EQUUS - - Eq Tack& Gear -

When your vet­eri­nar­ian draws blood from your horse for testing, you might no­tice that she’s fill­ing sev­eral dif­fer­ent tubes and care­fully la­bel­ing each one. You might also no­tice that she’s se­lect­ing tubes with rub­ber tops of dif­fer­ent colors.

There is a rea­son for this: Each color-coded tube comes pre­loaded with ad­di­tives to pre­pare the blood for dif­fer­ent types of testing. “We might use a tube that keeps the blood from co­ag­u­lat­ing, such as a pur­ple-top tube, for the CBC,” says Peggy Marsh, DVM, DACVIM, DACVECC, of Equine Med­i­cal As­so­ciates in Lex­ing­ton, Ken­tucky. Dif­fer­ent an­ti­co­ag­u­lants, in green or light blue tubes, might be used for other tests on blood plasma, which is the fluid that is left af­ter un­co­ag­u­lated whole blood cells have been re­moved via a cen­trifuge, leav­ing the platelets be­hind. Gray-topped tubes con­tain chem­i­cals that pre­vent the break­down of glu­cose.

A red-topped tube, which con­tains no an­ti­co­ag­u­lants, might be used to col­lect blood for serum tests, which is the fluid left be­hind af­ter all of the cells, in­clud­ing the platelets, have been re­moved by clot­ting. “Of­ten we’d keep just the serum when we want to look for cer­tain an­ti­bod­ies,” says Marsh. “There is a dif­fer­ence in the tests look­ing for an­ti­bod­ies ver­sus look­ing for the spe­cific or­gan­ism. This could re­quire a PCR [poly­merase chain re­ac­tion] test. For that, we need cells, so we’d col­lect blood for a PCR test in a pur­ple-top tube.”

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