Final answer:
The fluid portion at the top of a centrifuged blood sample that has been allowed to clot is known as serum. Serum differs from plasma in that it does not contain clotting factors, which are consumed during the blood clotting process.
Step-by-step explanation:
When a blood sample is centrifuged after clotting, the fluid portion that separates at the top of the tube is called serum. During the clotting process, the cellular elements and clotting factors such as fibrinogen come together at the site of injury or, in this case, at the bottom of the centrifuge tube. The serum is essentially the portion of blood plasma without the clotting factors, as they are consumed during the clotting process or trapped in the blood clot.
The plasma is the liquid component of blood that is present before the blood has clotted. It contains water, electrolytes, antibodies, and coagulation factors. When blood is allowed to clot prior to centrifugation, the coagulation factors are consumed, meaning the top liquid layer after centrifugation lacking these factors is defined as serum, not plasma. This is an important distinction as serum is used in various diagnostic tests where clotting factors could interfere with the results.