Final answer:
The liquid portion of blood that remains after it has clotted in a collection tube is known as serum. Serum is plasma without the clotting factors, and it is obtained from clotted blood after centrifugation.
Step-by-step explanation:
The liquid portion of blood that remains after the blood has clotted in a collection tube is the serum. Blood plasma refers to the liquid component that remains after the removal of blood cells but still contains clotting factors, which include fibrinogen among other components necessary for the clotting process. In contrast, serum is derived from plasma but is the portion that lacks clotting factors because it is what remains after the blood has clotted and the fibrinogen has been consumed in the clotting process. Thus, the difference between serum and plasma is the presence or absence of coagulation factors.
When blood is drawn into a collection tube and allowed to clot, and then it is centrifuged, the solid clot is at the bottom, and the serum is the clear liquid on top. Serum is used for various diagnostic tests as it contains antibodies and other important constituents except for the clotting factors which are used up when the blood clots.