C-Reactive Protein (CRP), also referred to as Pentraxin 1, is a non-glycosylated protein that belongs to the Pentraxin family, which also includes Pentraxin 2/SAP and Pentraxin 3/TSG-14.
It functions as an acute phase reactant, meaning it is produced by the liver and released into the bloodstream within a few hours following tissue injury, the onset of an infection, or other causes of inflammation.
Elevated CRP levels in the blood indicate that an inflammatory process may be occurring in the body.
While inflammation itself is not usually a direct problem, it can serve as a marker for a variety of health concerns such as infection, arthritis, kidney failure, and pancreatitis.
High CRP levels may also signal an increased risk of developing coronary artery disease, a condition that can lead to heart attacks.
A CRP test is a blood test specifically designed to measure the amount of CRP present in the blood.
Principle of CRP Test
The C-Reactive Protein (CRP) test operates on the principle of latex agglutination.
In this method, latex particles coated with human anti-CRP antibodies are combined with the patient’s serum.
If C-reactive proteins are present in the serum, they bind to the antibodies on the latex particles.
This binding triggers a visible agglutination (clumping) reaction.
The agglutination typically occurs within 2 minutes, indicating the presence of CRP in the sample.
Uses of CRP Test
CRP testing can be used to detect or monitor significant inflammation in individuals suspected of having acute conditions, such as serious bacterial infections like sepsis, fungal infections, and pelvic inflammatory disease (PID).
It is valuable for monitoring patients with chronic inflammatory conditions to identify flare-ups and/or assess the effectiveness of treatment; examples include inflammatory bowel disease, certain forms of arthritis, and autoimmune diseases such as lupus or vasculitis.
Measuring CRP levels is useful for monitoring the response to therapy.
The test is performed to check for infection after surgery, as CRP levels typically rise within 2 to 6 hours post-surgery and then decrease by the third day.
If CRP levels remain elevated beyond the third day after surgery, it may indicate the presence of an infection.
Procedure of CRP Test
Qualitative Test:
Bring all reagents and the serum sample to room temperature and gently mix the latex reagent before use.
Do not dilute the controls or the serum.
Place one drop of serum, one drop of positive control, and one drop of negative control on separate reaction circles of a glass slide.
Add one drop of CRP latex reagent to each reaction circle.
Mix each circle with a separate mixing stick and spread the fluid over the entire cell area.
Tilt the slide back and forth slowly for 2 minutes, preferably under artificial light.
Observe for visible agglutination.
Semi-Quantitative Test:
Prepare serial dilutions of the specimen using 0.9% physiological saline according to the specified table for the CRP test.
Perform the test on each dilution following the same procedure as for the qualitative test.
Result Interpretation of CRP Test
Positive: Presence of visible agglutination of latex particles, indicating that C-reactive protein is present at a significant and detectable level.
Negative: No visible agglutination observed.
Semi-Quantitative Test Interpretation: The CRP titre is determined by identifying the last serum dilution that still shows visible agglutination.
Calculation of Titre:
where D is the highest serum dilution showing agglutination, and 7 µg/ml is the test’s sensitivity.
Limitations of CRP Test
The strength of the agglutination reaction does not directly indicate the CRP concentration; weak reactions may occur with either slightly elevated or markedly elevated CRP levels.
Antigen excess (prozone phenomenon) can result in false-negative results; therefore, all negative sera should be rechecked by retesting at a 1:10 dilution.
Extending the reaction time beyond the specified limit may cause false reactions due to the drying effect.
Strongly lipemic or contaminated sera can lead to false-positive results.
Only serum should be used for this test.
A quantitative titration procedure is necessary for positive specimens to monitor increasing or decreasing CRP levels.
Patients with high titers of rheumatoid factors may produce positive results.
References
Healthline – C-Reactive Protein Test
R&D Systems – Quantikine ELISA: Human C-Reactive Protein (CRP) Immunoassay
Swemed Diagnostics – CRP Test
Genix Technology – RapidTex CRP Latex Test
University of Washington Medical Center – C-Reactive Protein
MLAB 1235 Immunology/Serology – Laboratory 7: C-Reactive Protein Testing
Lab Tests Online – C-Reactive Protein
Mayo Clinic – Tests and Procedures: C-Reactive Protein Test
MedlinePlus – C-Reactive Protein
MedicineNet – C-Reactive Protein (CRP)
eMedicineHealth – C-Reactive Protein Test (CRP)
Life Extension – Testing for C-Reactive Protein May Save Your Life
KidsHealth – Medical Tests & Exams: Blood Test: C-Reactive Protein (CRP)
WebMD – Digestive Health Centre: C-Reactive Protein (CRP)
WebMD – Heart Disease and C-Reactive Protein (CRP) Testing
ShareCare – What Do C-Reactive Protein (CRP) Test Levels Indicate?
Walk-In-Lab – C-Reactive Protein (CRP) Blood Test, Quantitative
Australian Prescriber – C-Reactive Protein
The New York Times Health Guide – C-Reactive Protein
About – What Is the C-Reactive Protein (CRP) Test?
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