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3 marks of the 15 in Part A, which runs for 30 minutes.
An injection question gives you an ampoule or a vial and asks for a volume. Reconstitution is where these get interesting, because the concentration you divide by is the one after the diluent has gone in, not the amount printed on the vial.
What this section asks about
Drawing up part of an ampoule, given a strength per millilitre
Reconstituting a powder and working out the concentration that results
Doses per kilogram that then have to become a volume
Recognising a volume too large for one intramuscular site
Where the marks go
3 marks of the 15 in Part A, which runs for 30 minutes.
Every numeracy question sits at the apply and analyse level: you are given a situation and asked to work something out, rather than to recall a fact.
Worked examples
The method, with the answer shown. Practise below once you have the shape of it.
Prescribed 60 mg. The ampoule reads 80 mg in 2 mL. What volume?
60 / 80 x 2
Answer: 1.5 mL
A 1 g vial is reconstituted to a final volume of 4 mL. Prescribed 750 mg. What volume?
Work from the concentration after reconstitution, not the powder.
1 g in 4 mL is 1,000 mg in 4 mL.
750 / 1,000 x 4
Answer: 3 mL
The powder displaces some volume, so the water added and the final volume are not the same number. Use the final volume the label states.
Prescribed 600 mg intramuscularly. The stock is 150 mg in 1 mL. What volume, and what does it mean?
600 / 150 x 1
Answer: 4 mL, which is more than one site will take
The arithmetic is the easy half. More than 2 to 3 mL into one intramuscular site in an adult is not given, so the answer is a volume and a decision to split it.
Where candidates lose marks
Dividing by the vial contents instead of the concentration after reconstitution.
Ignoring what the answer means in practice. More than 2 to 3 mL in one intramuscular site in an adult is divided between two sites.
Losing a decimal place on a small volume. 0.25 mL and 2.5 mL are a tenfold difference.
Sample questions
From our bank, with the explanation as a student sees it. These particular questions are kept out of the mock papers, so reading them here does not use anything up.
Sample 1 of 1
A patient is prescribed 450 mg of cefuroxime IV. The vial contains 750 mg of cefuroxime powder. After reconstitution with 6 mL water for injection, what volume in mL should be drawn up? (Give answer to one decimal place.)
The question gives you what you need. If it tells you the reconstituted concentration, use it; if it tells you the diluent volume, work the concentration out from it.
Are injection questions harder than tablet questions?
They have more steps, which is where the marks go rather than in the arithmetic. Write the steps down on the note board instead of holding them in your head.
Practise this section
Practice is filtered by section, so you can work on this one on its own. Every account gets 10 new questions a day without a card.