What is the primary characteristic of serum?

The primary characteristic of serum is that it is the liquid portion of blood that has been allowed to clot. When blood is drawn and allowed to clot, the solid components, or clot, separate from the liquid portion, resulting in serum. This serum contains various substances, including proteins, electrolytes, hormones, and waste products, but does not include clotting factors since these are consumed during the clotting process.

In contrast, other options refer to components that do not accurately describe serum. For instance, the presence of clotting factors is characteristic of plasma, not serum. Plasma is the liquid portion of blood that has not clotted, which contains clotting proteins. Additionally, serum does not contain platelets, as these are part of the cellular components eliminated when clotting occurs. Therefore, the definition of serum revolves specifically around its composition after coagulation has taken place.