Accepted answer
At 1 mg/mL a 1 mg dose is 1 mL — 100 units on a U-100 barrel — and no needle gauge changes that number. Gauge changes three other things: how long the draw takes, how much stays behind in the hub, and how much rubber you core out of the stopper. On the 30G scale a larger number is a finer needle, so a 30G needle is fine enough that a viscous solution draws slowly and a hurried draw pulls bubbles. If you are drawing 100 units at a time, the dead space matters more than the bore: a fixed-needle barrel loses microlitres, a luer hub loses tens of them, and at 1 mg/mL each microlitre is 1 µg.
Answering this needs to know the viscosity of what is being drawn, since a viscous solution through a fine needle is slow enough to encourage bad technique.
Drawing a viscous or foamy solution through a fine needle takes long enough that people rush the plunger, which causes more foaming. Using a wider drawing needle is the fix.
Very fine needles are more prone to bending and to blocking with any particulate, which is a practical argument for inspecting the solution before drawing.
Needle gauge to outer diameter correspondence is standardised and published; the inverse relationship between gauge number and diameter is the reason for the counter-intuitive labelling.
Angle the bevel and insert gently to avoid coring the stopper.
4Small correction: the units in the third paragraph should be micrograms, not milligrams. – kwn_analytical 13 days ago 5Would this be different for a peptide that foams? Mine does and I have never known why. – Dr_Lena_Ostrowska 2 months ago add a comment