Calculating concentration

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Hi folks,

basically I have a two part question from the book How to Master Nursing Calculations, which I can do the first part but can not get my head around they author has arrived set the answer for the second.

"10ml of sterile water is added to a vial containing 500mg of clarithromycin (Klaricid®). What is a) the concentration of the reconstituted drug in mg/ml? And b) the approximate concentration of the infusion fluid in mg/ml if the 10ml solution is added to a 250ml bag of 5% glucose."

Litle bit of background.

I haven't started my first year yet, I start in September, but it's been a while since I've been in education, so I got this book to refine my mathematics, which it has helped tremulously, along with a few other to help prepare me for uni. I've slowly been working my way through the tests and am dedicated to not moving on to the next section until I get 100% of the current sections correct or at least understand how the answer was obtained.

Part a I understand to be 500mg/10ml= 50mg/ml the book says this is correct and I'm happy I understand this part.

Part b be confuses me. Surely the total volume of the bag would be 260ml, but I'm not sure if to include the 5% glucose which I think means 5g/100ml.

I know now some questions add extra info to trip the student up and this one seems to do it job.

Thanks in advance

Andy

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you're 1st thought was on the right track --- would still be 500mg of the medication but the volume has changed. For the purpose of this calculation that fact that it's going into D5W doesn't matter.

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Specializes in MICU.

well, you have to calculate the concentration of 5g/250ml or 5000mg/250ml; the concentration would be 20mg/ml.

Now, the question says you should add the first concentration(50mg/ml) to the second concentration (20mg/ml).

so the total concentration is 70mg/ml. I hope that helps.

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For practical intent, the increase in total volume of 10 cc (from 250->260) is meaningless (if you calculate it, 500mg/250cc vs 500mg/260cc, the difference isn't clinically significant). They'e asking for total concentration of the drug only, so you don't need to consider the dextrose.

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Specializes in MICU.

ok climber_andy

pls change your username and also what is the correct answer in the book

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What's the matter with the name? ??

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Thanks folks!

Im sure things like this will be explained when I get to uni next month, but it's great to hear know how to work it out now. And hopefully will help me get through the rest of the book.

I appreciate you time.

Thanks again.

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Specializes in MICU.

i read one of the admin's comment that we should change our username to something that can not be traced back to us

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well, you have to calculate the concentration of 5g/250ml or 5000mg/250ml; the concentration would be 20mg/ml.

Now, the question says you should add the first concentration(50mg/ml) to the second concentration (20mg/ml).

so the total concentration is 70mg/ml. I hope that helps.

OK, ignore this post. You don't add concentrations of different things.

It doesn't matter that you're diluting in D5 or NS or LR... It just matters that you're diluting it into a total of 260 mL.

Completely ignore the referenced post... He's completely wrong.

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The answer in the book is 2mg/ml, which if you take into account the question states "approximate" as well as what GrnTea said

"if you calculate it, 500mg/250cc vs 500mg/260cc, the difference isn't clinically significant" It now makes sense but as I assumed you were to include the glucose when it mention "infusion fluid" I'm not sure what constitutes "clinically significant", but am sure I will find out.

I did try to do it the waya Loveofrn did by adding the two concentrations together. Which is what set me off on the mission to find out how I could have been so far out.

Loveofrn, I will change my username, most sites have a policy that you username shouldn't be identifiable to you, I may have overlooked that advice on setting up a profile.

Thanks again everyone.

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Specializes in MICU.
The answer in the book is 2mg/ml, which if you take into account the question states "approximate" as well as what GrnTea said

"if you calculate it, 500mg/250cc vs 500mg/260cc, the difference isn't clinically significant" It now makes sense but as I assumed you were to include the glucose when it mention "infusion fluid" I'm not sure what constitutes "clinically significant", but am sure I will find out.

I did try to do it the waya Loveofrn did by adding the two concentrations together. Which is what set me off on the mission to find out how I could have been so far out.

Loveofrn, I will change my username, most sites have a policy that you username shouldn't be identifiable to you, I may have overlooked that advice on setting up a profile.

Thanks again everyone.

Thanks for that, we learn new things everyday

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You don't add concentrations of different things.

Great! It's these little bits that I think will trip me up until I learn them.

Thanks

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