Which statement correctly differentiates serum from plasma?

• A. Plasma lacks clotting factors
• B. Plasma is obtained after coagulation and lacks fibrinogen
• C. Serum contains anticoagulants; plasma does not
• D. Serum is plasma after coagulation and does not contain fibrinogen

Answer: (D)

The difference between serum and plasma hinges on whether clotting factors are present. Serum is the liquid left after blood has clotted and the clot is removed; during clotting, fibrinogen is converted to fibrin and becomes part of the clot, so serum lacks fibrinogen and other clotting factors. Plasma, on the other hand, is obtained from blood drawn with an anticoagulant so clotting is prevented; after centrifugation you get the liquid portion that still contains fibrinogen and the other clotting factors. Therefore, saying that serum is plasma after coagulation and does not contain fibrinogen accurately describes this distinction. The other statements misstate which components are present or absent in plasma or serum.