Calculating concentration

[OutdoorEd]

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

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

Thanks

The question is, do you understand why?

It's really not a "bit of information" but rather a broad concept.

If you're trying to memorize bits, you're prone to making errors; if you understand the concepts, you'll make no errors except those related to misreading the problem or typographical ones.

[OutdoorEd]

The question is, do you understand why?

It's really not a "bit of information" but rather a broad concept.

If you're trying to memorize bits, you're prone to making errors; if you understand the concepts, you'll make no errors except those related to misreading the problem or typographical ones.

No, I do not yet fully understand why. I would take a guess to an explanation based on the above threads to, but it would probably be wrong. If it's not too much bother, I would appreciate an explanation. Or my book might explain it soon, failing that I start uni next month, although I am keen to learn as much as I can just now.

[Carpediem1012, BSN, RN]

Many IV drugs cannot be given "full strength, so must be diluted in a compatible dilution fluid. Things like D5W, Normal Saline etc are chosen based on compatibility and pt needs. But when you are calculating the CONCENTRATION of the DOSE of the drug being given, disregard what diluent is being used and focus only on the VOLUME. The question is actually very simple: how much drug is in the 250 mls of diluent?

500mg (dose) / 250 ml (volume) = 2mg/ml

The "clinical significant" part that GrnTea is referring to means that even if you added the 10 ml used to reconstitute to the 250 ml (=260), the 500 mg/260ml = 1.9, which you would round to 2 anyways. Unless this were a neonatal case or a drug that required the exact dosage (say mg/kg/ml), it is close enough for therapeutic range.

I hope that helps and doesn't confuse you more! :)

[SopranoKris, MSN, RN, NP]

Many IV drugs cannot be given "full strength, so must be diluted in a compatible dilution fluid. Things like D5W, Normal Saline etc are chosen based on compatibility and pt needs. But when you are calculating the CONCENTRATION of the DOSE of the drug being given, disregard what diluent is being used and focus only on the VOLUME. The question is actually very simple: how much drug is in the 250 mls of diluent?

500mg (dose) / 250 ml (volume) = 2mg/ml

The "clinical significant" part that GrnTea is referring to means that even if you added the 10 ml used to reconstitute to the 250 ml (=260), the 500 mg/260ml = 1.9, which you would round to 2 anyways. Unless this were a neonatal case or a drug that required the exact dosage (say mg/kg/ml), it is close enough for therapeutic range.

I hope that helps and doesn't confuse you more! :)

This explanation is spot on! Was just getting ready to post a similar response. OP, I hope Carpediem's post makes it clear. If you understand that concept, it will help you weed out extraneous information in the problem meant to throw you off.

[nurseprnRN, BSN, RN]

Taking Carpediem1012's concept one step further, it could be (for the sake of argument) that you could have two medications in a diluent bag. The concentration of each is calculated separately (e.g., Put into a 500cc bag imaginomycin 500mg = 1mg/cc and magicillin 3gm = 6mg/cc). You would have no need to add those two together (to make, in this imaginary example, 7mg/cc) because it's the individual active ingredient component that counts.

In reality this doesn't happen too often at ll. The corollary is that this is why in problems like this you ignore anything that comes in the carrier fluid itself, like the dextrose, lactate, electrolytes, etc., for purposes of calculating delivered doses of the active drug being carried.

[OutdoorEd]

This is great. It is slowly beginning to make sense, dose/volume of diluent=concentration, seems a lot simpler when you take all the extra nonsense. I think the more questions I come across throughout the book will aid me.

Thanks everyone!