Serum differs from blood plasma because

Serum differs from blood plasma because it lacks the blood protein fibrinogen. Both serum and blood plasma are components of blood that are obtained when blood is separated by centrifugation. However, plasma contains all the proteins normally found in blood, including fibrinogen, while serum is the fluid that remains after the blood has clotted and the fibrinogen has been removed. Therefore, while blood plasma contains fibrinogen and other blood proteins, serum lacks fibrinogen, which is why it is often used for diagnostic tests that require the measurement of blood clotting ability. Serum also lacks blood cells, but this is true of both serum and plasma, as these are removed during the centrifugation process.