Describe the main difference between serum and plasma when interpreting some laboratory results.

• A. Serum contains all clotting factors; plasma does not.
• B. Serum and plasma are identical in composition.
• C. Serum lacks clotting factors; plasma contains anticoagulant and coagulation factors; some analytes differ slightly due to fibrinogen and sample type.
• D. Plasma is derived after clotting.

Answer: (D)

The main idea is that whether the blood is allowed to clot or kept from clotting determines what the liquid portion contains. Serum is what you get after blood is drawn and allowed to clot, then the clot is removed; it lacks fibrinogen and other clotting factors that are consumed during clot formation. Plasma is obtained from blood drawn with an anticoagulant and then centrifuged before clotting occurs, so it still contains fibrinogen and the full set of clotting factors.

Because of this difference, some analytes can differ slightly between serum and plasma, partly due to the presence of fibrinogen and other clotting proteins in plasma and the effects of the anticoagulant used. Coagulation tests (like PT and aPTT) specifically require plasma because they depend on those clotting factors.

So the key distinction is that serum lacks clotting factors because clotting has occurred, while plasma contains anticoagulant-preserved clotting factors. The idea that plasma is derived after clotting is not correct.