A C-Reactive Protein (CRP) test is a common blood examination used to detect the presence of inflammation somewhere in the body. The test measures the concentration of a protein that the liver releases rapidly in response to various stimuli, acting as a general marker for infection or tissue damage. Achieving accurate results from this test depends heavily on collecting the blood sample using the correct container, which ensures the protein remains stable until it can be analyzed in the laboratory.
The Essential Tube Color for CRP Testing
The primary sample type required for a standard CRP test is serum, which necessitates the use of a collection tube designed to facilitate clotting. The most frequently used container is the Gold-top tube, also known as a Serum Separator Tube (SST), which contains a clot activator and an inert gel barrier. The clot activator accelerates the natural clotting process, while the gel creates a physical barrier between the heavier blood cells and the lighter serum after centrifugation. A plain Red-top tube, which contains no additive and also yields serum, is a suitable alternative for the test.
These serum-yielding tubes are preferred because the CRP protein is stable in serum, and the absence of an anticoagulant simplifies the separation process for many automated laboratory analyzers. However, some laboratory protocols may request plasma, which requires a tube containing an anticoagulant to prevent the blood from clotting. In these cases, a Mint Green-top tube containing Lithium Heparin or a Lavender-top tube containing EDTA may be used.
Understanding C-Reactive Protein
C-Reactive Protein is classified as an acute phase reactant, a type of protein whose concentration in the bloodstream changes significantly in response to inflammation. The protein is synthesized by the liver, primarily under the influence of pro-inflammatory signaling molecules called cytokines. This production is triggered rapidly and dramatically following the onset of tissue injury, infection, or other inflammatory events.
The function of CRP is rooted in the body’s innate immune system, where it acts as a general defense mechanism against pathogens and damaged cells. It works by binding to specific molecules found on the surface of dead cells and many types of microbes. Once bound, CRP acts as an opsonin, flagging the foreign or damaged material for destruction by immune cells and activating the complement cascade. The level of CRP can increase dramatically within hours of an inflammatory stimulus, making it a highly sensitive, though non-specific, marker for processes like bacterial infection or trauma.
Sample Processing and Analysis
Once the blood sample is collected in the appropriate tube, it must undergo a specific preparation process to isolate the serum or plasma for analysis. The tube is gently mixed, if it contains a clot activator or anticoagulant, and then centrifuged at a high speed. Centrifugation spins the heavier cellular components to the bottom of the tube, allowing the laboratory technologist to separate the liquid portion (serum or plasma) from the blood cells.
The separated sample is then analyzed using a technique called an immunoturbidimetric assay. This method utilizes the principle that the concentration of CRP in the sample can be quantified by measuring the cloudiness, or turbidity, it creates when mixed with a reagent containing anti-CRP antibodies. A specialized instrument measures the amount of light scattered, which is directly proportional to the amount of CRP present in the sample. If the sample cannot be tested immediately, it can be refrigerated for up to 72 hours or frozen for longer storage, which helps maintain the stability of the protein.
Clinical Context of CRP Results
The results of a CRP test provide physicians with valuable information about the presence and severity of inflammation, but they are not specific to any one disease.
Standard CRP Test
For a standard CRP test, a result below 1.0 mg/L is considered a normal level in a healthy adult. Levels significantly above this threshold, particularly those exceeding 10 mg/L, strongly suggest an acute inflammatory process, such as a serious bacterial infection, major trauma, or a flare-up of an autoimmune disease.
High-Sensitivity CRP (hs-CRP) Test
The high-sensitivity CRP (hs-CRP) assay uses the same basic principles but is calibrated to accurately measure much lower concentrations of the protein. The hs-CRP test is not used to diagnose acute infections, but rather to assess long-term, low-grade inflammatory risk, particularly concerning cardiovascular health. Results are categorized to stratify heart disease risk: below 1.0 mg/L suggests low risk, 1.0 to 3.0 mg/L indicates an average risk, and levels above 3.0 mg/L are associated with a high risk of future cardiovascular events. This distinction allows healthcare providers to use the CRP measurement as a tool for both acute disease monitoring and long-term risk assessment.