Concretely: 40 mg · 2 mL · 0.9% sodium chloride · cagrilintide.
I want to decide this in advance so that I am not deciding it under pressure later.
Assume I will follow the plan I write down, so I would like it to be a good one.
How would you structure this, and what thresholds would you set in advance?
It gives 20 mg/mL, and whether that is sensible depends on the dose you will draw from it. 40 ÷ 2 = 20 mg/mL in 0.9% sodium chloride. A 0.5 mg dose is then 2.5 units on a U-100 barrel and a 1 mg dose is 5 units. The smaller dose lands too low on the scale to read accurately — more diluent would buy resolution you cannot recover later.
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.
| 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.
Dead-space loss scales with the number of draws, not with the concentration, so a lower concentration spread over more draws loses proportionally less of the total peptide.
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.
edited 17 Dec 2024 by bac_or_bust — fixed an arithmetic slip in the third paragraph
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Browse resultsAnswering this needs the syringe you actually own, because the barrel graduations decide what "readable" means.
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.
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.
Nominal vial volumes in the standard 2R and 3R glass sizes have published brimful capacities well above the nominal fill, but the usable volume is bounded by the stopper displacement.
Do not change the diluent volume between vials of a titration without recalculating; it is the commonest source of a ten-fold error.
Check the vial can physically hold the volume before you draw it up.
Answer first: diluent volume sets concentration and therefore resolution on the syringe barrel, and resolution is free at reconstitution and impossible to recover afterwards.
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.
Write the concentration and the resulting units-per-dose on the vial label at reconstitution. The arithmetic that is obvious now will not be obvious at six in the morning three weeks from now.
U-100 means 100 units per millilitre by definition, so 1 unit is 0.01 mL and volume in millilitres times one hundred gives units. Every conversion here reduces to that.
Nothing here is medical advice, and research-use material is not approved for human use.
Choose the volume that puts your largest intended dose between 10 and 30 units. Everything else follows.
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