💉 Needles and syringes

Gauge by task — and why green means two different things

Needle gauge runs the opposite way to intuition: the bigger the number, the thinner the needle.

The trap is that needles and cannulae use the same colours for different gauges. A green needle is 21G. A green cannula is 18G. Blue needle 23G, blue cannula 22G. Asking for "a green one" across the room gets you whichever the other person was thinking of.

Needles

18G	PinkDrawing up only, especially viscous drugs and glass ampoules. Change the needle before injecting.
21G	GreenVenepuncture, IM injection in an adult, drawing up. The default.
23G	BlueIM in a small adult or child, subcutaneous, local anaesthetic infiltration.
25G	OrangeSubcutaneous, intradermal, skin wheal before a bigger needle.
27G	GreyIntradermal, fine work.

Syringes

1 ml	Insulin, heparin, small dosesInsulin syringes are marked in UNITS. Never draw insulin with anything else.
2–5 ml	Most IV drugs
10 ml	Flushes, IV pushThe minimum size for flushing a central line — a smaller syringe generates higher pressure and can rupture it.
20–50 ml	Bladder washout, syringe drivers, aspiration

Injection route

Intramuscular	21–23G, 90°Deltoid up to 1 ml; ventrogluteal or vastus lateralis for larger volumes.
Subcutaneous	25G, 45–90°Abdomen, outer thigh, upper arm. Rotate sites for anything repeated.
Intradermal	25–27G, 10–15°Mantoux, local wheal. A bleb should form — if it does not, you are too deep.

The thing that goes wrong

Never re-sheath a needle, and dispose of it yourself, into a sharps bin you can see before you start. Most needlestick injuries happen to the person who did not do the procedure.

If you do sustain one: encourage bleeding, wash with soap and water, do not scrub, then report immediately to occupational health or A&E out of hours. PEP for a high-risk exposure is time-critical.

Source: ISO 6009 needle colour coding; RCN injection technique standards

💉 Needles and syringes

Needles

Syringes

Injection route

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