Greiner's Vacuette CAT serum separator clot activator tubes contain a separating gel that migrates up to the interface between serum and clot during centrifugation, where it forms a stable barrier and separates the serum from the fibrin and cells.. 13 x 75 mm, 4 mL sterile, polyethylene, terephthalate (PET) serum tubes come with gold pull caps..