Am I an idiot? (reconstituting ampicillin)

Nurses General Nursing

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I feel like a complete idiot asking this question, but must admit that math was never my forte.

It rarely ever happens that a doctor orders IV Ampicillin, 260 mg dose to be given.

The 250 mg vial of Ampicillin says to add 2.5 ml preservative free sterile water and this will make a dilution of 250 mg/ml.

I suppose because of the powder in the vial, when we add 2.5 ml sterile water, it actually makes 2.6 ml in the vial.

So, an order for 260 mg would be 2.6 ml.

I believe that if I am able to draw up 2.6 ml from this vial then I have 260 mg in the syringe. I honestly believe that this is true. The other day a coworker told me I'm wrong, saying, "The bottle says there is only 250 mg in the vial, so you have to mix up two vials and take 2.5 ml from one and 0.1 ml from another one to get the 2.6 ml."

If she is correct then we are calculating all our doses incorrectly because we are not taking into account the extra 0.1 ml. ??!? I'm thinking that when we mix it according to the directions then that means we have 250 mg/ml... not 250 mg/vial.

What do you think?

I'm always wanting to learn things and can take criticism well. I just feel like I remember having to give 2.6 ml mixed this same way at another hospital who had a Pharm.D. working with us who said that if we can get the full 2.6 ml out of the vial for a 260 mg order then we should do it and not mix up another vial needlessly.

Help me please!

Many thanks in advance!

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You are making a solution with 250 mg of ampicillin in each ml. That's what it means when it says you are getting a concentration of 250mg/ml on the vial. It doesn't mean there is 250 mg of ampicillin in the vial, it means that once mixed with sterile water, each ml will have 250mg of ampicillin in it.

Once again, the amount of sterile water is unimportant for you in determining how many mls to administer in order to give the ordered dose (2.6 mls doesn't matter!). Each ml of that solution has 250mg of ampicillin, so that 2.6 mls has 650 mg of ampicillin in it (2.6X250). That means you have more than twice the necessary dose in the solution you mixed and you don't need to mix another. If you all have been giving your meds based on the notion that there are only 250mg in the vial, then you have been giving way too much ampicillin.

So, to give 260mg of ampicillin, you would need 1.04ml (260mg divided by 250 mg/ml). It's that old "dose ordered divided by the concentration available= amount to be given " calculation.

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Specializes in LTC.

I'm thinking that if the med in the vial is 250mg ,its 250mg regardless of how many mls the mixed volumn is. I'm also thinking that if mixed volumn is more than than 1ml you'd have to give the whole amount to equal the 250mg. It seems that by ending up w/ greater than 1 ml after mixing would mean that the mgs would be diluted. I'm thinking about Rocephin, once mixed its 2.1mls that is given to get the total mg dosage.

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Oh Fergus thank you for your answer!

I have too much benadryl in my system right now because I forgot to mention that when adding the 2.5 ml of sterile water the the vial that the concentration becomes 100 mg/ml. This makes a huge difference in the calculations!! See, I am an idiot! :rolleyes:

Therefore, 260 mg divided by 100 mg/ml equals 2.6 ml... That's where we were getting the 2.6 ml from. I completely see that if it was 250 mg/ml then the dose to draw up would certainly be 1.04 ml. :)

I came down with a cold 2 days ago and along with the lack of sleep and the benadryl I took an hour ago, I'm truly not thinking... or speaking.... clearly right now. I apologize for wasting your time.

So, with the new 100 mg/ml dilution, do you think drawing up the 2.6 ml from one vial would be a viable option?

Thanks so much! You're a real pal!!

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Specializes in LTC.

I learn something new everytime I visit this site.

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Specializes in NICU.
Oh Fergus thank you for your answer!

I have too much benadryl in my system right now because I forgot to mention that when adding the 2.5 ml of sterile water the the vial that the concentration becomes 100 mg/ml. This makes a huge difference in the calculations!! See, I am an idiot! :rolleyes:

Therefore, 260 mg divided by 100 mg/ml equals 2.6 ml... That's where we were getting the 2.6 ml from. I completely see that if it was 250 mg/ml then the dose to draw up would certainly be 1.04 ml. :)

I came down with a cold 2 days ago and along with the lack of sleep and the benadryl I took an hour ago, I'm truly not thinking... or speaking.... clearly right now. I apologize for wasting your time.

So, with the new 100 mg/ml dilution, do you think drawing up the 2.6 ml from one vial would be a viable option?

Thanks so much! You're a real pal!!

To make it easier, can you get Ampicillin 500mg vials? We use them for any dose over 250 mg, put 5ml saline in the vial, and 2.6 ml is 260mg. If it's less than 250, we use the 250mg vial, add 5 ml, and your ml dose is double the number of your mg's. 180mg will be 3.6ml.

I think I would ask your pharmacist if he thinks that you could get 2.6ml out of a 250 mg vial. Maybe he'll give you 500 mg instead.

Hope you soon get over your cold!

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Specializes in Med-Surg, Wound Care.

Call the doc and ask him if he really wanted 260mg!! :p

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The 250 mg vial of Ampicillin says to add 2.5 ml preservative free sterile water and this will make a dilution of 250 mg/ml.

So it doesn't say this, right?:) If it is 100mg/ml then there is no reason you can't get 2.6 mls from that one vial.

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You started with a vial of 250mg of ampicillin so this will obviously never change. If you put 0.9ml or 10ml in the vial it will still contain 250mg.

When reconstituting in the nicu you should use the IM dilution for accuracy then dilute down before giving to your patient. This takes the problem of volume expansion out of play.

Ex. add 0.9ml to the 250mg vial and your total volume will be 1ml (0.9ml of diluent +0.1ml of powder) = a concentration of 250mg per 1ml.

simply divid your desired dose by 250mg and you will get how many ml's you need to draw up.

ex. dose is 110mg. 110mg/250mg = 0.44ml You can then add diluent (qs or quantity sufficient) to bring it to the 100mg/ml. in this case you want a total of 1.1ml, so 1.1-0.44 =0.66ml.

You can also add 2.3ml to the vial to reconstitute the original vial as well to get 100mg/ml, but the way above is more accurate.

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Specializes in NICU, Infection Control.

You can't get 260mg from a 250 mg bottle. Use a 500mg bottle, dilute to 100mg/ml w/draw 2.6ml and you're in business.

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