Plasma is the supernatant liquid obtained after centrifugation of anticoagulated whole blood. Serum is the liquid obtained after clotting of whole blood sample collected in a plain tube. Some of the differences between the two are as follows: Plasma contains fibrinogen as well as all the other proteins, while serum does not contain fibrinogen. Plasma can be obtained immediately after sample collection by centrifugation, while minimum of 30 minutes are required for separation of serum from the clotted blood. Amount of sample is greater with plasma than with serum for a given amount of blood. Use of anticoagulant may alter the concentration of some constituents if they are to be measured like sodium, potassium, lithium, etc.