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Introduction to Pharmaceutical

Calculations

FTY 2021/2022 Development day 1: PM session

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Aim

To provide an overview of the Common Registration Assessment (CRA) Framework and practice opportunities for the range of calculation types encountered within the CRA.

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Learning outcomesüAppreciate the importance of the CRA

FrameworküIdentify the range of calculation question types

included within the CRAüDevelop strategies to answer the range of

different calculation question types üRecognise the importance of accuracy when

completing calculation questionsüAppreciate the need to work efficiently to

answer these question types within the CRAüReview common errors with calculations

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Webinar content

üCRA formatüCommon pharmaceutical

calculations mistakes and illogical answers

üUnit conversion and rounding üDose and dose regimensüDosage and unit conversionsüEstimations of kidney function

üConcentrations e.g. %w.v, %v/v

üMolecular weightüUsing provided

formulaeüPharamcokineticsüHealth economicsüQuantity to supply

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Performance standardsC1.5 Perform calculations correctly

10.2.3 Ensuring that safe and effective systems are in place to manage the risk inherent in the practice of pharmacy and the delivery of pharmaceutical services:

Common Registration Assessment (CRA) framework

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CRA frameworkThe CRA Framework outlines the topics you must be familiar with in order to pass the registration assessment. Accessible at: https://www.pharmacyregulation.org/53-registration-assessment-framework

Please familiariseyourself with the framework as soon as possible and refer to it regularly as you progress through the FTY

At all times we will endeavour to direct you to relevant sections of the framework during Development Days

Remember! The question writers will use the framework when setting your exam, so it is in your best interests to be coming from the same place.

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CRA formatThe CRA has two parts – Part 1 and Part 2.

• Part 1 consists of 40 calculations questions • 2 hours• Calculator permitted

• Part 2 consists of 120 questions in total; 90 single best answer questions and 30 extended matching questions • 2.5 hours• Up to 10 questions in Part 2 will require some

calculation• No calculator permitted

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CRA:Approved calculators for Part 1

Casio MX 8B-WE / MX 8B

(in black or white)

Aurora HC133

Aurora DT210

*Unapproved calculator models are NOT permitted in the assessment

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CRA:Types of pharmaceutical

calculationsDoses and dose

regimensDosage and unit

conversionsEstimations of kidney function

Displacement volumes and

values

Concentrations (eg. expressed as w/v, % or 1 in X)

Dilutions Molecular weight Using provided formulae

Infusion rates Pharmacokinetics Health economics Quantities to supply

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Common mistakes

Not reading question properly

Using the

wrong units

Decimal point error

Rounding errors

Running

out of time

Illogical answers

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Common mistakes: Illogical answers

Do the following scenarios make sense?

• a 2 week supply of 80500 mL of a liquid medicine for the treatment of epilepsy in a 7 year-old child

• diluting a 9% solution with water and getting a more concentrated solution

• adding 20000 g of a substrate to make a total of 100 g of an ointment

Always review your answers to check they are realistic and makes sense in real life.

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Common mistakes:Converting units

It is really important that you can switch between different units with ease – as often this is just a small step within a much larger calculation.

Micrograms (mcg)

Milligrams (mg)

Grams (g)

Kilograms (kg)

x1000

x1000÷1000

x1000

÷1000

÷1000

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Converting units quizQuestion Answer1. Convert 120mg to mcg

2. Convert 2.75kg to g

3. Convert 6g to mcg

4. Convert 2000mcg to g

5. Convert 30g to kg

6. Convert 400mg to g

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Common mistakes:Rounding numbers

• Important to identify at which stage in the question you need to round - End of question or earlier?- Typically round only once during the

course of the question

• Often questions provide specific rounding instructions- Eg. round the dose to the nearest 1mL or

5mL

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Common mistakes:Rounding numbers

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Rounding numbers quizQuestion Answer1. Round 77 mg to the nearest 5 mg

2. Round 87 kg to the nearest 25 kg

3. Round 865 micrograms to the nearest 100

4. Round 8.4 litres to the nearest whole number

5. Round 12,762 to the nearest thousand

6. Round 278,376 to the nearest ten thousand

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Common mistakes:Rounding decimals

Some decimal numbers go on for ever! To simplify their use, we decide on a cut off point and “round” them up or down.

If we want to round 2.734216 to two decimal places, we look at the number in the third place after the decimal, in this case, 4. If the number is 0, 1, 2, 3 or 4, we leave the last figure before the cut off as it is. If the number is 5, 6, 7, 8 or 9 we “round up” the last figure before the cut off by one. 2.734216 therefore becomes 2.73 when rounded to two decimal places.

If we are rounding to two decimal places, we leave 2 numbers to the right of the decimal point.

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Rounding decimals quizQuestion Answer1. Round 243.7684 to two decimal places

2. Round 1973.28 to one decimal place

3. Round 2.4689 to two decimal places

4. Round 0.99879 to two decimal places

5. Round 97.45 units to one decimal place

6. Round £456.347 to two decimal places

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CRA:Types of pharmaceutical

calculationsDoses and dose

regimensDosage and unit

conversionsEstimations of kidney function

Displacement volumes and

values

Concentrations (eg. expressed as w/v, % or 1 in X)

Dilutions Molecular weight Using provided formulae

Infusion rates Pharmacokinetics Health economics Quantities to supply

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Practise and practise and keep on practising!

