Vacutainer needle is a special double ended needle-it is usually screwed into a large hub, and has a needle on both ends-one end is inserted into the vein, other end into vacutainer which fits into the hub.
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Note how the little finger is poised to push the vacutainer into the needle inside the hub to fill the tube. No plunger needed as the vacuum within the tube will pull in blood on its own-when vacuum used up tube is filled.
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Once tube is filled, tube is removed from the needle, then entire assembly is removed.
If the tube’s vacuum has been breached or leaked…it wont work this is common in old tubes-so check the date!
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Butterfly adapters are commonly used to draw blood from small veins to in fractious animals who prefer less direct contact, these are attached to syringes or vacutainer tubes as shown below