerative colitis
  • Assess all rheumatic conditions and decide treatment modalities for inflammatory conditions
  • Detect postoperative complications such as infections
  • Detect a risk of cardiovascular diseases through hs-CRP
  • Predict the presence of tumours

CRP concentration ranges mentioned above may differ according to the method of analysis used and the laboratory in which the test is performed.

Conclusively, CRP is an important indicator of acute infections and chronic and acute inflammatory conditions; It also helps in the diagnosis of cardiovascular diseases and requires regular monitoring in heart patients. Depending on the health of the patient, a CRP test should be followed by other important laboratory investigations for diagnosing the condition accurately and deciding the correct line of treatment.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is provided from a purely educational perspective and is in no way a substitute for medical advice from a qualified doctor.  

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References

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  2. Fischbach FT. A manual of laboratory and diagnostic tests, protein chemistry testing/serum proteins: acute-phase proteins and cytokines, 7th ed, 2003 Lippincott Williams & Wilkins Publishers pp 388-389.
  3. Ferri FF, Ferri’s. Best Test: A practical guide to clinical laboratory medicine and diagnostic imaging, 4th ed 2019, Elsevier pp 171, 356, 361.
  4. Drew P. Oxford handbook of clinical and laboratory investigation, 4th Ed 2018. Oxford University press pp 348-349, 416-417, 744-745.
  5. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; C-Reactive Protein (Blood)
  6. Sproston NR and Ashworth JJ. Role of C-Reactive Protein at Sites of Inflammation and Infection.. Front Immunol. 2018 Apr 13;9:754. PMID: 29706967
  7. Shrotriya S, Walsh D, Nowacki AS, Lorton C, Aktas A, Hullihen B et al. Serum C-reactive protein is an important and powerful prognostic biomarker in most adult solid tumors.. PLoS One. 2018 Aug 23;13(8):e0202555. doi: 10.1371/journal.pone.0202555. eCollection 2018.