Key takeaways
- Milligrams from the old vial = units left ÷ 100 × old concentration.
- Remainder = dose − milligrams from the old vial.
- Units from the new vial = remainder ÷ new concentration × 100.
- Do not add the unit counts together to check. Add milligrams.
What this tool does
When an old vial has less left than one dose and the new vial is a different concentration, you draw part of the dose from each. This tool works out the units from each vial so the milligrams add up to the dose.
How the arithmetic works
- Old part (mg) = units remaining ÷ 100 × old concentration.
- New part (mg) = dose − old part.
- New units = new part ÷ new concentration × 100.
A worked example
Example. These numbers are made up to show the steps. They are not a label value and not a suggestion.
Made-up: dose 5 mg. Old vial 10 mg/mL with 20 units left. New vial 5 mg/mL.
- Old part = 20 ÷ 100 × 10 = 2 mg.
- New part = 5 − 2 = 3 mg.
- New units = 3 ÷ 5 × 100 = 60.
20 + 60 = 80 units, but that is not a check. 2 mg + 3 mg = 5 mg is.
What to check against your label
- Both concentrations, read off the vials.
- The units really left in the old vial.
- Whether your pharmacy or prescriber is happy with mixing from two vials in one dose. Some prefer you discard the remainder. [verify against pharmacy or label guidance]
- Beyond-use dates for both vials.
Frequently asked questions
Why not just use the new vial?
You can. The old remainder is then waste. This is a choice.
Does it matter which vial is drawn first?
The sums are the same. Follow whatever your pharmacy told you.
Is this for any medicine?
The arithmetic is. The decisions are medical.
How Reko does this in the app
Reko tracks what is left in each vial from the doses you log. When one runs low, you can see how much is left to plan the switch. The vial tracker is at the vial tracker page.
Reko records what you and your prescriber decide. It never suggests a dose, and this page is not medical advice.
- Pharmacy instructions for your product. [link]