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Yes, and No

September 10th, 2010

Rob Fahrni

Something we need to do here at Apple Core Labs is create a FAQ for RxCalc. I’ve been saying that for quite a while now but I just haven’t gotten around to it. It’s one of those things that falls to the bottom of the list, but it really is time I did it.

Why?

A bouquet of flowers.We need to get this hammered out so we avoid bad reviews to be quite honest. Our calculations are accurate but we continue to receive reviews that say otherwise. There is no one true method for deriving Pharmacokinetics calculations. There are many methods. To say RxCalc under doses is true, and false, all at the same time. If you’re a new Pharmacist, or married to a particular method, you may be extremely disappointed with the numbers RxCalc produces.

By creating a FAQ the hope is we’ll help remove that disappointment. RxCalc currently uses the following Creighton formulas for vancomycin calculations:

  1. Cockcroft and Gault equation using IBW for creatinine clearance (use ABW if less than IBW)
  2. Elimination rate constant: ke = (0.00083 * CrCl) + 0.0044
  3. RxCalc considers the total dose of vancomcyin per dosing interval determined by equation; in this case [(ln Cp-ln Ctr)/ke] for the interval and [Cp*Vd*(1-e-^(ke*tau))] for the dose.
  4. Vd is defaulted to 0.7 liters/kg, but can be changed to the users taste between 0.5-1 liter/kg

Vancomycin dosing is so variable, and there are so many methods, that it was difficult to chose just one. Methods for vancomycin kinetics include Bauer, Burton, Matzke, Moellering and Winter just to name a few. At the time RxCalc was designed the Apple Core Labs team felt most comfortable with the formulas dervied by Creighton. This choice was neither right nor wrong as vancomycin kinetics are as much an art form as they are a science. You will find many different methods used throughout the various schools of pharmacy and medical centers in the United States.

With that said, Apple Core Labs chose to offer an adjustable volume of distribution for RxCalc, in effect giving the end user an amount of flexibility in their calculations. The option to set your default volume of distribution from 0.5 liters/kg to 1.0 liters/kg can make a significant difference in the final outcome of your vancomycin calculations. RxCalc relies not only on numbers to perform calculations, but the end user’s clinical acumen and experience as well.

Apple Core Labs will continue to improve RxCalc by adding features and improving functionality. We appreciate any and all feedback, no matter how good or bad. One thing we would like to request from our end users is detailed information included with their comments. If we know what method the user is comparing the results generated by RxCalc to, we can better understand what the problem might be and how best to help.

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