Randomized problems for powdered vials. Each label tells you how much diluent to add and the concentration you get after mixing. Your job is to use the right one of those two numbers. Hard mode writes the order in grams.
Mix the vial as the label directs, then use the concentration after mixing, not the diluent volume: mL = ordered dose ÷ concentration. A 1 g vial mixed with 3.4 mL to make 250 mg/mL, with 500 mg ordered: 500 ÷ 250 = 2 mL.
Powder takes up space. When you add 3.4 mL of sterile water to a vial with 1 g of powder, you end up with about 4 mL of solution, not 3.4. The label accounts for this and gives you the final concentration, here 250 mg/mL. Work from that concentration and the displacement takes care of itself.
Students who use the diluent volume instead get 500 ÷ 1,000 × 3.4 = 1.7 mL, which is a 15% underdose. It is the most common reconstitution error on exams, and the practice tool calls it out by name when you make it.
| Order and label | Setup | Answer |
|---|---|---|
| 500 mg; 1 g vial; add 3.4 mL → 250 mg/mL | 500 ÷ 250 | 2 mL |
| 375 mg; 1 g vial → 250 mg/mL | 375 ÷ 250 = 1.5 | 1.5 mL |
| 0.75 g; 2 g vial → 330 mg/mL | 750 ÷ 330 = 2.27 | 2.3 mL |
| 175 mg; 500 mg vial → 100 mg/mL | 175 ÷ 100 = 1.75 | 1.8 mL |
Some vials offer a choice: a small amount of diluent for a concentrated solution, often for IM use where volume is limited, or a larger amount for a weaker solution meant to be diluted further for IV use. Read which instruction matches the route that was ordered, mix with that one, and use its concentration. Using the IM concentration with the IV volume, or the reverse, is a large error in either direction.
A multi-dose vial you have reconstituted gets a label: the date and time you mixed it, the concentration, and your initials. The prescribing information says how long it stays stable and whether it must be refrigerated. On an exam, a question may ask how many doses the vial holds: divide the total drug in the vial by the ordered dose and round down, since a partial dose cannot be given.
Rounding follows the usual volume rule: the nearest tenth above 1 mL, the nearest hundredth below it. Before you draw up, compare the volume with the vial: if the dose needs more than the whole vial holds after mixing, recheck the order, the concentration and your units.
Reconstitute as the label directs, then divide the ordered dose by the concentration after mixing. 500 mg ordered from a solution of 250 mg/mL is 2 mL.
The powder adds volume when it dissolves, so the final volume is larger than the diluent you added. The label's concentration after mixing already allows for this. Using the diluent volume underdoses the patient.
It is the extra volume the powder adds once dissolved. A 1 g vial mixed with 3.4 mL of diluent may end up as about 4 mL, so 0.6 mL is displacement.
The date and time it was mixed, the concentration, and your initials, plus any storage instruction from the prescribing information.