Vacuum Blood Collection Tube (Grey cap) Tube material: glass or plastic Volume of blood draw (ml): 1~10ml Tube Spec.(mm):12/13*75,12/13*100,13*55mm Tube with grey cap, called Glucose Tube/Sodium Fluoride Tube, with additive of potassium oxalate /sodium fluoride, sodium fluoride. used for the test of blood coagulation mechanism. The choice of the concentration depends upon the policies of the laboratories.