NMCMATE

Fluid balance calculations

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1 mark of the 15 in Part A, which runs for 30 minutes.

Fluid balance is arithmetic with a clinical judgement attached. Add the intake, add the output, subtract, and say whether the patient is running positive or negative. The marks are lost in what gets left out of the two columns.

What this section asks about

  • Totalling intake, including oral fluids, intravenous fluids and the volume that intravenous medicines carry
  • Totalling output, including urine, drains, vomit, stoma and estimated insensible losses
  • Reporting a balance with its sign, positive or negative
  • Percentage dehydration and fluid deficit from a change in body weight

Where the marks go

1 mark of the 15 in Part A, which runs for 30 minutes.

Every numeracy question sits at the apply and analyse level: you are given a situation and asked to work something out, rather than to recall a fact.

Worked examples

The method, with the answer shown. Practise below once you have the shape of it.

  1. Over 24 hours: oral intake 850 mL, intravenous fluids 1,000 mL, three 100 mL doses of intravenous antibiotics. Output: urine 1,450 mL, wound drain 200 mL, vomit 150 mL. What is the balance?

    1. In: 850 + 1,000 + 300 = 2,150 mL
    2. Out: 1,450 + 200 + 150 = 1,800 mL
    3. 2,150 - 1,800

    Answer: Positive 350 mL

    The antibiotics are intake. Three 100 mL doses is 300 mL in, and leaving them out turns a positive balance into a smaller one.

  2. The same figures, with insensible losses given as 500 mL. What is the balance now?

    1. In: 2,150 mL
    2. Out: 1,800 + 500 = 2,300 mL
    3. 2,150 - 2,300

    Answer: Negative 150 mL

    Report the direction, not just the size. "150 mL" is not an answer; negative 150 mL is.

  3. A patient whose usual weight is 60 kg now weighs 57.6 kg. What is the percentage fluid deficit?

    1. A kilogram lost is about a litre of fluid.
    2. 60 - 57.6 = 2.4 kg, so about 2.4 L
    3. 2.4 / 60 x 100

    Answer: 4%

Where candidates lose marks

  • Forgetting that intravenous antibiotics are intake. Three 100 mL doses is 300 mL in.
  • Dropping insensible losses when the question has given them to you.
  • Reporting the size of the balance without its direction.

Sample questions

From our bank, with the explanation as a student sees it. These particular questions are kept out of the mock papers, so reading them here does not use anything up.

Sample 1 of 2

Intake: IV 800 mL, oral 600 mL. Output: urine 1000 mL, drain 100 mL, insensible 400 mL. Balance in mL?

Give your answer in mL.

Answer:-100

Why

In = 1400. Out = 1500. Balance = -100 mL.

NMC Standards

Sample 2 of 2

Over 12 hours a patient takes 1200 mL of fluid, passes 900 mL of urine and vomits three times, measured as 250 mL, 175 mL and 300 mL. What is their fluid balance in mL?

Give your answer in mL.

Answer:-425

Why

Output is 900 mL of urine plus 725 mL of vomit, which is 1625 mL against an intake of 1200 mL, so the balance is minus 425 mL. Vomit is an output nobody prescribes and is the column most often left blank, which turns a negative balance into a positive one on paper.

NMC Standards of Proficiency Annexe B

Questions people ask

How much fluid balance is in the CBT?
It is the smallest of the six numeracy categories in the test specification, but it is on the paper, and it is the one people practise least.
How is percentage dehydration worked out?
Weight lost divided by the pre-illness weight, times 100. In a child, each 1% of body weight lost is roughly 10 mL per kilogram of deficit.

Practise this section

Practice is filtered by section, so you can work on this one on its own. Every account gets 10 new questions a day without a card.

Create an account All CBT sections How the bank is built

This is one of the fifteen sections the paper is built from. Work through the whole thing with the NMC CBT study guide.