Pharmacology Math

Pepper The Cat, BSN, RN · Jan 12, 2011

Its probably very basic math. MD orders 1 L of fluid over 8 hours - what rate do you run it at? (divide 1 L (1000 cc) by 8).

Tylenol 1000 mg ordered. You have 650mg tabs and 500 mg tabs. What do you give? (answer 2 500 mg tabs).

don't get too worked up. She going to teach you everything you need to know.

Just take everything one step at a time.

carolmaccas66, BSN, RN · Jan 12, 2011

Calm down, it won't be anything difficult like Pepper the Cat says.

Try to remember some formulas, for questions like:

1) 1000mls normal saline is to run over 8 hours, how many mls will run through per hour?

Well look at the question first, read it CAREFULLY and actually write down what you need to know. So, we have 1000mls which we can divide by 8 = 125. So, your answer is 125mls per hour for this fluid to run through. So, the formula is your amount (1000mls) divided by your time (8 hours) to get you the answer.

Also always use big pieces of paper for working out, give yourself plenty of space on the paper, and think about what ur doing.

2) A patient is ordered ordered 120mg Frusemide tablets. You only have 40mg tablets in stock. How many tablets will your patient require?

Well look at the question again - what you WANT is 120mg.

What you HAVE are 40mg tablets.

So, it's what you WANT over (divided by) what you've GOT. We can do it like this: 120 divided by 40 = 3. Your answer is 3 tablets.

Make sure with tablets that you have the same dosage for the whole calculation ie: it's in either micrograms (mcg) or milligrams (mgs) when working out, as you will confuse yourself with your answer. Otherwise you have to convert it (but won't go into that).

3) A patient needs an injection of 10mg of Drug X. Stock for Drug X comes in glass ampoules made up in 5mg in 5mls. How many mls will you be injecting?

Again, read the question slowly. So, what we HAVE is 5mg in an ampoule of 5mls of Drug X.

We WANT 10mg of this drug to give to the patient.

We HAVE 5mg IN 5mls.

So, if you look at this logically, you know if you gave 5mls of the drug, your patient will only be getting 5 MILLIGRAMS, but we want 10mg to make up our dosage for our patient. So logically, we need another 5mls to make up our 10mg dosage. So we give 10mls of Drug X.

You can do this as what you WANT, 10mg, divided by 5mg

TIMES

5mls divided by one (you always have a one there when there is nothing else to put in the formula).

10 x 5

__________________

5 x 1

The two fives cancel each other out, so our answer is 10.

It's hard explaining on paper, so hope it's not too confusing, and I've tried to use easy numbers to work with. Most ampoules may have 2mg in 5mls, or even 2.5mg in 5mls so that's when it gets a bit tricky. But if you practice your formulas it won't matter what the numbers are - you will be able to work it out.

But the way a question is worded can also throw you. Try to look at it logically, and just write down on a blank piece of paper the numbers you need. It's just simple maths really.

Go to a large bookstore and they will have some, simple nursing calculation books - they only cost about $10AUD through Angus & Robertson or through your university. Buy one that has all the formulas worked out step by step so you don't get confused trying to work it all out.

And always round up, ie: drop per minute may work out to 62.5 so round up to 63dpm, or if the answer is under 5, round down to the nearest whole number.

Phew! Hope I didn't confuse you. But do get a good calc book and start practicing. If I passed all mine - and I was an absolute dunce at maths - you can do. You just have to practice, practice, practice like I did.

Good luck!

petethecanuck · Jan 12, 2011

Do you have access to eDose? Lots of good practice questions and "how to" step by step explanations there.

Fiona59 · Jan 12, 2011

To put it the easiest way possible. Know your fractions and decimal calculations.

Spritz · Jan 12, 2011

I use the formula drug ordered

drug on hand

Ex: 150 mg Demerol s/c Q2-3h prn pain. Amps of 100mg/1ml available

150mg = 1.5 ml of Demerol You will use 2 vials and waste 50mg of Demerol

100mg

I always double & triple check my calculations and if I am not 110% sure, I ask a co-worker to check with me. I always get someone else to read insulin orders with me and confirm. I was a pharmacy tec before going into nursing and it's vitally important to be 110% with your meds.

Yours in nursing