What characteristic distinguishes serum from plasma?

Serum is distinguished from plasma primarily by its origin following the blood coagulation process. After blood is drawn and allowed to clot, the liquid that separates from the clot is known as serum. This is in contrast to plasma, which is the liquid component of blood that remains when an anticoagulant is added to prevent clotting.

Serum does not contain clotting factors, which are removed when the blood clots and the clot is then removed. This means that serum lacks fibrinogen and other coagulation proteins that are abundant in plasma, making it less viscous and free-flowing. This characteristic of serum being the liquid that remains after clotting plays a pivotal role in various laboratory tests and analyses where clotting factors might interfere with the results.

The other options describe various aspects of serum but do not accurately define the fundamental characteristic that distinguishes it from plasma. For instance, serum is not necessarily thicker than plasma; while it does often appear yellow due to the presence of bilirubin or other pigments, color can vary. Finally, serum does not contain anticoagulants since it is derived from clotted blood, while plasma contains these agents to prevent coagulation.