serum

Whole blood is allowed to clot in a tube without anticoagulant and then centrifuged; the supernatant is serum. Compared with plasma (collected in EDTA, citrate, or heparin), serum lacks fibrinogen and most clotting factors II, V, VIII, and XIII, which are consumed in the clot. Serum is used for electrolytes, BUN/creatinine, liver and cardiac enzymes, hormones, drug levels, and serologic antibody testing. Plasma is required when fibrinogen or coagulation factor measurement is needed (PT, PTT). A high-yield distinction is that the difference between plasma and serum is essentially the presence of fibrinogen and consumed coagulation factors.