Accepted answer
At 6.67 mg/mL a 1 mg dose is 0.15 mL — 15 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 27G scale a larger number is a finer needle, so a 27G needle is fine enough that a viscous solution draws slowly and a hurried draw pulls bubbles. If you are drawing 15 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 6.67 mg/mL each microlitre is 6.67 µg.
The short version: 21G to 23G to draw, 29G to 31G to inject, and never the same needle for both.
A 30G or 31G needle through a butyl stopper leaves a track that reseals, which is why fine-gauge repeated entry is tolerable and coarse-gauge repeated entry is not.
Reading a lyophilised cake
| Appearance | Interpretation | Action |
|---|
| Intact opaque puck, proud of base | Cycle ran correctly | Proceed |
| Slumped to one side | Shipped before fully dry, or vibration | Usually usable; note it |
| Glassy translucent film | Collapse above glass transition | Test before use |
| Melt-back ring at stopper | Thermal excursion in transit | Test before use |
| No visible cake at all | Very low fill, or nothing there | Weigh it; query the supplier |
Concretely, 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.
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.
If a solution will not draw through a 25G needle, the problem is the solution rather than the needle.
Gauge numbers run backwards. Higher number, thinner needle.