Fluid Balance Calculations for the NMC CBT

Fluid balance is the least mathematical calculation in Part A: addition, subtraction, and a minus sign. That makes it the topic people under-prepare, and the exam compensates with detail. The marks are lost in missed chart lines, not failed arithmetic.

The method:

• Total the intake: oral fluids, IV fluids, IV drugs, NG or PEG feed, flushes.
• Total the output: urine, vomit, drains, stoma, anything measurable.
• Balance = in minus out.

Positive balance, more in than out. Negative, more out than in. Neither is automatically bad; a dehydrated patient being rehydrated should run positive, and a heart failure patient on diuretics should run negative. When the CBT asks what a balance “suggests”, it wants that context, not alarm at any non-zero number.

Where the marks leak

Unlisted output. The question lists urine 1,900 mL, vomit 300 mL, drain 250 mL. The wrong option +850 is what you get by subtracting only the urine. Every line in the stem is there to be used. Tally them on the whiteboard.

Sign errors. Half the option set is usually the correct magnitude with the wrong sign. After subtracting, say the direction in words: “more out than in, so negative”. Ten seconds, one mark saved.

Rate-times-duration intake. Feeds and infusions arrive as rates. An NG feed at 75 mL/h for 20 hours contributes 1,500 mL to intake. Multiply before you total.

Two clinical numbers the exam attaches

The urine output floor: 0.5 mL/kg/hour for adults. An 80 kg patient must pass at least 40 mL/h; persistently less is oliguria and, in a NEWS2-era question, an escalation trigger. The calculation appears both forwards (what’s the minimum?) and backwards (is 25 mL/h acceptable for this patient?).

And the weight shortcut: over days rather than weeks, 1 kg of weight change is roughly 1 L of fluid. It’s why heart failure and renal patients get daily weights. A 1.5 kg two-day drop with poor intake reads as roughly 1,500 mL down.

Insensible losses (sweat, respiration) are real but unmeasurable, so charts ignore them unless a question supplies an estimate. Use only the numbers the stem gives you.

Fluid balance leans on the same careful-reading discipline as NEWS2 scoring, and the two share exam stems. Five questions below, then a change of pace with BMI.