What is the difference between serum and plasma?

• A. Serum is the liquid after clotting; plasma is the liquid with anticoagulant; serum lacks fibrinogen; plasma contains fibrinogen.
• B. Serum contains fibrinogen.
• C. Serum is thicker than plasma due to more cells.
• D. Serum and plasma are the same.

Answer: (A)

The key idea is what each liquid contains after the blood is drawn. Serum is what you get after allowing a blood sample to clot and then removing the clot; because the clotting process uses up clotting factors, serum does not contain fibrinogen or other clotting factors. Plasma, on the other hand, is the liquid part of blood collected with an anticoagulant; the blood doesn’t clot, so all clotting factors, including fibrinogen, remain in the liquid portion. This difference in fibrinogen presence is what most clearly separates serum from plasma. In practice, plasma is used when you need to measure clotting factors or run coagulation tests, while serum is often used for many chemistry and immunology tests where those factors aren’t needed.