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.”