Clinical Laboratory & DiagnosticsAug 14, 2026·6 min read
C-Reactive Protein (CRP) Test: Rapid Biomarker for Acute Inflammation
Explore the CRP latex agglutination test. A highly sensitive acute-phase reactant used to promptly detect inflammation, tissue injury, and assess response to treatments.
1. Abstract & Brief Introduction
The C-Reactive Protein (CRP) test measures the level of CRP, an acute-phase protein synthesized by the liver in response to interleukin-6 (IL-6) during inflammation. CRP levels rise rapidly within hours of tissue injury, infection, or inflammation, making it a highly sensitive but non-specific biomarker.
2. Test Objectives
Detect the presence of acute inflammation or tissue injury.
Differentiate between bacterial infections (markedly elevated) and viral infections (mildly elevated or normal).
The manual qualitative/semi-quantitative method uses Latex Agglutination: Polystyrene latex particles coated with anti-human CRP antibodies are mixed with the patient's serum. If CRP is present in the serum at levels above the detection limit (usually 6 mg/L), the antigen-antibody reaction causes the latex particles to clump together (macroscopic agglutination).
Note: Automated labs use Immunoturbidimetry, where the formation of antigen-antibody complexes increases the turbidity of the sample, measured photometrically.
4. Laboratory Equipment
Glass slide with black background (Agglutination slide)
Mechanical rotator (set at 100 rpm) or manual rocking
Disposable applicator sticks
Precision pipettes (20 µL, 50 µL)
5. Reagents & Solutions
CRP Latex Reagent (Latex particles coated with anti-CRP).
Positive Control Serum.
Negative Control Serum.
Normal Saline (for serial dilutions in semi-quantitative testing).
6. Sample & Pre-analytical Protocol
Parameter
Details & Protocol
Specimen
Serum.
Container
Plain tube (red top) or SST (yellow top).
Stability
Stable for up to 8 days at 2-8°C, or 3 months at -20°C.
Interferences
Extreme lipemia, heavy hemolysis, or high rheumatoid factor (RF) levels may cause false positives.
7. Complete Step-by-Step Procedure
Step
Action & Execution
Step 1
Bring all reagents and the patient's serum to room temperature.
Step 2
Place 50 µL (or one drop) of the patient's serum in a designated circle on the agglutination slide.
Step 3
Place 50 µL of Positive Control and 50 µL of Negative Control in separate circles.
Step 4
Gently mix the CRP Latex Reagent vial and add 50 µL (one drop) to each circle next to the serum/controls.
Step 5
Using a clean applicator stick for each circle, mix the latex and serum well and spread over the entire area of the circle.
Step 6
Rock the slide gently manually or place on a mechanical rotator at 100 rpm for exactly 2 minutes.
Step 7
Observe immediately for macroscopic clumping (agglutination) under a strong light source.
In cases of extremely high CRP, agglutination may be falsely negative (requires dilution).
9. Reference Ranges
Parameter
Normal Range
Qualitative Test
Negative (< 6 mg/L)
Quantitative (Automated)
< 5 mg/L or < 0.5 mg/dL (depending on lab units)
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The C-Reactive Protein (CRP) test measures the level of CRP, an acute-phase protein synthesized by the liver in response to interleukin-6 (IL-6) during inflammation. CRP levels rise rapidly within hours of tissue injury, infection, or inflammation, making it a highly sensitive but non-specific biomarker.
2. Test Objectives
Detect the presence of acute inflammation or tissue injury.
Differentiate between bacterial infections (markedly elevated) and viral infections (mildly elevated or normal).
The manual qualitative/semi-quantitative method uses Latex Agglutination: Polystyrene latex particles coated with anti-human CRP antibodies are mixed with the patient's serum. If CRP is present in the serum at levels above the detection limit (usually 6 mg/L), the antigen-antibody reaction causes the latex particles to clump together (macroscopic agglutination).
Note: Automated labs use Immunoturbidimetry, where the formation of antigen-antibody complexes increases the turbidity of the sample, measured photometrically.
4. Laboratory Equipment
Glass slide with black background (Agglutination slide)
Mechanical rotator (set at 100 rpm) or manual rocking
Disposable applicator sticks
Precision pipettes (20 µL, 50 µL)
5. Reagents & Solutions
CRP Latex Reagent (Latex particles coated with anti-CRP).
Positive Control Serum.
Negative Control Serum.
Normal Saline (for serial dilutions in semi-quantitative testing).
6. Sample & Pre-analytical Protocol
Parameter
Details & Protocol
Specimen
Serum.
Container
Plain tube (red top) or SST (yellow top).
Stability
Stable for up to 8 days at 2-8°C, or 3 months at -20°C.
Interferences
Extreme lipemia, heavy hemolysis, or high rheumatoid factor (RF) levels may cause false positives.
7. Complete Step-by-Step Procedure
Step
Action & Execution
Step 1
Bring all reagents and the patient's serum to room temperature.
Step 2
Place 50 µL (or one drop) of the patient's serum in a designated circle on the agglutination slide.
Step 3
Place 50 µL of Positive Control and 50 µL of Negative Control in separate circles.
Step 4
Gently mix the CRP Latex Reagent vial and add 50 µL (one drop) to each circle next to the serum/controls.
Step 5
Using a clean applicator stick for each circle, mix the latex and serum well and spread over the entire area of the circle.
Step 6
Rock the slide gently manually or place on a mechanical rotator at 100 rpm for exactly 2 minutes.
Step 7
Observe immediately for macroscopic clumping (agglutination) under a strong light source.
