Zosyn Dose Calculations

[sueoct79]

Help. I can’t wrap my brain around how they got the dose of Zosyn for a baby. The order is to give 100mg/kg of piperacillin to baby who weighs 0.8kg. Concentration of Zosyn is 45g/ml. Each 4.5g vial of Zosyn contains 4g of piperacillin and 0.5g of tazobactam. The dose they want to give is 90mg of Zosyn. I’m trying to figure out the med math, but having a hard time understanding.

[chare]

When working this type of problem, as you frequently have considerably more information than you need the first task is to determine what exactly you are being asked to solve.  In this vignette, you are being asked to calculate the Zosyn dose for a 0.8 kg infant.  Of the information provided above, what do you need to do this?

And, there seems to be some confusion in the information provided.  At my facility, Zosyn is dosed using the piperacillin component, as when you wrote to “give 100 mg/kg of piperacillin.”  Then you state write that “The dose they want to give is 90mg of Zosyn.”  While the latter is correct, this is not how Zosyn is dosed, at least in my experience.  To avoid confusion, if I wrote this, I would not use both in the same vignette.

[sueoct79]

24 minutes ago, chare said:

When working this type of problem, as you frequently have considerably more information than you need the first task is to determine what exactly you are being asked to solve.  In this vignette, you are being asked to calculate the Zosyn dose for a 0.8 kg infant.  Of the information provided above, what do you need to do this?

And, there seems to be some confusion in the information provided.  At my facility, Zosyn is dosed using the piperacillin component, as when you wrote to “give 100 mg/kg of piperacillin.”  Then you state write that “The dose they want to give is 90mg of Zosyn.”  While the latter is correct, this is not how Zosyn is dosed, at least in my experience.  To avoid confusion, if I wrote this, I would not use both in the same vignette.

I guess I’m trying to figure out how they got 90mg?

[chare]

37 minutes ago, sueoct79 said:

I guess I’m trying to figure out how they got 90mg?

Your ordered dose is 100 mg/kg of piperacillin.  How many mg of tazobactam are there with each 100 mg piperacillin?

[sueoct79]

I’m not sure.  Each 4.5g vial of Zosyn contains 4g of piperacillin and 0.5g of tazobactam

[chare]

5 minutes ago, sueoct79 said:

I’m not sure.  Each 4.5g vial of Zosyn contains 4g of piperacillin and 0.5g of tazobactam

The ratio of medication is provided 4000 mg piperacillin:500 mg tazobactam.  Using this ratio, 100 mg piperacillin:X mg tazobactam.

[sueoct79]

OK, I know, I apologize.  Its so frustrating that I am not comprehending. I am old school and need it written out. Is there any way you can “show the work” on how you get 90 mg?  

[chare]

Part of the confusion, I think, is that the dose is being described two ways.  First, you have an order to administer "100mg/kg of piperacillin to baby who weighs 0.8kg."  In my experience, pediatric Zosyn dosing is most frequently ordered using the piperacillin component, which is what is being done here.  

Then, you are being told that they want the patient to receive 90 mg of Zosyn, which is the sum of total doses of both the piperacillin and the tazobactam contained in the weight based dose of Zosyn in the above paragraph.  While this is technically correct, in my experience I have never seen Zosyn dosed this way. 

Your vial contains 4.5 g Zosyn, or a ratio of 4 g piperacillin to 0.5 g tazobactam.  This can either be written as 4.5 : 0.5, or 4.5/0.5.  As your ordered dose is 100 mg/kg piperacillin, I would determine the amount of tazobactam associated with 100 mg piperacillin, as 100 mg/kg piperacillin is the ordered dose.  

When you set up the problem you can do it one of two ways.

First, set the problem up: 4000 : 500 = 100 : x.  Using this format, multiply the outer values (4000, x) and the inner values (100, 500) and solve for x.  Or, you could set the problem up as 4000/500 = 100/x.  Using this format, cross multiply (4000, x) and (100, 500), and again solve for x.  Either format will give you the amount of tazobactam associated with 100 mg piperacillin.  Adding the two gives the total dose of Zosyn per kg.   

ETA: If pharm math is difficult for you, you might find the attachment this post helpful.  Although written for pharmacy techs, much of the information is helpful for nurses.

[sueoct79]

OK.  Thank you.  So, it’s the sum of the piperacillin (80mg) and tacobactam (10mg). That’s where my confusion was, I guess.  I didn’t realize you have to add the two to get the total mg for the Zosyn.   Thank you again