Posted in Pharmacology

In the past few days, I’ve received a few desperate pleas from nursing students struggling with dosage calculations. In each case, the problem has been the exact same, so let’s clear things up!

Answer the right question!

Yes, you can use dimensional analysis to solve ANY dosage calculation problem. However, you have to be super careful that you are solving the RIGHT question. Some questions will be written to trick you into automatically solving a completely different question, and thus getting the answer wrong. Take the question below, for instance. At very first glance, a rushed and nervous student might solve the problem as though they are trying to determine the ml/hr rate to program on the pump. NOT SO! Read it through carefully and then determine what the question is actually asking:

Mrs. Reynolds has been admitted for severe epigastric pain and coffee ground emesis. The physician orders BarfNoMore 8mg/hr continuous IV infusion. BarfNoMore is available as a 10ml vial containing a concentration of 4mg/ml. The continuous dose requires 200 mg of BarfNoMore to be diluted into a 500 mL bag of normal saline. What volume (in mL) of BarfNoMore is required for the continuous infusion? 

If your answer is 20ml, then you have answered the wrong question. You have likely set up your equation like this:

8mg/1 x 10ml/4mg = 20 ml

That’s because you just glanced at the question…found your “ordered dose” of 8mg and then plugged in the conversion factor. Which is fine if you’re solving a ml/hr question. Which you are not. Look at it again.

Notice that the question is…how much BarfNoMore do you need to mix into your 500ml bag of NS? That’s it! It’s so simple, your over-worked and stressed brain made waaaay too much of this question! So…go ahead and set up your calculation…but first do a quick little math to determine that your conversion factor is a 10ml vial with 40mg of BarfNoMore (at 4mg/ml). The VERY FIRST thing you need to determine is….what is your ordered dose? Not the 8mg/hr (though it would be if the question asked about the ml/hr rate)….but instead, you want to know the TOTAL DOSE to make the 500ml bag have the correct concentration of drug. Yes…it’s 200mg. Start there:

200mg/1 x 10ml/40mg = 50 ml.

Let’s do another one:

Sometimes the reconstitution instructions will be thrown at you to confuse you. Like this:

The new resident on your hospitalist team orders 150mg of PeeMore IV q 12 hours. The pharmacy has sent up a 3g vial of PeeMore with instructions to reconstitute with 9.6ml NS for a final volume of 10ml. How many ml of PeeMore will  you give for each dose? 

Again, look at what the question is asking…don’t get tripped up with the 9.6ml! Just determine your conversion factor by figuring out that  you have 3000mg of PeeMore in a 10ml vial. That’s it!

150mg/1 x 10ml/3000mg = 0.5

I hope that helps you guys rock your dosage calc quizzes! I am in the process of creating another dosage calc quiz for you with trickier questions…so stay tuned for that! If you are getting stumped on questions, send ’em my way and I’ll do what I can to help you through them. Just type it in below and I’ll get back to you as soon as I can!

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