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# drug calc question... is the answer key wrong?

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Nurse math experts: what are your thoughts on this?

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.

This appears inaccurate because it seems to be based on the assumption that cefazolin has a density of 1 g/mL.... However, when you look at the drug monograph, by inference, cefazolin appears to have a higher density than this assumption! i.e. according to the monograph, we should add 2.5 mL to a 1 g vial to achieve a concentration of 330 mg/mL. (If the density was 1 g/mL, then the concentration would actually be 286 mg/mL).... Therefore if we add 9 mL to a 1 g vial, the concentration is going to be > than 1 g/10 mL!! In other words, if you add 9 mL to a 1 g vial, you might not actually end up with 10 mL. It might be 9.3, 9.5, 9.7, or even 10.3 mL depending on the drug's characteristics (I've assumed density, but it could also be solubility and other characteristics)............ which would mean calculation errors...........

For this reason, I usually just reconstitute according to the monograph, draw up the required dose, and then dilute further as necessary to the desired concentration. In my opinion, it's not a good idea to calculate using the first method (that the answer key has used....)

Can anyone explain?

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