Medication calculations for NZ nursing students: worked examples

Calculation errors are the failure mode nursing students fear most, and reasonably so. The good news is that calculation is the most trainable thing on the exam, it is a small set of methods applied carefully.

Reading a prescription: NZ abbreviations

Decode the frequency before you calculate anything. Most errors in this area are comprehension errors wearing arithmetic clothing.

Getting the units right first

The second most common setup error, after frequency, is a unit mismatch between what is prescribed and what is held. Convert before you calculate, never during.

Write microgram out rather than abbreviating it. That is a safety convention rather than a stylistic one, and it is the sort of detail a New Zealand paper expects and a United States practice set will not teach you.

Working out a maximum daily dose

A prescription reads: Paracetamol 1 g PO QID PRN. What is the maximum in 24 hours from this prescription?

QID is four times daily, so 1 g × 4 = 4 g in 24 hours. Two things make this question worth practising. First, PRN means the patient may take less, but the prescription still sets a ceiling. Second, that ceiling needs checking against the recognised maximum daily dose, and against anything else the patient is taking that contains the same drug.

The portable rule: decode the frequency first, then check the total against the maximum.

Calculating a dose from a stock concentration

Prescribed 375 mg. Stock is 250 mg in 5 mL. What volume do you draw up?

Wanted over held, times the volume it is in: (375 ÷ 250) × 5 = 7.5 mL.

Sanity-check every answer against the stock. 375 mg is one and a half times 250 mg, so the volume must be one and a half times 5 mL. If your arithmetic disagrees with that estimate, trust the estimate and redo the arithmetic.

Infusion rates

1,000 mL over 8 hours. What is the rate in mL/hour?

1,000 ÷ 8 = 125 mL/hour. For drops per minute, multiply the hourly volume by the drop factor and divide by 60. Write the units next to each number as you go: unit tracking catches more errors than re-checking arithmetic does.

Paediatric weight-based dosing

15 mg/kg for a child weighing 18 kg. 15 × 18 = 270 mg.

Simple arithmetic, high stakes. The errors that occur here are almost always in the setup: using an adult dose, using pounds, or missing that a dose is per day rather than per administration.

A checking routine that catches most errors

1. Decode the prescription in words before touching a number: drug, dose, route, frequency, and whether it is regular or as required.
2. Convert units so prescribed and held are in the same one.
3. Estimate the answer's rough size, and whether it should be more or less than the stock volume.
4. Calculate, writing the units beside every number.
5. Compare against the estimate. If they disagree, the arithmetic is wrong, not the estimate.
6. Ask whether the answer is plausible as a volume you would actually draw up or an amount you would give.

Step five is the one people skip and the one that catches order-of-magnitude errors, which are the errors that matter. Step six is the one that catches everything else.

Where do calculation errors come from?

• Misreading the prescription, especially frequency abbreviations.
• Unit mismatches: mg and g, mL and L, kg and lb.
• Not estimating first, so an order-of-magnitude error goes unnoticed.
• Using overseas conventions from practice material written for another country.
• Doing it in your head because the numbers looked easy. They usually are; that is not where the error is.

How should you practise?

Little and often, with the method written out rather than done in your head. Speed comes from confidence in the method, and confidence comes from having written it out enough times that you no longer need to.

Fifteen minutes a day for a fortnight will move this further than a full day of it, which is why the six-week revision shape gives calculations a daily slot rather than a block. For where calculations sit in the whole examination, see the state final guide.