Personally, the choice of the size of the IV should depend upon the condition of the patient. A critical patient warrants a larger bore IV due to the possibility of fluid resuscitation, or some potentially irritating medications.
If the patient is there for a less critical reason, a 20 would do fine. I've used 22's and 24's in the elderly with those fragile spider veins that blow the minute you hit them.
I believe proper cleansing of the size and proper dressing is more important for infection than the size of the cannula.
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