Which statement best describes the relationship between serum and plasma as described in the material?

• A. Serum is plasma plus clotting factors.
• B. Plasma contains clotting factors.
• C. Serum contains red blood cells.
• D. Plasma contains no red blood cells.

Answer: (A)

Clotting factors determine the difference between serum and plasma. When blood is allowed to clot, the clotting factors are used up in forming the clot, so the liquid that remains (serum) lacks those factors. Plasma is the liquid portion collected with an anticoagulant to prevent clotting, so it still contains the clotting factors. Therefore, the best description is that plasma contains clotting factors. It’s not that serum has them; it’s that serum is depleted of them after coagulation. Serum is the acellular liquid after clotting, with the cells and the clot removed; a note about red blood cells is consistent with that, but the central distinction hinges on the presence or absence of clotting factors in the two fractions.