In the medical setting, obtaining a blood sample is often the first step toward diagnosis and monitoring, a procedure known as phlebotomy. This process relies on specialized collection tubes to ensure the sample remains suitable for testing. One of the most frequently used tools for this purpose is the Serum Separator Tube (SST), which plays an instrumental role in preparing blood for laboratory analysis. The choice of the correct tube directly impacts the accuracy and reliability of the test results.
Identifying the Serum Separator Tube (SST)
The Serum Separator Tube is designed to yield a clean sample of serum, which is the liquid portion of blood after it has clotted. Phlebotomists easily recognize this tube by its distinct color-coded stopper, typically gold or a marbled red and grey, often referred to as a “tiger-top.” Inside the sterile, vacuum-sealed tube are two primary components that allow for this specialized function. The first is a clot activator, usually micronized silica particles that coat the inside wall. The second is a thixotropic gel barrier that rests at the bottom of the tube. Serum differs from plasma because it is collected from blood that has been allowed to clot, meaning it lacks clotting factors like fibrinogen.
The Mechanism: How SSTs Process Blood Samples
The SST’s function begins immediately upon blood collection, utilizing the two internal components to prepare the sample for testing. The minute silica particles act as a surface to initiate the body’s natural coagulation cascade, significantly speeding up the clotting process. This rapid activation allows the whole blood sample to fully clot, often recommended to be between 30 and 60 minutes at room temperature. The blood must be allowed to clot completely to ensure that all cellular components are trapped within the fibrin mesh.
Once the clotting process is complete, the tube is placed into a centrifuge. Centrifugation separates the blood’s components based on their density. The heavier elements, which include the red blood cells and the clot, are forced to the bottom of the tube. The lighter, clear serum remains suspended at the top.
The thixotropic gel is the final part of the separation mechanism. Its density is carefully calibrated to be intermediate between the heavier cellular components and the lighter serum. As the tube spins, the gel barrier shifts upward, forming a stable, solid physical barrier between the heavier clot and the serum. This barrier prevents the serum from mixing back with the blood cells, which could otherwise contaminate the sample. The resulting clear serum can then be easily extracted for laboratory analysis.
Common Laboratory Tests Requiring SST Samples
The purpose of the SST is to obtain high-quality serum, which is necessary for a wide variety of diagnostic tests that measure dissolved substances in the blood. The most frequent use is for routine chemistry panels, which measure a range of substances like electrolytes, glucose, and kidney function markers.
Types of Tests
SST samples are required for several other major categories of testing:
- Liver function tests, which analyze enzymes such as Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST).
- Hormone testing, including assays for thyroid function or reproductive hormones.
- Lipid profile tests, which measure cholesterol and triglyceride levels to assess cardiovascular risk.
- Specialized tests, such as therapeutic drug monitoring (TDM) and many serological tests for infectious diseases.
The clean serum sample provided by the SST ensures accurate measurement of analytes or antibodies, safeguarding the integrity of the diagnostic information.