SERUM TUBES In order to obtain serum samples from plastic tubes, the tube must have coagulation activator added. As the plastic suface alone is insufficient to trigger the coagulation within an acceptable time, LYBCON VACUTAINER serum tubes have silica particles added for this purpose. CLOTTING TIMES The recommended minimum time for the coagulation of samples from patients who have not been treated with anticoagulants is 30- 45 minutes for serum (red).