The tubes used for blood collection are in a vacuum which forces blood from the vein through the needle and into the tube. The various tubes are covered with a standardized color-coded plastic cap that indicates the additives that each tube contains. These additives can include anticoagulants (EDTA, sodium citrate, heparin), gels with intermediate density between blood cells and serum, or other substances that preserve certain chemicals or substances within the blood, which preserve the blood for processing in the medical lab. We provide here a comprehensive list to ensure that the proper tube selection is made for its corresponding test, since using the wrong one may render the blood sample unusable. Source
