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Hi there

I'm a nursing student and I have a question regarding Ceftriaxone dosing calculation. I'm not sure if I should start with the 500mg vial reconstituted with 1.0ml of lidocaine or the 1g vial reconstituted with 2.1 ml for lidocaine. Both the 500 mg and 1g vial will give me 350mg/ml after reconstitution but I don't understand how to get the remaining 50 mg needed for the order. I've tried to figure this out with dimensional analysis and I just confuse myself over and over. I'd really appreciate any sage wisdom that can be offered.

Thank you in advance!

500mg reconstituted in 1.0 ml

That means 1.0 ml has 500mg

So each 0.1ml had 50 mg

Draw up 0.8 ml and you get 400mg

0.8ml x 50mg - 400 mg

Hope I'm doing it correctly

Your math is wrong. If you take 500mg and add 1 ml, you have 500 mg/ml. Not 350.

Same for diluting 1000 mg with 2ml. You still end up with 500 mg/ml.

In real life you will never dilute with 2.1 ml. Where did you come up with that number?

Be simple. Redo the math step by step.

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Your vial has 500mg of Ceftriaxone in it. You could add any volume of dilutent to it -- the concentration will then be 500mg in ______ mL. (however many mL of dilutent was added)

So think about your route of administration. In this example, for IM dosing ... you want a pretty small volume to be injected IM. Dilute your Ceftriaxone with 1-2mL and administer 4/5 or 80% of the reconstituted volume.

You could add 1mL - 500mg in 1mL = 400mg in 0.8mL

Or 2mL - 500mg in 2mL = 400mg in 1.6mL

The vial states for every 1ml there is 350 mg after reconstitution. The 2.1 is listed on the vial.

DIRECTIONS FOR USE: Intramuscular Administration: Reconstitute Rocephin powder with the appropriate diluent (see COMPATIBILITY AND STABILITY).

Inject diluent into vial, shake vial thoroughly to form solution. Withdraw entire contents of vial into syringe to equal total labeled dose.

After reconstitution, each 1 mL of solution contains approximately 250 mg or 350 mg.

This is where I'm confused. The vial states there is 350mg/ml after reconstitution.

You would need to draw up 1.1 mL.

1mL/ 350 mg x 400mg / dose = 1.14 mL/dose

If you go with 350 mg/mL as per the vial. You would just draw up more than a mL to make up the additional 50 mg.

Thank you! That's the result I keep getting. I just wanted to be sure. Thank you. I don't think a lot of people realize that there is 350mg/ml after reconstitution and this causes confusion. I appreciate your response.

Ok. The original question you posted was interpreted as a simple math problem, that's why people are telling you your math is wrong. We are going off the information you posted where you said you had a 500mg or 1gm vial.

If you have specific dilution guidelines that are different from logical mathematics (as in this case, possibly due to chemistry of the product or inability to fully remove all the contents from the vial), then you should have explained that better.

If you are going with the concentration of 350 mg/ml, then dimensional analysis gives you the following:

1ml/350mg x 400mg/1= 400/350 = 1.14ml

So, since you need more than the 1ml you are injecting to reconstitute the 500mg vial, you should choose to use the 1gm vial in order to be able to draw up the correct amount.

This is actually quite common with reconstituted medications, as the reconstituted powdered medication adds to the total volume. Quite often, the enclosed instructions include two diluent volumes, one to be used if reconstituting for IM use and the other if reconstituting for IV use, so it is important to read the material closely.

In the example included in the original post, diluent volumes were provided for both the 500 and 1000 mg vials of Ceftriaxone; both of which yield a final concentration of 350 mg/mL. The total volume in the 500 mg vial is going to be ~1.4 mL, and for the 1000 mg vial the total volume is going to be ~2.8 mL.

You would be able to withdraw the dose from either vial. When I have multiple options to choose from, I typically use the smallest vial that provides the ordered dose.

If you work in the ER, you'll be doing this calculation on the job. And usually with the 350 vial.

## 1000mg of rocephin

So if you have 1000mg of rocephin that equals 1 gram even after the dilution with 3.5ml of lidocaine is that correct? That's what the pharmacist was telling me

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