Plasma is an anticoagulant and serum is a clot activator

therefore when blood mixes with plasma it cannot become or

make a clot, but when it is mixed with serum it clots. Many

anticoagulants contain calcium which binds with blood,

other contain potassium oxalate which can act as a

preservative for the blood.

When blood is mixed with serum it will clot faster because

many contain thrombin which coagulates faster and other

thixotropic gel which acts as thrombin.