American Society for Clinical Pathology (ASCP) Histotechnician Certification Practice Exam

Which plasma protein is not found in serum?

1. Albumin

2. Globulin

3. Fibrinogen

4. Serum amyloid P

The correct answer is: Fibrinogen

Fibrinogen is a plasma protein that is not found in serum because it is involved in blood clotting and is consumed when a blood clot forms. When blood is allowed to clot, fibrinogen is converted to fibrin, which participates in the formation of the clot. After the clotting process, the liquid that remains is serum, which lacks fibrinogen and other clotting factors. Therefore, in serum, fibrinogen is absent, making it distinct from plasma, which still contains this protein. On the other hand, albumin and globulin are both proteins that remain in serum after the clotting process because they are not involved in coagulation. Serum amyloid P is also part of the serum protein composition but serves a different role related to immune response and inflammation, further establishing that fibrinogen is the only protein not found in serum post-clotting.