Alan Nathan offers guidance on calculations in the RPSGB registration examination
One of the most important skills of a pharmacist is performing calculations, necessary not only for working out doses and quantities for formulae and dilutions but, particularly in hospitals, performing or checking calculations for doctors and nurses administering medicines, and carrying out pharmacokinetic calculations.
To help ensure that pharmacists are competent for these tasks, calculations form part of the RPSGB’s registration examination. A brief explanation of the exam and the question formats, plus some sample questions, are provided in this article. It is hoped that it will give an idea of what the exam is like to pharmacists who qualified before the registration examination began some 15 years ago, as well as providing a little extra practice for pre-registration trainees ahead of their exam at the end of this month.
Structure of the examination
The examination consists of two multiple choice question (MCQ) papers, taken on the same day. The first, in the morning, is a ‘closed book’ paper (ie no reference materials are allowed), and contains 90 questions to be answered in 90 minutes.
The second, in the afternoon, is an ‘open book’ paper for which candidates may use the BNF, Drug Tariff and Medicines, Ethics and Practice and the Professional Standards and Guidance Documents booklet that accompanies it, as references.
This paper contains 80 questions to be answered in 150 minutes, and includes a 20-question section of calculation questions, to be done without calculators.
Structure of the questions
Four types of question are used; all require one correct option to be selected from five possibilities.
1. Simple completion
A question is followed by five suggested answers (A to E) from which the correct one must be selected.
2. Multiple completion
A question or statement is followed by three options, numbered 1-3, one or more of which is or are correct. Candidates must choose if:
• 1, 2 and 3 are correct (answer A)
• 1 and 2 only are correct (B)
• 2 and 3 only are correct (C)
• 1 only is correct (D)
• 3 only is correct (E)
(Options 1 and 3 only and 2 only cannot be correct answers)
3. Classification
These are groups of between two and four questions, consisting of three to five lettered headings (A,B,C etc) followed by a list of two to four numbered questions (most often five headings followed by three questions). For each numbered question, the one heading that is most closely related to it must be selected. Each heading may be used once, more than once, or not at all.
4. Assertion/reason
Questions consist of a statement in the left hand column of the page followed by a second statement in the right hand column. The candidate must decide whether the statements are true or false and then choose:
• If both statements are true and the second statement is a correct explanation of the first statement (ie if both can be linked with “because”) (Answer A)
• If both statements are true but the second statement is not a correct explanation of the first statement (B)
• If the first statement is true but the second statement is false (C)
• If the first statement is false but the second statement is true (D)
• If both statements are false (E)
Alan Nathan is a former member of the RPSGB’s registration examination board
Questions
All the necessary information to answer the questions is included within them. The five questions should be answered, without a calculator, within 10 minutes.
1. Simple completion
You are asked to prepare 250ml of a solution of potassium permanganate such that 10ml diluted to 1 litre will give a 1 in 10,000 solution. How much potassium permanganate do you require?
A. 2.5g
B. 10g
C. 25g
D. 100g
E. 250g
2. Multiple completion
A woman with eclampsia has been prescribed an intravenous infusion of magnesium sulphate. Below is an extract from her prescription:
“Magnesium sulphate 20% in 250ml sodium chloride 0.9%.
A loading dose of 4g magnesium sulphate to be given over 10 minutes, followed by 1g every hour.”
A nurse asks you to check her calculations. Which of the following is/are correct?
1. To give 4g of magnesium ions over 10 minutes, the infusion rate should be set at 120ml/hr
2. To give 1g of magnesium ions every hour the infusion rate should be set at 30ml/hr
3. The first 250ml infusion bag should last for a maximum of
4 hours 30 minutes
3. Classification (For reasons of space, only one of the three questions that would normally be set is given here)
Possible answers:
A. 0.02g
B. 0.2g
C. 2g
D. 20g
E. 200g
Select from A to E. Which one of the above is the correct quantity to prepare?
1. 200ml of a 1 in 1,000 solution of chlorhexidine gluconate
2. 2 litres of a 0.01% solution of chlorhexidine gluconate
4. Assertion/reason
A local GP occasionally writes prescriptions for Gentian Mixture, Alkaline BP. You keep all the ingredients in stock and make up the mixture extemporaneously in response to individual prescriptions. You receive a prescription for 500ml of the mixture. The formula requires 5ml of double-strength chloroform water per 10ml dose. You do not keep double-strength chloroform water but use concentrated chloroform water instead. (Chloroform water contains 0.25% chloroform. Concentrated chloroform water contains 10ml chloroform/100ml.)
