The following question relates to the ‘calculation’ sections of the General Pharmaceutical Council (GPhC) registration assessment framework. It is designed to help you practise calculations you may be asked to perform when checking and dispensing prescriptions as a pre-reg or, if you are already a pharmacist, to help you support your pre-reg as they prepare for the assessment.
Question: quantity of amoxicillin suspension to supply
A three-year-old girl, weighing 14kg, is prescribed amoxicillin 250mg/5ml oral suspension BP at a dose of 420mg three times daily x 252ml for acute otitis media. The instructions advise adding 64ml of potable water and shaking until all contents are dispersed. The reconstituted suspension has a shelf-life of seven days and should be stored at 2-8°C.
Click here for the correct working out and answer
This is one method to work out the answer to this calculation:
Reconstituted antibiotic concentration = 250mg/5ml = 50mg/1ml
As mentioned in the question, 50mg of antibiotic is in each millilitre. We can work out how many millilitres (V1) per dose (420mg) are required.
420mg/V1 = 50mg/1ml
420mg = (50mg/1ml) x V1
(420mg x 1ml)/50mg = V1
V1 = 8.4ml
To calculate how many days the suspension lasts, you need to work out total daily volume used.
8.4ml x 3 doses per day = 25.2ml per day
The total volume prescribed is 252ml
252ml/25.2ml = 10 days
- The question is asking for the number of days of treatment
- Additional information is available in the question, such as patient age, volume of potable water and fridge temperature. You should ensure you recognise the information relevant to the question – you may want to highlight or circle key figures.
An example answer grid with instructions on how to use the grid can be found here.
Babir Malik is a teacher practitioner at Bradford University and lead at the Green Light Campus masterclass.
Mr Malik does not set or write questions for the GPhC registration assessment and he does not imply that these calculations will come up in the exam. However, the questions relate to the GPhC framework. The author adds that there may be other ways to work out these questions, but it does not matter how you work out a question as long as you get to the correct answer.
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