Checkyou answered the question that was asked.

you used the correct units and number of decimal places in your answer.

your answer makes sense in real life.

Don’t overcomplicate

the question or your method of calculating the answer.

and keep the process as simple as possible.

and use a method you are comfortable with and have practiced

Identify the numbers and other information which you need from the question.

Read every question very carefully - at least twice.

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Doses and dose regimensA 9 week old infant of average weight is prescribeddrug X123 as part of a clinical trial. There is noknown paediatric dose for the drug, and the onlyinformation available is that the 9 week old babyshould receive 15 % of the adult daily dose, and thatdrug X123 should be given tds. The adult total dailydose is 1,250 mg. Drug X123 is available as an oralsolution containing 25 mg/12 mL.

What volume of drug X123, in mL, should the childbe given per dose?

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Doses and dose regimens

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Dosage and unit conversions

Baby L is 4 months old and weighs 11lbs. Their doctor has prescribed folic acid at a dose of 500mcg/kg once daily.

How many mLs of folic acid 2.5mg/5mL oral solution would Baby L require each day? Give your answer to 2 decimal places.

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Dosage and unit conversions

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Estimations of kidney function

You have been asked to calculate the creatinine clearance for a 53-year-old woman who weighs 77.5 kg. Her most recent serum creatinine is 175 micromoles/L.

Using the formula provided below, calculate the creatinine clearance, in L/hour, to one decimal place.

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Estimations of kidney function

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Concentrations

Concentration What this means in practice

W% w/v Wg in 100mL

X% w/w Xg in 100g

Y% v/v YmL in 100mL

Z% v/w ZmL in 100g

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In your pharmacy you have a stock solution of drug A with aconcentration of 18.75% w/v. Drug A is used as a mouthwashat a concentration of 0.25% w/v. You are requested to supply150 mL of a solution of intermediate strength, such that thepatient will dilute this solution 1 in 50 to get the correctconcentration immediately before use.What is the concentration, in % w/v, of the intermediatesolution?

Concentrations

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Concentrations

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Molecular weight

Whilst working in the pharmacy aseptic unit you are asked to add 7.5 mmol of potassium ions to a batch of intravenous fluids.

How many millilitres of potassium chloride injection 15% w/v will you need to add? Give your answer to two decimal places.

(atomic weight of potassium = 39; atomic weight of chloride = 35.5)

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Molecular weight

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Using provided formulaeMrs P is 20 weeks pregnant and during a routine appointment with her midwife, has been diagnosed with iron-deficiency anaemia. The doctor would like to aim for a haemoglobin level of 12 g/dL and prescribes a course of iVenofer® (intravenous iron). You look in Mrs P’s medical notes and find the following information:

You find the following formula for calculating iron replenishment:

What is the total dose of iron needed for Mrs P? Give your answer to the nearest 10mg.

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Using provided formulae

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Body Mass Index (BMI)

Height (m) x Height (m) * Often this formula is NOT provided –please memorize

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Vancomycin is administered by IV infusion andimmediately after administration, its plasma level iscalculated to be 136mcg/mL.If the drug’s half-life is 8 hours, what will theplasma concentration be, in mcg/mL, after 40hours? Give your answer to one decimal place.

Pharmacokinetics

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Pharmacokinetics

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Health economics

You have been auditing the prescribing of high cost antihypertensive drugs in your GP practice during 2020. Captopril is available as 50 mg tablets or 50 mg capsules. Tablets cost £0.04 per tablet, whereas capsules cost £2.10 per capsule. In the year 2020, 10,512 tablets were prescribed compared to 1,488 capsules.

What would have been the cost saving, in £s, if all prescriptions for captopril in 2020 were for tablets? Give your answer to the nearest £100.

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Health economics

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A patient with type 2 diabetes is started on metformin. The consultant prescribes a starting dose of 500 mg at breakfast for 1 week, then increased to 500 mg two times daily for a further week, followed by a maintenance dose of 500mg three times daily.

How many metformin 500 mg tablets will the patient need for the first 56 days?

Quantity to supply

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Quantity to supply

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CRA:Types of pharmaceutical

calculationsDoses and dose

regimensDosage and unit

conversionsEstimations of kidney function

Displacement volumes and

values

Concentrations (eg. expressed as w/v, % or 1 in X)

Dilutions Molecular weight Using provided formulae

Infusion rates Pharmacokinetics Health economics Quantities to supply

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Learning outcomesüAppreciate the importance of the CRA

FrameworküIdentify the range of calculation question types

included within the CRAüDevelop strategies to answer the range of

different calculation question types üRecognise the importance of accuracy when

completing calculation questionsüAppreciate the need to work efficiently to

answer these question types within the CRAüReview common errors with calculations

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Before you go…üPractise, practise, practise!üWorksheets and Pre-requisite worküONTracküAdditional pharmaceutical calculation resources

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ResourcesüMCQs in Pharmaceutical Calculations (Johanne Barry and Ryan F. Donnelly)

ü Introduction to Pharmaceutical Calculations (Judith A. Rees, Ian Smith, Jennie Watson)

üPharmaceutical Calculations Workbook (Judith A. Rees, Ian Smith)

üONTrack

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• Development day 214th(G1) 15th(G2) 16th(G3) September 2021• Pre-requisites

available on the FTY hub from 26th August 2021

• Remember to complete the mandatory learning by 26th August • Patient Medication

review and record • Medical calculations

• September – 4 mandatory recorded lectures to be completed by 24th

September

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