SST full form in medical is the Serum Separation Tube, a type of blood collection tube used to collect a blood sample for diagnostic testing. It is typically made of plastic and has a rubber stopper at one end. The tube contains a gel separator that allows for the separation of serum (the clear, yellowish fluid that is left behind after blood has clotted) from whole blood. When the tube is filled with blood and allowed to stand for a certain period of time (usually about 30 minutes to an hour), the blood will begin to clot. As the blood clots, the serum will separate out and rise to the top of the tube, above the gel separator.
Serum separation tubes are commonly used in diagnostic testing because serum can be easily separated from other components of whole blood, and it is stable at room temperature for several days. This makes it an ideal sample for many types of laboratory tests.