Passing Part A of the NMC CBT: Drug Calculations, Infusions and Fluid Balance

What Part A looks like

The NMC CBT combines two parts in one sitting of 115 questions in 3 hours. Part A numeracy comes with a 30-minute allowance for 15 questions. The booklet suggests up to 2 minutes per numeracy question, which is generous if you do not stall. You get an on-screen calculator and an erasable note board. You work on the board and type the answer.

The real Part A is constructed response, which means there are no options to choose from. A multiple-choice question lets you test your answer against four numbers. A typed answer gives you no such check. That changes how you practise: you must be able to judge whether an answer is sensible without options to guide you. The Pearson VUE practice test sets the entry rules: enter the number only, with the correct significant digits, use the standard unit abbreviation such as mL with no added s, and do not add the drug name. Our question bank converts Part A items to four options because the platform needs it, so also practise typed answers on the free Pearson VUE test.

The mix gives you a target. Oral medicines and injections together are 7 of 15 questions, nearly half the paper, and they rest on one formula. Master that formula and conversions and you have secured most of your 13.

The five conversions

Almost every error in drug calculation starts with a unit. Know these without thinking.

• 1 kg = 1,000 g
• 1 g = 1,000 mg
• 1 mg = 1,000 micrograms
• 1 microgram = 1,000 nanograms
• 1 L = 1,000 mL

Write micrograms and nanograms in full in your working and, in practice answers, as UK safe-prescribing practice asks. Using the abbreviation "mcg" is discouraged in UK practice because of confusion with mg. To move from a larger unit to a smaller one, multiply by 1,000. To move from smaller to larger, divide by 1,000. Always check direction with a sense check: a gram is a big quantity, a microgram very small.

The three formulas

1. Dose to give. (prescribed dose divided by stock strength) multiplied by stock volume. Prescribed dose and stock strength must be in the same unit.
2. Infusion rate. mL per hour equals volume in mL divided by time in hours.
3. Drip rate. drops per minute equals volume in mL multiplied by drops per mL, divided by time in minutes.

The drop factor is printed in the question. Standard clear-fluid giving sets are commonly 20 drops per mL, blood sets 15 and paediatric micro-drip sets 60, but use the figure given. Do not substitute a figure from memory.

Worked examples

These are our own examples, written to show method. They are not exam questions.

Example 1: oral liquid from a stock strength

A patient is prescribed 750 mg of an oral liquid. The stock is 250 mg in 5 mL. Units already match. Dose to give is 750 divided by 250, which is 3, multiplied by 5 mL: 15 mL. Sense check: the dose is three times the stock strength, so three times the stock volume.

Example 2: a conversion first

A dose of 0.25 mg is prescribed. The stock is 125 micrograms per tablet. Convert 0.25 mg to micrograms: 0.25 multiplied by 1,000 is 250 micrograms. Then 250 divided by 125 is 2 tablets. The error here is dividing 0.25 by 125 without converting, which gives a tiny wrong number.

Example 3: weight-based dose

A drug is prescribed at 15 mg per kg for a patient weighing 70 kg. The dose is 15 multiplied by 70, which is 1,050 mg. Stock is 500 mg in 10 mL. Then 1,050 divided by 500 is 2.1, multiplied by 10 mL: 21 mL. Sense check: 1,050 mg is a little over twice 500 mg, so a little over 20 mL.

Example 4: infusion rate

1 litre of fluid is to run over 8 hours by volumetric pump. Convert 1 litre to 1,000 mL. Rate is 1,000 divided by 8, which is 125 mL per hour.

Example 5: drip rate

500 mL is to run over 4 hours by gravity set with 20 drops per mL. Time in minutes: 4 multiplied by 60 is 240. Drops per minute is 500 multiplied by 20, which is 10,000, divided by 240: 41.67, so 42 drops per minute if the question asks for the nearest whole number. Always follow the rounding instruction in the stem.

Example 6: duration and volume remaining

An infusion of 500 mL runs at 80 mL per hour. Duration is 500 divided by 80, which is 6.25 hours, or 6 hours 15 minutes. Another infusion starts at 1,000 mL and runs at 125 mL per hour. After 3 hours, 375 mL has been given (125 multiplied by 3), so 625 mL remains.

Example 7: syringe pump

A syringe contains 50 mg of drug in 50 mL, which is 1 mg per mL. The prescribed rate is 10 mg per hour. Rate is 10 divided by 1, which is 10 mL per hour.

Example 8: percentage strengths

A 0.9% solution contains 0.9 g in 100 mL. That is 900 mg in 100 mL, or 9 mg per mL. A 500 mL bag contains 4.5 g. Adrenaline 1 in 10,000 is 1 g in 10,000 mL, which is 0.1 mg per mL, so 1 mg is 10 mL.

Example 9: fluid balance

Input is 1,850 mL. Output is 1,200 mL. Net balance is 1,850 minus 1,200: positive 650 mL. State the sign when the question asks for balance.

A repeatable method

1. Underline the unknown. Decide whether the answer is mL, tablets, mL per hour, drops per minute or something else.
2. Write every number with its unit. Do this on the note board before calculating.
3. Convert. Bring all quantities to matching units.
4. Estimate. Say whether the answer will be bigger or smaller than the stock volume, and roughly how much.
5. Calculate on the calculator. Enter the whole expression carefully, then clear it.
6. Check a second way. Work backwards, or use a different route such as per-mL strength.
7. Type the answer in the exam format. Number only, correct digits, standard unit abbreviation.

Where marks are lost

The NMC does not publish error analyses, so these are the failure points we plan for, based on how the questions are built.

• Skipping the conversion. The most common route to an answer that is wrong by a factor of 1,000.
• Hours against minutes. A drip rate uses minutes, a pump rate uses hours.
• Missing the drop factor. The question prints it. Read it every time.
• Inverting the fraction. Dividing stock by prescribed dose instead of the other way round. The estimate step catches this.
• Rounding too early. Keep full figures until the last step, then round as instructed.
• Typing errors. An extra zero, a dropped decimal point, or adding "mL" when the box wants the number only.
• Missing the clinical sense. Some questions ask whether a prescribed dose is unsafe. Know when to stop and query, such as a paracetamol total above 4 g in 24 hours. Confirm limits in the BNF.

A four-week numeracy ramp

After week 4, keep one Part A set in every study day. Skill fades quickly if unused. If you fail a Part A practice set, do not do a fresh set at once. Review each error and rewrite the method, then retest after a day.

Common questions about the calculator

The on-screen calculator is provided, and you also have the note board. Practise with an on-screen calculator on a screen, not a handheld one, because the key layout differs. Use brackets for division of a product, such as 500 times 20 divided by 240, or break the problem into two steps and write the intermediate result. Do not trust a number you have not estimated first.

Part A links to Part B. Medicines safety, fluid balance and intravenous therapy also feature in Platform 4 and Platform 6. See the Part A domain page for the subtopic list, and the study plan for where to place numeracy blocks.