Accepted answer
It gives 5 mg/mL, and whether that is sensible depends on the dose you will draw from it. 10 ÷ 2 = 5 mg/mL in 0.9% sodium chloride. A 0.5 mg dose is then 10 units on a U-100 barrel and a 1 mg dose is 20 units. Both land in a readable part of the barrel, which is the whole point of choosing the volume deliberately.
Start from the dose you intend to draw and work backwards to the volume that puts it in a readable part of the barrel.
The other direction: 10 mg in 3 mL is 3.33 mg/mL, and a 0.5 mg dose becomes 0.15 mL, or 15 units. More barrel, easier reading, and a larger fraction of the vial volume lost to dead space across the same number of draws.
Dead space by syringe type
| Configuration | Dead volume | Loss at 5 mg/mL | Over 20 draws |
|---|
| Fixed-needle insulin syringe | 3–5 µL | 15–25 µg | 0.3–0.5 mg |
| Low-dead-space, detachable | <2 µL | <10 µg | <0.2 mg |
| Standard luer-lock + 30G | 35–60 µL | 175–300 µg | 3.5–6 mg |
| Luer-lock + 21G drawing needle | 70–100 µL | 350–500 µg | 7–10 mg |
Concretely, content matters. If the same 10 mg vial assays at 94 per cent content, you have 9.4 mg. In 2 mL that is 4.7 mg/mL, and a nominal 0.5 mg draw of 10 units actually delivers 0.47 mg — a six per cent shortfall that no amount of careful drawing will fix.
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.
A concentration calculated to three decimal places from a diluent volume measured to one is false precision.
Check the vial can physically hold the volume before you draw it up.
I have seen exactly this failure mode twice and both times it was the diluent volume. – esther_vandeVelde 6 months ago Reading the leading edge of the stopper rather than the shoulder is worth a sentence of its own. – thabo_maseko 5 months ago add a comment