Which statement about serum is NOT true?

Which statement about serum is NOT true?

• A. Serum is plasma minus fibrinogen.
• B. Serum does not contain clotting factors.
• C. Serum lacks fibrinogen.
• D. Serum is a form of plasma that contains fibrinogen.

Answer: (D)

The key idea here is the difference between serum and plasma and what happens to clotting factors during clot formation. When blood clots, fibrinogen is converted to fibrin to form the clot, and the clot traps cells and many clotting factors. The liquid left behind after this clotting is serum, which therefore lacks fibrinogen and most other clotting factors. Plasma, by contrast, is the liquid component of blood drawn with an anticoagulant, so it still contains fibrinogen and the other coagulation factors. So serum is effectively plasma minus fibrinogen (and minus the clotting factors that get used up in the clot). That’s why a statement claiming serum contains fibrinogen is not true. The other statements align with this concept: serum does not contain clotting factors, and serum lacks fibrinogen.

The key idea here is the difference between serum and plasma and what happens to clotting factors during clot formation. When blood clots, fibrinogen is converted to fibrin to form the clot, and the clot traps cells and many clotting factors. The liquid left behind after this clotting is serum, which therefore lacks fibrinogen and most other clotting factors. Plasma, by contrast, is the liquid component of blood drawn with an anticoagulant, so it still contains fibrinogen and the other coagulation factors. So serum is effectively plasma minus fibrinogen (and minus the clotting factors that get used up in the clot). That’s why a statement claiming serum contains fibrinogen is not true. The other statements align with this concept: serum does not contain clotting factors, and serum lacks fibrinogen.