Serum is the blood fluid freed of cells and clotting proteins. It is the supernatant liquid obtained after coagulation and centrifugation of the blood in a "dry" tube, i.e. without coagulation inhibitors. Unlike plasma, which is obtained by simple centrifugation without prior coagulation (i.e. taken from a tube containing anticoagulants), serum is free of coagulation factors and fibrinogen, which are consumed by coagulation. Thus, plasma is the liquid component of blood that contains serum and clotting elements. When the latter are eliminated, only the serum remains.

Unlike plasma, serum contains neither red nor white blood cells nor fibrinogen. It is poor in proteins, particularly those that form the other coagulation factors (which it contains in small quantities). Fibrinogen or "factor I" (FI: factor one) is a coagulation factor, a protein in blood plasma that turns into fibrin during coagulation. This fibrinogen will pass into the form of fibrin, which will allow coagulation