Winged Infusion Sets, or "Butterfly" needles are very small needles used in venipuncture (phlebotomy) of very young or elderly patients or patients with small veins and arteries. They are often used with people who have spasticity.
Since the butterfly needle uses a flexible tube, there is less chance of the patient causing damage if they move during the procedure. Butterfly needles are usually 23 gauge needles, with two plastic 'wings' on either side, flexible tubing, and another needle, generally sheathed with some sort of rubber like material at the other end. The phlebotomist attaches a hub or a syringe to the end of the set that has the sheathed needle. This is the same type of hub used for a routine venipuncture. The needle is held by the 'wings' and placed into the vein, generally at a fairly shallow angle. The wings allow the phlebotomist to grasp the needle very close to the end, to ensure accuracy. When the needle is successfully placed into the vein, a 'flash' can be seen. The 'flash' is a small amount of blood that flows into the tubing when the needle enters a vein. The phlebotomist can then push vacuum tubes into the hub and draw blood as usual.
Butterfly needles are most commonly used when available veins are very small, fragile, or when they are in a place that would make using a normal evacuated tube system difficult. Butterfly needles are also used on very shallow veins, because the design allows the needle to be inserted at a much more shallow angle than an evacuated tube system allows. Butterfly needles are nearly always used when drawing blood from the hand, wrist, or other places on the body where veins are very close to the skin. Since a butterfly needle doesn't pull quite so hard, and the needle is smaller, the blood flows more slowly in most cases, which can reduce the chances of delicate veins collapsing. It is advisable, however, to use a syringe with a butterfly, rather than vacuum tubes directly, when drawing tubes that require a full draw, such as light blue sodium citrate tubes (which are most commonly used for coagulation tests). This is due to the fact that a small amount of blood will remain in the tubing when there is no vacuum remaining in the tube.