# need help with these 2 drug calc problems

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hi everyone,

So i am going over practice problems and i came across these two. I am used to solving problems the dimensional analysis way,but i can't seem to set up these 2 using dimensional analyis!HELP!

1. order: cefazolin 1gm

available: cefazolin 500 mg vial powder

directions: add 2mL sterile water to vial to total 2.6 mL.

how much total volume will you add to the IV bag for each cefazolin dose?

2. The physician has ordered 10million units of penicillin -G IVP q12h .The 20 million unit vial of powder has these instructions: add 40mL of sterile water. How many milliliters equal 10,000,000 units?

886 Posts

1000mg = 500mg

X 2.6ml

You set it up as a proportion. Be certain that the numerators and denominators match. In other words, both numerators in this example are mg and the denominators are ml.

Question number two will be set up the same way.

886 Posts

1000mg = 500mg

X 2.6ml

You set it up as a proportion. Be certain that the numerators and denominators match. In other words, both numerators in this example are mg and the denominators are ml.

Question number two will be set up the same way.

Okay, I can't get the X and 2.6ml to line up...the 2.6ml should be under the 500mg part. You cross multiply and solven for X.

49 Posts

1. 1000/x = 500 mg /2.6 mL, cross multiply: answer is 5.2 mL

2. 10million units/ x = 20 million units/40 mL = 20mL. i got it!

thanks parker bean! by the way i was wondering,how do i know if i should use proportion as to dimensional analysis? is it just if one doesn't work,then try the other method?

886 Posts

This is a worksheet I give my students to help them...

dosagecalculations.pdf

788 Posts

1. 1000/x = 500 mg /2.6 ml, cross multiply: answer is 5.2 ml

2. 10million units/ x = 20 million units/40 ml = 20ml. i got it!

thanks parker bean! by the way i was wondering,how do i know if i should use proportion as to dimensional analysis? is it just if one doesn't work,then try the other method?

i prefer solving for x this way:

1. 500 mg/2.5 ml = 1000 mg/x ml

2. 20 million units/40 ml = 10 million units/x ml

2 Articles; 5,114 Posts

i always teach people to look at the problem before they start setting up equations. at very least, it will at least give you a ballpark range. this will also be invaluable when you take nclex...but i mostly care because it's invaluable when you take care of actual sick people.

in number 1, you have to give 1gm, and you have 500mg vials at hand. how many vials of 500mg make 1gm? if you can't look at that and say, "um, two," then we have a problem. if you do see that you need the contents of two vials, then reconstitute your two vials, each of them coming up as 2.6ml, and how many mls do you have now? if you can't say, "um, twice 2.6 = 5.2," we still have a big problem. (in reality, in this example it totally doesn't matter how much diluent you add to the vials to draw the drug out and put it into the bag. you just need all the contents of both vials. right?)

in number 2, you have 20m units and you want 10m units. so you want what fraction of the 20m? if you can't say, "um, half," then again, we have a problem. no matter how much fluid you add to the 20m units, how much of it do you want to give half of the drug in it? again, if you can't say, "um, half," we have a problem. so if it's 40cc when it's reconsituted, how much is half of that? (let's all say this together now!) "um, 20cc." yep.

now, before you think i'm being snotty about this, let me tell you about the student i had who had a 100cc bag with 10,000 units of heparin in it. that's how much heparin per cc? "um, 100 units." good.

she had to give 880 units per hour. just eyeballing this tells you that if there are 100 units per cc, then 850 units is gonna be a little less than 9cc, right? (in point of fact, it's 8.5cc). so why did she figure that she was going to need a lot more bags sent up from pharmacy because she was going to give 850 cc per hour? because she set up her equation wrong and couldn't figure out decimals for toffee, sure (and why, oh why didn't she work in the payroll department and pay me using her same formula? i could have retired...), but also because she didn't do the quick-and-dirty reasonableness test.

sure, there are plenty of calculations you can't do in your head. find a good way to think of them (there have been several here; i had one my students loved because it never fails and it's intuitive and simple) and use it all the time. but for heaven's sake, look at what you're doing first, before you dive in.

2 Articles; 5,114 Posts

here's the med calcs example i mentioned: (i hope this attaches)

Med calcs.pdf

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