Yellow Top SST Tubes. These tubes contain a special clot activator chemical and contain a gel barrier at the bottom of the tubes. After centrifugation, this gel barrier positions itself between the separated serum and red blood cells. This helps to keep the parameters stable for up to 48 hours after centrifugation. Yellow top tubes are often used in Chemistry, Immunology, and Serology tests, as well as for samples where sample transportation is required before testing.