What is the difference between serum and plasma?

• A. Plasma is the liquid containing anticoagulant.
• B. Serum is the liquid after clotting; plasma is the liquid with anticoagulant and fibrinogen intact.
• C. Serum contains fibrinogen; plasma does not.
• D. Serum and plasma are identical.

Answer: (B)

The key idea is how the blood is treated before separating the liquid. If blood is drawn into a tube with an anticoagulant, it won’t clot, so after centrifugation the liquid portion is plasma. Plasma keeps the clotting factors, including fibrinogen, intact because the coagulation cascade was blocked by the anticoagulant.

If blood is drawn without an anticoagulant and is allowed to clot, a clot forms and traps cells and clotting proteins. After centrifugation, the liquid above the clot is serum, which has had the fibrinogen and most other clotting factors used up in the clot, so they’re not present in serum.

That's why serum is the liquid after clotting, and plasma is the liquid with anticoagulant present and fibrinogen (and other clotting factors) still intact.