23g needles, also known as butterflies, are used when a person’s vein is much narrower than average. It is light blue color coded. Despite the phlebotomist’s efforts to anchor the vein, the patient may be unwell or have minimal sites to choose a vein from, requiring a smaller needle.
Small children and infants are the most common patients to be drawn on using a 23g needle also as their veins are naturally much thinner than those of an adult. Some adult veins, however, do require the use of a butterfly.
23g needles are part of a winged infusion system (butterfly), not the multi-sample needle ETS system. Needles smaller than a 23g might hemolyze the red blood cells, and the sample could not be processed for testing. For instance, a 25g needle is better suited for intramuscular injections than blood draws.