SST (Serum Separator Tubes), often referred to as gel tubes, are blood collection tubes used in medical laboratories. They are designed to separate the serum from the blood after centrifugation.ST (Serum Separator Tubes), commonly known as gel tubes, are specialized blood collection tubes used in clinical laboratories to separate blood components for testing. They contain a gel that acts as a barrier between the serum and the blood cells once the blood sample is centrifuged.