What does a serum separator tube contain and why is it used?

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Serum separator tubes are designed to give you clean serum by both speeding clot formation and creating a barrier between serum and the cellular/clot material. The clot activator helps blood clot reliably and quickly, so you get a solid clot rather than a prolonged, incomplete clot. After the sample is centrifuged, the thixotropic gel moves to the interface and forms a physical barrier between the serum (the liquid on top) and the clot with blood cells below. This separation reduces cellular or clot residue from mixing back into the serum, preserving the integrity of the sample for chemistry tests and making it easier to aliquot and store.

Other tubes are used for plasma or have different additives: sodium fluoride in some tubes is for inhibiting glycolysis during glucose testing, heparin tubes preserve plasma, and tubes with no additives don’t provide serum separation.