Help with med calculations

Nursing Students Student Assist

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Looking for some advice from nurse math lovers! Thanks in advance :)

When reconstituting a drug, I generally use the information supplied on the monograph or drug insert. (i.e. Add 2.5 mL to a 1 gram vial to achieve a concentration of 330 mg/mL... or whatever). After I reconstitute, then I take the dose I need and dilute it further....

However, the nursing department is coming up with a new way of reconstituting... simply by adding 9 mL to a 1 g vial..... i.e. In order to dilute cefazolin to 1 g/10 mL: the answer key states -- add 9 mL to a 1 g vial to achieve a concentration of 1 g/10 mL.

My interpretation: This might work if the required dose = the entire 1 gram. However, for smaller doses, it would be inaccurate to assume that the final concentration is 1 g/10 mL (because this would mean that the drug would have to have a density of 1g/mL.... but usually the drug's density is > 1 g/mL, which can be inferred from the reconstitution information on the monograph!!) i.e. If we add 2.5 mL to a 1 gram vial, if the density of the drug is 1 gram/mL, we should have a final concentration of: 1000 mg/(2.5 mL + 1 mL)... but this is LESS than the ACTUAL concentration on the monograph. Likewise, 1 g/10 mL is LESS than the ACTUAL concentration..... This wouldn't matter if we need the entire 1 gram, but it WILL MATTER if we need a SMALLER dose!

Does anyone understand what I am talking about? If you do, please help me understand further! Personally, I think it is MORE ACCURATE to reconstitute ACCORDING to the MONOGRAPH rather than simply adding 9 mL, especially when smaller doses are required and we do not know for sure that the drug's density is 1 gram/mL!

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

what? the drugs density? if you add 9 cc to the vial and have 1gram(or 1000mg)/10 ml the drugs concentration is 100mg/1ml and quite easliy broken into smaller doses. i'm no math genius but i think you are over thinking this.

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I know it "sounds" crazy, but try figuring out what the concentration would be if you added 2.5 mL to a 1 gram vial on your own and then check to see if your calculation agrees with the monograph.... It doesn't add up to what you would think.... In fact, the calculated concentration ends up being quite a bit less than the concentration indicated on the monograph... This must be because the drug is DENSER than we are assuming it is! (Unless there's ANOTHER reason for the discrepancy that I'm not aware of)

This doesn't matter if you plan on using the entire 1 gram. However, if you have a small dose and you ASSUME the concentration is 100 mg/mL, it's not accurate.

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

I still am not clear what you are saying....I think you are over thinking this way too much. Why would the dose not be 100mg/ml? Denser??? I am still not following and I think you are riving yourself unecessarily crazy.

If there is 1gram in 10 mls them there is 100 mgs in 1 ml. It can be nothing else. Medicine is not an exact science even though we pretend it is....CEFAZOLIN (ANCEF ®) - Intravenous (IV) Dilution

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follow the directions on the vial/product information insert. then if you want your solution more diluted for some reason, you can add more diluent in the syringe, once it's out of the vial.

where your problem lies is this: adding 9ml to a 1gm vial might not give you 10ml. it might only be 9.75ml or 9.3ml. it might even give you 10.2ml. if you draw it all up and it's less than 10ml, you can add more diluent to make the 10ml, and then take a subset of that for a reliable smaller dose. if it's more than 10ml, return to the original: follow the directions on the label.

if your nursing department doesn't see it that way, they'll have you over- or under-dosing at least some of the time. if this doesn't make sense to them, draw pictures of the different outcomes until it does.

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Thanks Grn Tea!!!

That's exactly how I see it too... but it has been SO HARD to explain this to others in my program....

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where your problem lies is this: adding 9ml to a 1gm vial might not give you 10ml. it might only be 9.75ml or 9.3ml. it might even give you 10.2ml.

... this is a clue that we should not assume the density of the drug is 1 g/ml...

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if the drug in the vial is in powdered or semisolid form, there is no "ml."

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It may seem that way, but a powdered or semisolid substance still has a mass, a volume, and a density. However, it might make more sense to say that the density of the drug is more than 1g/cm*3.

If we really wanted to reconstitute without the monograph, we would need to know the density of the drug, correct? Because we don't have that information, we need the monograph.

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

Thanks GrnTea........I was not seeing this. I see what the OP is saying now but I am still baffled about needing the density of the drug.....I do understand somewhat but I still think you are driving yourself unnecessarily crazy OP.

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It might seem like I'm driving myself crazy "unnecessarily;" however, this "crazy" thinking has helped me understand why it is so important to follow the monograph. I like to know why I do what I do. If I can make sense of it, it is safer. Period.

It was the thought about drug density that led to this realization in the first place. So I'm grateful for that thought!

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

I'm not criticizing you........I am glad you realize why it is important to follow the monograph provided by the inset/maker. There are many of these fine details in nursing and you are using you critical thinking skills.

I wish you the best in school!!!:loveya:

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