Serum lacks the fibrinogen and the following clotting factors:

Clot Retraction and Expression of Serum Within a few minutes after a clot is formed, it begins to contract and usually expresses most of the fluid from the clot within 20 to 60 minutes. The fluid expressed is called serum because all its fibrinogen and most of the other clotting factors have been removed; in this way, serum differs from plasma. Serum cannot clot because it lacks these factors. Serum lacks the fibrinogen and the clotting factors II,V,VIII.