Reconstitution
Concentration after adding diluent to a powdered vial — and the volume to draw up.
Final volume 10 mL — label the vial with this concentration, the date, and the time.
Delivers 400 mg at 100 mg/mL.
Divide the amount of drug in the vial by the final volume after adding diluent. A 1000 mg vial reconstituted with 10 mL gives 1000 ÷ 10 = 100 mg/mL, so a 400 mg dose is 400 ÷ 100 = 4 mL. Add the powder’s displacement volume if the label states one.
Turning a powder into a measurable dose
Many drugs, particularly antibiotics, are supplied as a dry powder because they are unstable in solution. Before they can be given they are reconstituted: a stated volume of sterile water or sodium chloride is injected into the vial and swirled until dissolved. Only then does the vial have a concentration, and only then can a partial dose be drawn accurately.
Two numbers matter afterwards. The final concentration is what you write on the vial label, and the volume to draw is the ordered dose divided by that concentration. If the whole vial is being given, no second step is needed — draw the lot.
Displacement volume
The powder itself occupies space. Adding 10 mL of diluent to a vial whose powder displaces 0.5 mL leaves 10.5 mL of solution, not 10 mL, which lowers the concentration slightly. Manufacturers handle this in one of two ways: either the label states the displacement, or it states the diluent volume that yields a stated final concentration. Where the label tells you what the reconstituted vial will contain, use that figure directly and leave displacement at zero.
Final volume = diluent added + powder displacement. If the manufacturer states the concentration after a given diluent volume, use their figure rather than calculating it.
Worked example: 1000 mg vial, 10 mL diluent, 400 mg ordered
Reconstitute, work out the concentration, then draw the dose:
- 1 Read the vial strength. The label states 1000 mg of dry powder.
- 2 Add the stated diluent volume. The label directs 10 mL of sterile water. Inject it and swirl until fully dissolved — do not draw up a cloudy or particulate solution.
- 3 Work out the final volume. With no stated displacement, the final volume is 10 mL. If the label gave 0.5 mL displacement it would be 10.5 mL.
- 4 Divide to get the concentration. 1000 mg ÷ 10 mL = 100 mg/mL.
- 5 Divide the ordered dose by the concentration. 400 mg ÷ 100 mg/mL = 4 mL to draw up.
- 6 Label the vial. Write the concentration, the date, and the time of reconstitution on the vial before storing any remainder.
Concentration for common vial and diluent combinations
Concentration = vial amount ÷ final volume, assuming no stated displacement.
| Vial strength | Diluent added | Concentration | 250 mg dose is |
|---|---|---|---|
| 500 mg | 2 mL | 250 mg/mL | 1 mL |
| 500 mg | 5 mL | 100 mg/mL | 2.5 mL |
| 1 g (1000 mg) | 10 mL | 100 mg/mL | 2.5 mL |
| 1 g (1000 mg) | 4 mL | 250 mg/mL | 1 mL |
| 2 g (2000 mg) | 10 mL | 200 mg/mL | 1.25 mL |
| 250 mg | 5 mL | 50 mg/mL | 5 mL |
Stability, storage, and multi-dose vials
A reconstituted vial has a much shorter life than the dry powder — often hours at room temperature and a limited number of days refrigerated, depending on the drug. That is why the concentration, date, and time go on the label: the next person to use the vial needs both figures, and a vial without them cannot safely be used.
Where a dose needs more than one vial, reconstitute each vial as directed and combine the volumes rather than doubling the diluent in a single vial. And if the ordered dose is very close to a whole vial, check whether the protocol rounds to vial sizes — drawing 9.6 mL of a 10 mL vial wastes little but adds an avoidable measurement step.
This tool is a study aid for practising calculations. Reconstitution must follow the manufacturer’s instructions for that product, and doses must be independently verified by a qualified professional.