Accurate medical laboratory testing requires precise procedures, starting with the correct collection device. The integrity of the blood specimen and the reliability of the resulting data depend on this fundamental requirement. For the C-Reactive Protein (CRP) assay, the specific tube chosen is crucial for preparing the blood for analysis.
The Essential Blood Collection Tube
The most common blood collection device used for measuring C-Reactive Protein is the Serum Separator Tube (SST), which is easily identified by its Gold or “Tiger Top” stopper. This tube is designed to yield serum, the liquid portion of the blood that remains after clotting factors have been consumed. The Gold Top tube contains two key components necessary for this process: a clot activator, which speeds up the natural clotting process, and a gel barrier material.
The gel barrier is a silicone-based substance at the bottom of the tube before processing. Once the blood has clotted and the tube is spun, this gel moves to form a stable physical separation between the heavier cellular components and the lighter liquid serum. Separating the serum or plasma from the cells is necessary because the blood cells can alter the concentration of certain analytes over time, leading to inaccurate results.
Some laboratories may alternatively use a Mint Green Top tube, which contains Lithium Heparin, to yield plasma instead of serum. Heparin is an anticoagulant that prevents the blood from clotting, allowing the sample to be processed quickly. However, the serum method using the Gold Top SST remains a widely adopted standard, especially when the CRP test is ordered as part of a larger chemistry panel.
Understanding C-Reactive Protein
C-Reactive Protein is a protein synthesized predominantly by the liver in response to signaling molecules released during inflammation or tissue injury. It is categorized as an acute phase reactant, meaning its concentration in the bloodstream rises rapidly, sometimes hundreds of times above normal, within hours of the initial stimulus. This makes CRP a sensitive, though non-specific, indicator of systemic disturbances.
The biological function of CRP involves binding to substances released from damaged cells and certain pathogens, such as phosphocholine. Once bound, CRP acts in the body’s innate immune system to activate the complement cascade, a defense mechanism that helps clear away dead or damaged cells and microbes. This protein acts as an early warning signal, coordinating the initial inflammatory response.
Physicians use the CRP test as a general marker to detect the presence of inflammation, infection, or tissue damage within the body. Elevated levels can suggest a variety of conditions, including bacterial infections, autoimmune disorders like rheumatoid arthritis, or recent surgical trauma. Monitoring CRP levels over time is also useful for tracking the effectiveness of treatment, as a decrease in the protein concentration suggests that inflammation is subsiding.
Preparing the Sample for Laboratory Analysis
The correct tube choice is only the first step; proper sample handling protocols are equally important for accurate CRP results. For the standard Gold Top SST, the blood must be allowed time to clot fully before the next stage. This clotting typically requires a minimum of 20 to 30 minutes at room temperature, though this must not exceed a two-hour window from the time of collection.
Following clotting, the tube is placed into a centrifuge, which spins the tube at high speed to separate blood components based on density. During this process, the heavier red and white blood cells are forced to the bottom of the tube. The gel barrier moves to sit immediately above the packed cells, creating a stable, physical seal that separates them from the clear, straw-colored serum containing the C-Reactive Protein.
The resulting serum must be separated from the cells and tested within a specific time frame to maintain the integrity of the CRP measurement. Although the sample is relatively stable, laboratories prefer to perform the analysis within 72 hours if the serum is refrigerated. If testing is delayed beyond this period, the serum may need to be frozen to ensure the reported CRP value accurately reflects the patient’s status.