I'm making this harder than it is

Coffee Nurse, BSN, RN · Jul 27, 2018

I'm taking a math quiz for a new job. Nothing like getting stuck on a test. It's nice when I'm at work, and a co-worker is available to solve it with me.
The question states: There's a new order for Ampicillin 150 mg IV q 12 h. Suppose you have available Ampicillin 500 mg vials. You add 4.8 ml of Sterile water to make a final concentration of 100mg/ml. How many mls of Sterile Water must you add to your final syringe for a Final Volume of 5 mls?
I think you can only add 4.8mls of the sterile water period. But it's a tricky question asking how much water to add to the final syringe. Any help is much appreciated!

That's a weirdly worded question, I think they're expecting you to dilute what you draw out of your reconstituted vial without explicitly saying so. If you reconstitute to 100 mg/ml as directed, your 150 dose would be 1.5 ml. How much additional sterile water would you add to make it 5 ml then?

Sour Lemon · Jul 27, 2018

What the hell kind of question is that?

JKL33 · Jul 27, 2018

Basic math...

Any confusion would've been cleared up by their final sentence being something like, "After reconstituting the powder as directed you wish to further dilute the patient dose to a total volume of 5ml. How much sterile water will you add?"

tlware · Jul 27, 2018

It doesn't make sense, does it? Once the med has been reconstituted, you can't keep adding sterile water to the syringe. That would make it too diluted and the wrong dose.

Sour Lemon · Jul 27, 2018

It doesn't make sense, does it? Once the med has been reconstituted, you can't keep adding sterile water to the syringe. That would make it too diluted and the wrong dose.

It wouldn't make it the wrong dose, but it's weird, pointless, and just doesn't make sense to me.

Coffee Nurse, BSN, RN · Jul 27, 2018

It doesn't make sense, does it? Once the med has been reconstituted, you can't keep adding sterile water to the syringe. That would make it too diluted and the wrong dose.

Further dilution doesn't change the dose. The dose is whatever you initially draw up.

JKL33 · Jul 27, 2018

It doesn't make sense, does it? Once the med has been reconstituted, you can't keep adding sterile water to the syringe. That would make it too diluted and the wrong dose.

No...

Adding water doesn't change the mg of medication in whatever your final solution is, although the concentration changes obviously.

If you have a (now) liquid medication (because you just mixed it) with a concentration of 100 mg (of med) in 1 ml, they want you to do the math shown to you above in response #1. The dose prescribed for your patient is 150 mg, and using what you now have available (100mg/1 ml), your patient gets 150 mg or 1.5 ml.

They are trying to be tricky. You have to come up with the 1.5 ml answer first in order to know that, from there, you can add 3.5 ml SW in order to end up with 150 mg/5ml in your final syringe.

Without saying anything about the med, the question, usual practices, etc., you could dilute that 1.5 ml in a bathtub and it would still contain 150 mg of medication.

tlware · Jul 27, 2018

Ohhhhh!!! I see it now.

I didn't show you the full question because I wan't seeing how the previous part played a role. Now that JKL33 spelled it out for me, it's so very easy to see. Part A of the question did ask me to get the 1.5mL dose.

thank you for the clarification!! Boy do I feel silly

donsterRN, ASN, BSN · Jul 28, 2018

Boy do I feel silly

Don't. It happens to everyone.

Good luck at the new job!