Accepted answer
It gives 8 mg/mL, and whether that is sensible depends on the dose you will draw from it. 40 ÷ 5 = 8 mg/mL in 0.9% sodium chloride. A 0.5 mg dose is then 6.3 units on a U-100 barrel and a 1 mg dose is 12.5 units. Both land in a readable part of the barrel, which is the whole point of choosing the volume deliberately.
Aim for a dose volume somewhere between about 10 and 30 units on a U-100 barrel and the reading problem disappears.
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.
Concentration and unit conversion at a glance
| Vial | Diluent | Concentration | 0.25 mg | 0.5 mg | 1 mg | 2.5 mg |
|---|
| 5 mg | 1 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 5 mg | 2 mL | 2.5 mg/mL | 10 u | 20 u | 40 u | 100 u |
| 10 mg | 1 mL | 10 mg/mL | 2.5 u | 5 u | 10 u | 25 u |
| 10 mg | 2 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 10 mg | 3 mL | 3.33 mg/mL | 7.5 u | 15 u | 30 u | 75 u |
Units are U-100 insulin units, where 1 unit = 0.01 mL. Divide dose by concentration for millilitres, then multiply by 100.
The underlying point is that for a dose that will change during titration, choose the volume for the largest intended dose rather than the first, so the whole schedule fits on one barrel without a mid-vial recalculation.
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.
Write the concentration on the label at reconstitution, in units per dose.
5The dead-space number surprised me until I did the multiplication across twenty draws. – rhian_prydderch 10 months ago 4Reading the leading edge of the stopper rather than the shoulder is worth a sentence of its own. – fresh_bac 8 months ago add a comment