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The acellular fluid component of blood in which the cells are suspended. Serum differs from plasma in that the serum is derived from clotted blood and is therefore deficient or devoid of clotting factors.

It practical terms, serum is collected in tubes that either accelerate clotting, or simply allow the blood to clot naturally. The clotting process results in consumption of the clotting factors and formation of a clot. The sample is centrifuged causing the more dense clot to precipitate at the end of the tube. Being less dense, the straw-coloured serum remains at the top of the tube as the supernatant.

Agitation of the tube can cause the clot to disperse and mix with the serum. To prevent this, some collection tubes contain a gel which has an intermediate density between serum and the clot. Centrifugation causes the clot to migrate to the end of the tube, but it is separated from the serum by the gel layer, keeping the serum free from contamination by blood cells.

Blood tests that use serum, especially to detect antibodies, are loosely known as 'serology', although in some cases plasma can be used interchangeably.