A gel barrier tube is a specialized device used in phlebotomy designed to improve the quality of a blood sample for laboratory analysis. The primary function of this tube is to efficiently separate the liquid component of the blood—either serum or plasma—from the heavier cellular components after collection. Creating a physical partition ensures the integrity of the sample, which is crucial for accurate diagnostic results. The gel barrier streamlines the pre-analytical phase of testing, making sample preparation more efficient. This separation method is widely used for a large variety of clinical chemistry and serology tests.
Anatomy of the Tube and Its Contents
The core of the gel barrier tube is an inert, silicone-based polymer material situated at the bottom of the collection vessel. This substance is chemically unreactive with blood components, meaning it will not interfere with the subsequent laboratory tests. The gel is classified as thixotropic, a property that allows it to temporarily change its viscosity, thinning out when subjected to mechanical stress, such as the spinning force of a centrifuge.
Beyond the gel, the tubes contain a specific chemical additive that determines whether the sample yields serum or plasma. Serum Separator Tubes (SSTs) include a clot activator, such as micronized silica particles, which accelerate the natural clotting process of the blood. Plasma Separator Tubes (PSTs) contain an anticoagulant, like lithium heparin, which prevents clotting by inhibiting the coagulation cascade. The gel’s density is carefully calibrated to be intermediate between the cellular components and the fluid phase of the blood.
How the Gel Creates a Stable Barrier
The mechanism of separation relies on a physical process called centrifugation, where the collected blood is spun at high speeds. This spinning action generates a powerful centrifugal force, which separates the blood’s components based on their individual densities. The heaviest components, which include the red and white blood cells, are forced to the bottom of the tube, forming a dense pellet.
As the centrifugal force acts upon the tube, the lighter serum or plasma floats to the top, while the thixotropic gel moves between these two layers. The gel temporarily liquefies under the stress of spinning, allowing it to migrate upwards until its intermediate density balances the surrounding components. Once the centrifuge stops, the gel reverts to its semi-solid state, forming a stable, solid wall between the cellular components and the liquid sample. This stable barrier prevents the separated fluid from re-mixing with the cells, which maintains sample stability during transport and storage and prevents cellular metabolism from altering analyte concentrations.
Identifying Different Gel Barrier Tubes and Their Use
Gel barrier tubes are identified primarily by the color of their stopper, which indicates the additive within and the resulting sample type.
Serum Separator Tubes (SSTs)
SSTs are commonly recognizable by a gold or mottled red-and-gray stopper, and they yield serum after the blood is allowed to clot and is then centrifuged. These tubes are routinely used for various chemistry panels, serology tests, and hormone assays, where the absence of clotting factors is preferred for the analysis.
Plasma Separator Tubes (PSTs)
PSTs often feature a light green stopper and contain an anticoagulant like lithium heparin, which ensures the blood does not clot. When these tubes are centrifuged, the gel separates the plasma—the liquid component containing clotting factors—from the blood cells. Plasma samples are required for specific tests, including some drug monitoring and rapid chemistry analyses, because the time-consuming clotting step is avoided.