Working out the ratio of liquid medicine in a pharmacy

Speaker 1: Hi Anthony.

Speaker 2: Hello.

Speaker 1: Please can you prepare this prescription for cefaclor, which comes as a liquid?

It's for a 4-year-old child, I've checked the prescription and the dose is correct for their age.

However the prescription only says how many milligrams the patient needs as one dose, and not how many millilitres of the liquid medicine that is.

So can you work out how many millilitres would make one dose?

Speaker 2: Sure. What's the concentration of the drug to liquid?

Speaker 1: The concentration we've got in stock is 250mg to 5ml.

Speaker 2: OK, I'll work on that now and get back to you for sign-off.

Speaker 1: OK great.

Speaker 2: (TO AUDIENCE) OK, this medicine has 250mg of the active drug in every 5ml of liquid.

That's 250 to 5. So using this ratio and the prescription information I should be able to work out how many millilitres to give for one dose.

Speaker: Right, so I've got to work out how much of this liquid medicine to give this patient.

But their prescription only tells me the dose they need to take in milligrams and not how many millilitres of the liquid medicine they need to take.

OK, so in this case, one dose is 62.5mg of the drug.

Now I need to work out exactly how many millilitres that equates to.

I know this liquid medicine is made up of a ratio of 250 to 5.

There's 250mg of the drug in every 5ml.

They only need to take 62.5mg which is quite a lot less.

So I can already tell that the dose in millilitres will also have to be quite a lot less than 5ml.

To work out how much less, I need to find how many times 62.5mg fits into 250mg.

So that's 250 divided by 62.5 equals 4.

So, I know that to make 250 become 62.5, I have to divide it by 4.

And I know that whatever you do to one side of the ratio, you then have to do to the other.

So to find my millilitres, I need to divide 5ml by 4 on this side too.

So that's it, one 62.5mg dose of this medicine is 1.25ml.

Speaker 1: Hi Sherita.

Speaker 2: Oh hi Anthony, how did you get on?

Speaker 1: Yeah I got on alright.

Speaker 2: Can I have a quick check before I sign it off?

Speaker 1: OK.

Speaker 2: How did you find the single dose amount?

Speaker 1: I started by using the ratio finding out how to get 250mg to become a 62.5mg dose.

I found out you need to divide it by 4 to get the dose.

So then I did that to the other side of the ratio, and that gave me the 1.25ml dose yeah?

Speaker 2: That sounds great. That's signed off. Thanks Anthony.

Speaker 1: Thank you.