My job as a tutor is to help people feel more confident. Maths can be a source of stress, especially if a lot of your skills haven’t been tested since school. I often work with nurses who do drug calculations as part of their jobs or as an essential part of their v100 or v300 courses. I’ve even written an eBook on drug calculations for busy nurses.
Here I want to share a few tips on drug calculations involving conversions and doses. Hope these help:
How to convert units in drug calculations for nurses
Converting between grams (g) and milligrams (mg) or between litres (l) and millilitres (ml) may already be part of your day-to-day routine. Perhaps you have been doing the same conversion, with the same chemicals, for many years. This provides you with a real-life example of how important it is to get the maths right.
In prescribing tests like v300, conversions appear in their own questions, but also crop up as subtle (sneaky!) steps in more complicated questions.
Take a look at this picture:
It shows you all you need to know to convert grams or litres into their “milli”, “micro” and “nano” equivalents. We follow the “down” arrows to convert into smaller units (for example from grams to milligrams) and follow the “up” arrows to convert into larger units (from milligrams into grams). We always find our starting point on the diagram from the information we are given in the question.
Real-life example 1:
Convert 10 milligrams into micrograms.
On the chart, we start at milligrams (because we are given 10 of them in the question) and then travel to micrograms, using a “x1000” arrow, so…
10 milligrams x 1000 = 10,000 micrograms.
How to do calculate prescribed doses in drug calculations for nurses
Another type of question you may have been doing in a particular way for years. Dosage questions usually involve medicine in the form of tablets or liquids.
Tablets have a strength associated with them. This “stock strength” is measured per tablet, such as 2mg (per tablet).
Liquid medicine has a stock strength, too, but because there is usually more than one dose in each bottle, we are supplied with a stock volume, too. This tells us how much drug is in a certain amount of liquid – for example 2mg in 5ml. This means a 5-millilitre volume will supply 2 milligrams of the drug.
Real-life example 1 (for tablets):
You are prescribing 60mg of a drug. The stock available is 15mg. How many tablets do you require?
We use this equation to help with tablet prescriptions:
First we extract the useful information from the question:
You are prescribing 60mg of a drug. The stock available is 15mg. How many tablets do you require?
Prescription given: 60mg
Stock strength: 15mg
Now we can put the information into the equation:
60 ÷ 15 = 4 tablets
A more complex example of a drug calculation for nurses
Nursing numeracy tests often test our ability to think on our feet. Some more advanced questions combine the different techniques we’ve already seen. But don’t worry – we have a plan!
A more difficult example question:
Your patient is prescribed 0.75mg of Digoxin. 250mcg tablets are available. How many tablets will you give?
This might look like a straightforward “dose” question. But currently each tablet has its strength written in micrograms (mcg), while the prescription is in milligrams (mg). Before we can calculate the number of tablets, we need to convert the prescription from mg to mcg, so everything is in the same units.
Step 1: Conversion (see above): To covert from mg to mcg we multiply the amount in mg by 1000
0.75mg x 1000 = 750mcg
Now we have all the information we need to work out the dose.
Step 2: Finding the dose (see above):
= 750mcg (our prescription in micrograms from step 1) ÷ 250mcg (the tablet’s strength) = 3 tablets
More help with drug calculations for nurses
We’ve only covered a couple of the maths skills we need for drugs calculations for nurses. But hopefully you see there’s no mystery to it, it’s about applying steps one after another until it becomes normal and useful.
The full ebook “Drug calculations for busy nurses”, contains lots more guides to tackling different types of drug calculations questions, together with practice examples, tests and answers.
If you’d like to work through some practice tests or boost your confidence with maths, maybe you’d like to get in touch for some one-to-one online tutoring? I’ve helped many nurses prepare for their V100 and v300 drug calculations tests and I’d be delighted to help.
Good luck,
John
Dr John Ankers is a specialist biology and bio-maths tutor.
