- C-reactive protein (CRP) is one of several ‘acute-phase’ proteins formed by the liver and released into the blood circulation within a few hours after injury or response to inflammation.
- The normal CRP level of human body is less than 10mg/L.
- The increased high level of CRP in blood gives a sign that there can be inflammatory processes occurring in the body.
- Though inflammation itself isn’t a typical problem it indicates other health concerns like infection, arthritis, kidney failure and pancreatitis.
- Patients with high CRP levels are at increased risk for coronary artery disease. It can cause a heart attack.
- CRP test helps in detection, diagnosis, and prognosis involving tissue damage and inflammation.
- Elevated CRP levels have been associated with:
- acute bacterial infections
- tumors
- myocardial infarction
- post-operative necrosis
- acute rheumatic fever
- rheumatoid arthritis
- Methods of CRP determination include capillary precipitation, Ouchterlony immunodiffusion, radial immunodiffusion, particle immunoassay, and particle agglutination of the latex particles.
Table of Contents
Principle
- The C-reactive protein test is based on the principle of latex agglutination.
- Reaction occurs between the human C-reactive protein and the corresponding anti-human CRP antibodies.
- In the case of a positive reaction, within 2 minutes, visible agglutination of latex particles can be observed.
Materials required
- Sample (serum)
- Positive and negative control
- Glass slide with six reaction circle
- Mixing sticks
- Test tube
- Saline
- A uniform suspension of polystyrene latex particles coated with anti-CRP antibodies.
Procedure
Qualitative Method
- Take all reagents and serum samples to room temperature before the test starts and mix latex reagent gently before use. Do not dilute the controls and serum.
- Place one drop of serum, positive control, and negative control on a separate glass slide using a (disposable pipette) dropper.
- Add one drop of CRP latex reagent to the drop of test specimen (serum) on each slide.
- Using the mixing stick mix the serum and CRP latex reagent uniformly over the entire circle.Immediately start a stopwatch, and rock the slide gently back and forth for 2 minutes.
- Observe the clump (agglutination) macroscopically.
Semi-quantitative method:
- Prepare the serial dilution of samples 1:2, 1:4, 1:8, 1:16, 1:32, 1:64, and so on by using normal saline.
- Pipette each dilution of the serum sample and control (positive control and negative control) on to separate reaction circle.
- Add one drop of CRP latex reagent to each drop of diluted serum (sample) and controls.
- Using the mixing stick mix the sample and latex reagent uniformly over the entire circle.
- Immediately start a stopwatch, and rock the slide gently back and forth (120 rpm/minute) for 2 minutes.
- Observe the clump (agglutination) macroscopically.
Results and interpretation
- A positive reaction is indicated by agglutination.
- False-negative results may occur when there is an excess antigen. In such conditions, the test should be repeated. When such a prozone effect is suspected, a diluted serum sample should be used.
- The presence of agglutination indicates a CRP concentration equal to or greater than the reagent sensitivity (mg/L CRP) . The titer, in the semi-quantitative method, is defined as the highest dilution showing a positive result.
Calculation:
- The approximate CRP concentration in the patient sample is calculated as follows: Sensitivity (Indicated on the label of the latex vial) x CRP Titer = mg/L
For example, sensitivity (indicated on the label of the latex reagent vial) is 6 mg/L the semi-quantitative method, multiplication of dilution factor with 6 mg/L will yield the approximate level of CRP in the serum sample.
| Dilution concentration | (mg/L) |
| 1:1 | 6 |
| 1:2 | 12 |
| 1:4 | 24 |
| 1:8 | 48 |
| 1:16 | 96 |
| 1:32 | 192 |
Expected values:
- The normal adult level of CRP is reported to be less than 12 mg/L. A trace level of CRP had been reported in the sera of apparently healthy adults and normal children. The CRP level can increase significantly (greater than 10 fold) above the normal values with the onset of a substantial inflammatory stimulus.
Limitation
- A reaction time longer than two minutes may lead to false-positive results due to the drying effect.
- Only fresh serum specimens should be used, plasma must not be used since fibrinogen may cause non-specific agglutination of the latex.
- False-negative results can occur due to the prozone phenomenon. So, all negative sera need to be checked by retesting at a 1:10 dilution.
References:
- Vanderschueren, S., Deeren, D., Knockaert, D. C., Bobbaers, H., Bossuyt, X., & Peetermans, W. (2006). Extremely elevated C-reactive protein. European Journal of Internal Medicine, 17(6), 430–433. https://doi.org/10.1016/j.ejim.2006.02.025
- Aryal, S (2022). C-Reactive Protein (CRP) Test- Uses, Procedure and Result Interpretation. Retrieved 3 May 2022, from https://microbiologyinfo.com/c-reactive-protein-crp-test-principle-uses-procedure-and-result-interpretation/
- C-Reactive Protein Test: Purpose, Procedure, and Results. (2022). Retrieved 3 May 2022, from https://www.healthline.com/health/c-reactive-protein
