= Blood serum
<Blood serum> is the liquid recovered after <blood> has clotted and the clot and <blood cells> have been separated. In particular, <fibrinogen> has been converted into the <fibrin> network of the clot. <Blood plasma> is instead the liquid portion of blood whose clotting has been prevented, so it retains <fibrinogen>.<Serum in the NCI Dictionary of Cancer Terms>[Definition: liquid after removal of cells and clotting proteins]{ref}
A two-tube comparison makes the distinction concrete. Allow one sample to clot before <centrifugation>; prevent clotting in the other and then centrifuge it. Both yield liquid fractions, but the preparations have undergone different biochemical processes. <Blood serum> still contains many <proteins>, including <antibodies>; it is not a protein-free version of <blood>. Consequently an assay validated for one specimen type should not be interpreted as though sample preparation were irrelevant.
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