Understanding the White Substance in Blood Vials: What Is It?

The white gel barrier in serum separator tubes (SST, "tiger top" or gold top) is a thixotropic polyester gel.

Function

1. During centrifugation, gel liquefies (thixotropy) and migrates to the plasma/serum–cell interface.
2. Upon stopping, gel solidifies → forms stable physical barrier.
3. Separates serum (above) from clot/red cells (below).
4. Prevents remixing during transport/handling.

Key Properties

• Inert — does not adsorb analytes (validated per CLSI).
• Density between serum (1.026) and cells (1.090).
• Allows direct sampling on analyzer without manual aliquoting.

Tube Types

• SST (gold/tiger): Clot activator + gel → serum.
• PST (light green): Heparin + gel → plasma.

Proper centrifugation (speed, time, temperature) per manufacturer ensures clean separation. Hemolysis, lipemia, icterus can affect gel performance.