Common Registration Assessment, Part 1

Oral rehydration fluid deficit calculations for the GPhC assessment

Three original practice questions, with the working shown. Read the stated prescription or plan before calculating.

Published 15 October 2026. Independent revision material, not GPhC questions, not for patient care.

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NICE CG84 covers gastroenteritis in children under five. For clinical dehydration its oral rehydration recommendation includes 50 mL/kg deficit replacement over four hours, as well as maintenance fluid. Deficit volume alone is not the total requirement.

NICE recommends low-osmolarity solution, frequent small amounts and regular clinical assessment. An hourly arithmetic average is not a fixed infusion instruction. Shock, persistent vomiting and other red flags require clinical assessment.

Each question fixes the values needed for its arithmetic. Write the unit beside every step and distinguish a single dose from a daily total or course supply. All prescriptions and patients here are fictional; these are revision materials, not for patient care.

The method

  1. Confirm the child is under five and read the management plan supplied.
  2. Multiply weight in kg by 50 mL/kg for the deficit volume.
  3. Keep maintenance separate unless its volume is supplied or another formula is requested.
  4. For an arithmetic hourly average, divide the deficit volume by four.
  5. Do not treat an average as an infusion rate or replace frequent small amounts and clinical assessment.

Three practice questions

Work each one on paper first, then open the answer. All patients and prescriptions are fictional.

Question 1

For a fictional 12 kg child under five with clinical dehydration, calculate deficit replacement at 50 mL/kg over 4 hours, excluding maintenance.

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Answer: 600 mL

Working: 12 × 50 = 600 mL over 4 hours. Maintenance is additional and excluded.

Question 2

For a fictional 8 kg child, calculate the arithmetic average deficit volume per hour when 50 mL/kg is allocated over 4 hours, excluding maintenance.

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Answer: 100 mL/hour

Working: 8 × 50 = 400 mL; 400 ÷ 4 = 100 mL/hour. This is an average, not a clinical delivery instruction.

Question 3

For a fictional 10 kg child, a stated plan includes deficit replacement at 50 mL/kg plus a separately prescribed 160 mL maintenance volume over the same 4 hours. What total volume is specified?

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Answer: 660 mL

Working: 10 × 50 = 500 mL deficit; 500 + 160 = 660 mL. The maintenance figure is an exercise input, not derived from CG84.

NICE CG84 recommendations

https://www.nice.org.uk/guidance/CG84/chapter/recommendations

Where marks are lost

  • Using a remembered strength or schedule instead of the stated product.
  • Reporting a daily total when the question asks for one dose or the course.
  • Treating arithmetic as evidence that a real prescription is clinically suitable.

Frequently asked questions

Are these official assessment questions?

No. They are original revision exercises, not official GPhC questions and not for patient care.

Can a quantity calculation justify a formulation switch?

No. Follow the formulation and prescription in the exercise. Real switches and dose changes need clinical checks.

How should I check the answer?

Check the requested unit, then work backwards from the quantity to the stated dose and duration.

Sources

The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.

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More calculation topics

See also the formula sheet, the eight sample questions and the approved calculator page.