Reconstitution dosage problem

[Mursely]

The question says, "The physician orders 250 mg Cefazolin IM q 8 hours. What volume will you administer?"

Label says, "Cefazolin for injection, USP 1 gram"

"Preparation of solution: For IM use - Add 2.5 ML sterile water for Injection. SHAKE WELL. Resulting solution provides an approximate volume of 3 ml (330 mg per ml)"

Here is what I did,

Desire/Have

250 mg/330 mg = 0.76 ml. Now, to me that seems wrong because it looks like it's underdosed, so I decided to multiple 350 mg by 3 ml, and got 750 mg/330 mg = 2.27 ml.

The correct answer is 2.27 ml, right?

[203bravo, MSN, APRN]

Think it out a little.

you want to give 250 mg and every mL has 330 mg right? so without any math at all you should know that you are going to give less than 1 mL.

I can't even attempt to figure out where you got 350 mg x 3 mL??? but 2.27 mL would be 749 mg.

[Mursely]

Think it out a little.

you want to give 250 mg and every mL has 330 mg right? so without any math at all you should know that you are going to give less than 1 mL.

I can't even attempt to figure out where you got 350 mg x 3 mL??? but 2.27 mL would be 749 mg.

okay, so it was 0.76 ml. I misread the question. thanks

[Mursely]

By the way, how do you do a question like this:

"The physician orders Mandol 500 mg IM stat. The 1 Gram vial that is available has the following directions for reconstitution. "Add 3 mL of an approved diluent. Provides an approximate volume of 3.5 mL" How many ml will you administer?

I am confused at this particular one, it doesn't provided me the me the mg after dilution.

[loriangel14, RN]

If it is a 1 gram vial it doesn't matter how much dilutent you add, it will still be 1 gram. Diluting it doesn't change how many grams are in the bottle.

[loveofrn, BSN, RN]

If it is a 1 gram vial it doesn't matter how much dilutent you add, it will still be 1 gram. Diluting it doesn't change how many grams are in the bottle.

But how many would you administer...am sure not 1g..so DA comes into play

[203bravo, MSN, APRN]

By the way, how do you do a question like this:

"The physician orders Mandol 500 mg IM stat. The 1 Gram vial that is available has the following directions for reconstitution. "Add 3 mL of an approved diluent. Provides an approximate volume of 3.5 mL" How many ml will you administer?

I am confused at this particular one, it doesn't provided me the me the mg after dilution.

1 g = 1000 mg so 1000 mg / 3.5 mL = 286 mg per Ml

500 / 286 = 1.75 mL

which makes perfect sense --> 500 mg is 1/2 of 1 g... so we know before we start the problem that we are going to give half the solution... 3.5 mL / 2 = 1.75 mL

[loriangel14, RN]

But how many would you administer...am sure not 1g..so DA comes into play

Yes but that's not what Mursley asked. He asked how many mg after dilution. It doesn't matter how much you add, it's still one gram. the concentration is all that changes.

[203bravo, MSN, APRN]

the real secret to medical math is do not make it harder than it needs to be :)

[Esme12, ASN, BSN, RN]

Take out the medical part...it's just math.

[nurseprnRN, BSN, RN]

Read the question and change it into more familiar units. So:

"The physician orders Mandol 500 mg IM stat. The 1 Gram vial that is available has the following directions for reconstitution. "Add 3 mL of an approved diluent. Provides an approximate volume of 3.5 mL" How many ml will you administer?

becomes:

"You need to get 500 jellybeans for your third grader's picnic stat! You go to the candy store, and they have bags that have 1000 jellybeans in 3 1/2 quart bags. You buy a bag. How do you know how to get 500 jellybeans out of the bag?"

If you can't see that 500 jellybeans is half of 1000 jellybeans, so you just need to divide the total volume in the bag by 2, well, I can't make it any more clear.

[KelRN215, BSN, RN]

But how many would you administer...am sure not 1g..so DA comes into play

All you need to do is divide the volume by 2. You have 1000 mg in 3.5 mL, you want to give 500 mg so you give half of 3.5 mL since 500 is half of 1000.