First statement: You need 50ml of concentrated chloroform water for 500ml of Gentian Mixture, Alkaline BP.
Second statement: Concentrated chloroform water is 20 times stronger than double-strength chloroform water.
Worked answers
1. Simple completion
Directions: Each of the questions in this section is followed by five suggested answers. Select the best answer in each case.
You are asked to prepare 250ml of a solution of potassium permanganate such that 10ml diluted to 1 litre will give a 1 in 10,000 solution. How much potassium permanganate do you require?
A. 2.5g T
B. 10g F
C. 25g F
D. 100g F
E. 250g F
Have to prepare a concentrated solution that the patient will dilute before use. ‘Dilution factor’ = 10ml g 1litre = 10ml g 1,000ml = 100
Therefore strength of concentrated solution = 1 in 10,000 x 100 = 1/10,000 x 100 = 1/100 = 1g in 100ml
Therefore for 250ml, need 250/100 x 1 = 2.5g
2. Multiple completion
A woman with eclampsia has been prescribed an intravenous infusion of magnesium sulphate. Below is an extract from her prescription:
“Magnesium sulphate 20% in 250ml sodium chloride 0.9%.
A loading dose of 4g magnesium sulphate to be given over 10 minutes, followed by 1g every hour.”
A nurse asks you to check her calculations. Which of the following is/are correct?
1. To give 4g of magnesium ions over 10 minutes, the infusion rate should be set at 120ml/hr T
Concentration of magnesium sulphate = 20% = 20g in 100ml
To give 4g (in 10 mins), volume required = 4/20 x 100 =20ml
20ml in 10mins = 20 x 6 = 120ml/hr
2. To give 1g of magnesium ions every hour the infusion rate should be set at 10ml/hr F
To give 1g (in 1 hr), volume required = 1/20 x 100 = 5ml/hr
3. The first 250ml infusion bag should last for a maximum of
4 hours 30 minutes F
250ml bag contains 2.5 x 20 = 50g magnesium sulphate
Infusion rate = 4g in 10 mins, then 1g/hr
After 10 mins bag contains: 50 - 4 = 46g
46g @ 1g/hr = 46 hours
Therefore contents of bag will last 46 hours 10 mins
Key = D (1 only correct)
3. Classification
Directions: Each group of questions below consists of five lettered headings followed by a list of numbered questions. For each numbered question select the one heading which is most closely related to it. Each heading may be used once, more than once, or not at all.
A. 0.02g
B. 0.2g
C. 2g
D. 20g
E. 200g
Select from A to E. Which one of the above is the correct quantity to prepare?
1. 200ml of a 1 in 1,000 solution of chlorhexidine gluconate
1 in 1,000 = 1g in 1,000ml
Therefore in 200ml = 200/1,000 = 1/5 = 0.2g
Key = B
2. 2 litres of a 0.01% solution of chlorhexidine gluconate
0.01% = 0.01g in 100ml
Therefore for 2 litres, require 2,000/100 x 0.01 = 20 x 0.01
= 0.2g
Key = B
4. Assertion/reason
A local GP occasionally writes prescriptions for Gentian Mixture, Alkaline BP. You keep all the ingredients in stock and make up the mixture extemporaneously in response to individual prescriptions. You receive a prescription for 500ml of the mixture. The formula requires 5ml of double-strength chloroform water per 10ml dose. You do not keep double-strength chloroform water but use concentrated chloroform water instead. (Chloroform water contains 0.25% chloroform. Concentrated chloroform water contains 10ml chloroform/100ml.)
First statement: You need 50ml of concentrated chloroform water for 500ml of Gentian Mixture, Alkaline BP.
Amount of double strength chloroform water required would be: 500 x 5/10 = 250ml. From information given, concentrated chloroform water is 40 times stronger than chloroform water (10%/0.25%), therefore 20 times stronger than double strength chloroform water. Therefore, quantity of concentrated chloroform water required is: 250/20 = 12.5ml
Therefore, statement is False
Second statement: Concentrated chloroform water is 20 times stronger than double-strength chloroform water.
True (see calculation above)
Key = D
