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A vacutainer is a sterile glass tube with a colored rubber stopper creating a vacuum seal inside of the tube facilitating the drawing of a predetermined volume of liquid.
Vacutainer tubes may contain additives designed to stabilize and preserve the specimen prior to analytical testing. Tubes are available with a safety-engineered closure (rubber stopper), with a variety of labeling options and stopper colors as well as a range of draw volumes.

The purple top tube became an issue in the O.J. Simpson murder case when the defense alleged that small droplets of blood found at the crime scene contained the preservative EDTA. Had this been true, it would have meant that the droplet might have been taken from the purple top tube used to collect Simpson's blood and planted at the crime scene.

Drawing blood using a vacutainer. The clear plastic sheath around the top of the tube holds the needle. The orange guard will shield the needle after it is withdrawn from the patient to protect from accidental needle sticks.

Vacutainer needle are double ended, with one side being encased in a thin rubber coating for safety. When the needle is screwed into the translucent plastic needle holder, the rubber needle is inside the holder, and the exposed needle will be inserted into the vein. When a Vacutainer tube is inserted into the holder, its rubber cap is punctured by the inner needle and the vacuum in the tube pulls blood through the needle and into the tube. The filled tube is then removed and another can be inserted and filled the same way. The amount of air evacuated from the tube predetermines how much blood will fill the tube before blood stops flowing.