Common Registration Assessment, Part 1

Hypertonic saline bolus calculations in children

Three original questions on 2 mL/kg boluses of 2.7% sodium chloride with a 100 mL cap, with the working shown.

Published 14 October 2026. Independent revision material, not GPhC questions, not for patient care.

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NICE guideline NG29 says that if acute symptomatic hyponatraemia develops in term neonates, children and young people, the fluid status should be reviewed, immediate expert advice sought, and action considered. The action is a bolus of 2 mL/kg (maximum 100 mL) of 2.7% sodium chloride over 10 to 15 minutes.

A further 2 mL/kg bolus (maximum 100 mL) can follow if symptoms persist. If they persist after the second bolus, the plasma sodium is checked and a third bolus can be considered. Plasma sodium is measured at least hourly. This is an emergency hospital recommendation for acute symptomatic hyponatraemia only. NG29 lists headache, nausea and vomiting, confusion, irritability, lethargy, reduced consciousness, convulsions, coma and apnoea as symptoms, and says not to manage acute hyponatraemic encephalopathy with fluid restriction alone. It is not a dispensing rule and does not apply to asymptomatic hyponatraemia. The calculation is weight times 2 mL/kg, then a cap check, then a rate.

The method

  1. Multiply the weight in kg by 2 mL/kg.
  2. Compare with the 100 mL maximum and use whichever is lower.
  3. For a rate, divide the volume by the time in minutes for the fastest and slowest ends, then multiply by 60 for mL/h.
  4. For grams of sodium chloride, 2.7% w/v means 2.7 g in 100 mL, so multiply the volume in mL by 2.7 and divide by 100.
  5. Add boluses if the question asks for a total.

Three practice questions

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

Question 1

A child weighs 18 kg. What is one bolus of 2.7% sodium chloride, and what range of rates in mL/h covers giving it over 10 to 15 minutes?

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Answer: 36 mL, at 144 to 216 mL/h

Working: 18 × 2 = 36 mL. Over 15 minutes: 36 ÷ 15 = 2.4 mL/min, 144 mL/h. Over 10 minutes: 36 ÷ 10 = 3.6 mL/min, 216 mL/h.

Question 2

A young person weighs 60 kg. What bolus volume is given?

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Answer: 100 mL, because the maximum applies

Working: 60 × 2 = 120 mL, which is over the 100 mL maximum, so 100 mL is used.

Question 3

A child weighs 30 kg and has two boluses of 2 mL/kg of 2.7% sodium chloride. What is the total volume and the total grams of sodium chloride?

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Answer: 120 mL, containing 3.24 g of sodium chloride

Working: One bolus: 30 × 2 = 60 mL. Two boluses: 60 × 2 = 120 mL. 2.7 g in 100 mL: 120 × 2.7 ÷ 100 = 3.24 g.

Exam approach

Multiply by 2 mL/kg, check the 100 mL cap, then work out the rate or the grams only if asked.

Where marks are lost

  • Forgetting the 100 mL cap for heavier patients.
  • Using 2.7% as 2.7 mg/mL instead of 27 mg/mL.
  • Giving the volume in 1 hour instead of 10 to 15 minutes when working out the rate.
  • Overlooking that the guidance calls for immediate expert advice, hourly plasma sodium and reassessment of symptoms between boluses.

Frequently asked questions

What does NG29 say about hypertonic saline for hyponatraemia?

For acute symptomatic hyponatraemia, review fluid status, seek immediate expert advice, and consider a 2 mL/kg bolus (maximum 100 mL) of 2.7% sodium chloride over 10 to 15 minutes.

How often is plasma sodium measured?

NG29 says to measure the plasma sodium concentration at least hourly, then decrease the frequency as symptoms resolve.

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.