Drug calculation for nurses

Drug calculations
Drug calculations appear to be impossibly difficult, unless you break them down into small steps. They are vitally important to get right, yet they are so easy to get wrong. This paper will now look at some commonly used drug calculations and the way that mistakes can happen.

Type A calculations
When the dose you want is not a whole ampoule.

For example:

- Prescription states 200mg (milligrams)

- You have an ampoule of 500mg (milligrams) in 4ml (millilitres).

What volume contains the dose you need?

If you have an ampoule of 500mg in 4ml, and you need 200mg, it can appear to be a daunting calculation. The first step is to find out what volume contains 1mg (4/500) and then multiply it by how many mg you want (200).

The easy way to remember this is the famous nursing equation:

'What you want, over what you've got, times what it's in'

In this instance:

200mg x 4ml / 500mg = 1.6ml

The common error here is to get it upside down, and divide what you've got by what you want. This fortunately gives you a stupid answer, which is obviously wrong, in this case 10ml. You already know that you need a fraction of an ampoule and not two and a bit ampoules, which highlights the error.

To help make sure you get it the right way up, remember WIG:

What you Want x what it's In / What you've Got

Converting units
All weights, volumes and times in any equation must be in the same units. With weights the unit changes every thousand. For example, you need 1000 micrograms (mcg) to make 1 milligram (mg) and 1000 milligrams to make one gram (g) .

Type B calculations
These are infusion rate calculations.

For example:

- Prescription states 30 mg/hour

- You have a bag containing 250mg in 50ml

Therefore, at what rate (ml/hr) do you set the pump?

These are the same as type A calculations, only once you have worked out the volume that contains the amount of drug you need, you set the pump to give that amount per hour.

In this instance, work out how many ml contain ONE mg of drug

Using the WIG equation

30 x 50 / 250 = 6ml

Therefore the calculation shows that, to give 30mg per hour, the infusion pump rate would need to be set at 6ml per hour.

This calculation is straightforward when the rate you want (30mg/hour) and the amount of the drug in the bag (250mg) are both in the same units (mg).

However, if the infusion required that 600 micrograms were to be infused each hour instead, this would first need to be converted into mg before the infusion rate was calculated, that is, 600 micrograms = 0.6mg.

The equation for infusion rate calculation is dose stated in prescription (milligrams per hour) times volume in syringe (in millilitres) divided by the amount in the syringe (in milligrams) equals the infusion rate (millilitres per hour), or:

Dose (mg/hr) x volume in syringe (ml) / Amount in syringe (mg) = Infusion rate