American Medical Certification Association (AMCA) Phlebotomy Practice Exam 2025 - Free Phlebotomy Practice Questions and Study Guide.

The substance in a Serum Separator Tube (SST) is thixotropic gel. This gel plays a crucial role in separating serum from the cellular components of blood after centrifugation. When the SST tube is spun in a centrifuge, the thixotropic gel moves to the interface between the serum and the blood cells, forming a barrier that prevents contamination of the serum by the cells.

This separation is important because it allows for accurate testing of serum samples without interference from the cells, ensuring that lab results are reliable. Clot activators are often present in SSTs to facilitate the clotting of the blood prior to centrifugation, but the defining feature of an SST is the thixotropic gel that provides the separation function.

In contrast, EDTA is an anticoagulant used in tubes designed for plasma collection and is not present in SSTs, while other types of tubes may contain different anticoagulants for various tests. Thus, thixotropic gel is the distinctive and essential component of an SST.