Common Registration Assessment, Part 1
Electrolyte content over time
A regimen that changes after a few days, and a total in mmol over a set number of hours. Three original questions.
Published 5 October 2026. Independent revision material, not GPhC questions, not for patient care.
One of the Pharmaceutical Journal's dose and dosage examples asks how many mmol of sodium a baby receives over 144 hours from infant powder sachets, with a regimen that changes after two days. The skill is converting the time period and counting doses in each phase.
Work in days, count the doses for each phase, multiply by the content of one dose and add the phases. State the unit as the question asks.
The method
- Convert the time period to days if the regimen is given per day.
- List each phase of the regimen with its doses a day and its days.
- Multiply doses by the content of one dose, phase by phase.
- Add the phases and state the unit, usually mmol.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A patient takes 1 sachet four times a day for 3 days, then 1 sachet three times a day. Each sachet contains 1.15 mmol of sodium. How much sodium in mmol is taken in 168 hours?
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Answer: 27.6 mmol
Working: 168 hours is 7 days. Days 1 to 3: 4 × 3 = 12 sachets. Days 4 to 7: 3 × 4 = 12 sachets. Total 24 sachets × 1.15 = 27.6 mmol. Convert hours to days first.
Question 2
Each modified-release tablet contains 8 mmol of potassium (a value for this exercise). A patient takes 2 tablets three times a day for 4 days. How many mmol of potassium is that?
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Answer: 192 mmol
Working: 2 × 3 = 6 tablets a day. 6 × 4 = 24 tablets. 24 × 8 = 192 mmol, which is 48 mmol a day.
Question 3
A fluid contains 150 mmol/L of sodium (a value for this exercise). A patient receives 2 L a day for 3 days. How many mmol of sodium is given?
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Answer: 900 mmol
Working: 2 L × 150 mmol/L = 300 mmol a day. 300 × 3 = 900 mmol.
The journal's sodium example
The tutorial's example gives a baby one infant powder sachet three times a day for two days, then twice a day, and asks for the mmol of sodium in 144 hours to one decimal place. The questions on this page use different regimens and values.
Where marks are lost
- Using hours as days, for example counting 168 doses in 168 hours.
- Applying the second regimen to the whole period.
- Mixing mmol per sachet with mg per sachet.
Frequently asked questions
How do I handle hours?
Divide by 24 to get days, then count the doses a day in each phase.
Are the electrolyte contents real?
They are given in each question for the exercise. Check actual contents in a current source in practice.
Sources
- The Pharmaceutical Journal, medication maths: how to solve dose and dosage calculations (video tutorial, updated 25 March 2025)
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
See also the formula sheet, the eight sample questions and the approved calculator page.