The formula behind the number
Reconstitution math is two steps. First you find the concentration of the reconstituted vial, then you find the volume that carries the dose. The syringe reading is a third step only because insulin syringes are marked in units rather than millilitres.
- Concentration (mg/mL) = vial strength in mg ÷ bacteriostatic water in mL.
- Volume (mL) = dose in mg ÷ concentration in mg/mL. A dose given in mcg is divided by 1,000 first.
- Units on a U-100 syringe = volume in mL × 100.
A worked example
A 5 mg vial reconstituted with 2 mL of bacteriostatic water, against a 250 mcg dose set by a healthcare professional:
| Step | Calculation | Result |
|---|---|---|
| Concentration | 5 mg ÷ 2 mL | 2.5 mg/mL |
| Dose in mg | 250 mcg ÷ 1,000 | 0.25 mg |
| Volume | 0.25 mg ÷ 2.5 mg/mL | 0.1 mL |
| Syringe units | 0.1 mL × 100 | 10 units |
So the draw is to the 10 unit mark. Change the water volume and the mark moves: the same 5 mg vial reconstituted with 1 mL instead gives 5 mg/mL, and the same 250 mcg dose lands at 5 units.
How to reconstitute a vial
- 1Read the vial's total strength
The label shows total peptide in the vial, in mg. It is the whole vial, not one dose.
- 2Set the bacteriostatic water volume
This volume, set with your healthcare professional, fixes concentration. More water gives a lower, easier draw.
- 3Add the bacteriostatic water slowly
Aim the stream at the vial wall, not the powder, since peptides are fragile in solution.
- 4Let it dissolve without shaking
Swirl gently or simply wait. Agitation is generally avoided.
- 5Calculate the draw in units
Divide strength by water for concentration, divide dose by that, then multiply by 100 for units.
- 6Record the reconstitution for later
Strength, water volume, and date make every later dose calculable. Peptmate stores this setup once.
Common reasons the number looks wrong
- Dose entered in mg instead of mcg. A 250 in the mcg field is 0.25 mg. A 250 in a mg field would be a thousandfold different.
- Vial strength read as a per-dose amount. The mg on the label is the whole vial.
- A syringe that is not U-100. A U-40 or U-50 barrel uses a different scale, so the × 100 step does not apply.
- Water volume estimated rather than measured. The concentration is only as accurate as the diluent volume, and every later draw inherits that error.
