Accepted answer
At 10 mg/mL a 0.25 mg draw is 2.5 units on a U-100 barrel and a 2.4 mg draw is 24. Those two numbers decide it, because the concentration is only sensible relative to the smallest and largest volumes you will actually measure with it. 2.5 units is too little of the scale to read honestly — half a graduation is 20 per cent of that dose — so going lower in concentration buys resolution you cannot get back after reconstitution. The other consideration is time: a vial you will finish in a fortnight can be concentrated, and a vial you will draw from for months should be split at reconstitution instead.
Total vial volume is a physical constraint: most 2 mL vials will not take 3 mL of anything.
Vial headspace is the hard constraint. A nominal 2 mL vial typically holds a little over 2 mL to the shoulder; adding 3 mL is not an option and attempting it wastes the lot.
Worked example. A 10 mg vial reconstituted with 2 mL gives 5 mg/mL. A 0.5 mg dose is 0.5 ÷ 5 = 0.1 mL, which on a U-100 syringe is 10 units. Reconstitute the same vial with 1 mL and the concentration doubles to 10 mg/mL, the same dose becomes 0.05 mL, and you are now reading 5 units instead of 10 — the same dose at half the resolution.
Insulin syringe barrel graduations are typically 1 unit on a 0.3 mL barrel, 1 unit on a 0.5 mL barrel and 2 units on a 1 mL barrel, which is why the barrel size changes what is readable.
Write the concentration on the label at reconstitution, in units per dose.
edited 5 Feb 2025 by lyoph_cake — added a caveat about sampling