Why I wanted RxCalc

By | July 28, 2009

I have a couple of passions when it comes to pharmacy. The first is a love of pharmacy technology. Very few pharmacists have an appreciation for the “operations” side of pharmacy which includes automated dispensing cabinets, automated carousels, automated TPN compounders, Pharmacy Information System, etc. These tools are absolutely necessary if we want to get pharmacists out of the physical pharmacy and at the bedside where they belong. My second passion is a little less known discipline known as pharmacokinetics. I have no idea why I like pharmacokinetics; I just do. Some kids like PB&J and some don’t. It’s just the way it is.

Vancomycin and aminoglycoside (especially vancomycin) pharmacokinetics are very popular in hospitals, and are part of a select group of drugs that physicians prefer pharmacists to handle. Doing pharmacokinetic consults isn’t difficult per se, but can involve lots of numbers and equations. Many people find it a bit tedious and boring. There is also considerable variability in methods used for performing pharmacokinetic calculations. For example there are several existing pharmacokinetic models for vancomycin including Bauer, Matzke, Winter, Moellering, etc. Some of the models are more popular than others, but each has merit. As I’ve heard many times, “there are many roads to Rome.”

Pharmacists typically choose a favorite pharmacokinetic model and then alter the model to fit their needs based on years of clinical experience. I’m no different. Even though I was taught pharmacokinetics by Mike Winter at the UCSF School of Pharmacy, I prefer the vancomycin formulas derived by someone else. Please don’t tell Mike, he’s considered one of the best in the business and might revoke my alumni card if he found out.

Not all pharmacokinetic “starts” and adjustments require advanced calculations. Often times, years of experience and a good eye are all you need. However, there are times when you need a little more. Since the days of the Palm Pilot, I’ve always desired a portable pharmacokinetics program. It always made sense to me to use the technology at my finger tips to make my job easier. I’ve written several pharmacokinetic calculators, including simple Microsoft Excel spreadsheets and a little Java Script tied to a web front end, but I never got the opportunity to create a portable version. The iPhone (and iPod Touch) changed all that. The iPhone is a compelling device with a great user interface and unheard of popularity among healthcare professionals. My desire for a portable pharmacokinetics calculator, and the appearance of the iPhone, resulted in the creation of RxCalc.

RxCalc is designed for pharmacists based on my experiences over the past ten years. While not perfect, RxCalc does what I need. It’s clearly a work in progress. I don’t use it all the time because it’s simply not necessary. I still do a lot of “guess work” here and there, but rely on RxCalc when I need something to handle the more advanced calculations. Like every piece of software ever used, there is room for improvement. I have received lots of good, constructive feedback from several users. Combined with my desires, the feedback has created quite a list of future “enhancements” I’d like to see in RxCalc over the next several months. Among these include High Dose Extended Interval (a.k.a. once-daily) aminoglycoside dosing, single dose vancomycin kinetics, the option to change units (i.e. lbs, kg, cm, inches, etc), the ability to select different dosing models, alternate color schemes or themes and a more user friendly interface. Unfortunately, I don’t have the skill set to make the changes myself. That’s why God gave me a brother. I come up with the ideas and he does all the work to make it a reality. We make a great team.

I mentioned in a previous post that it may be time for me to put away my calculator in favor of newer technology. Pharmacokinetics was the last thing I really needed my calculator for. Well, with the creation of RxCalc I’ve finally put my calculator in the drawer and have been using pen, paper and my iPod Touch almost exclusively for a few weeks now. I must say, so far the results have been encouraging.

2 thoughts on “Why I wanted RxCalc

  1. terry fagan MD (icumath)

    Curious to know how RxCalc computes the vancomycin drug clearance from creatinine clearance. Which equation did you pick

    I have assumed vanco clearance equals CCR for a longtime, and clinically this has seemed to work fine. RxCalc does give lower doses than I generally use.

  2. Jerry Fahrni Post author

    Since, as you allude to in your comment, vancomycin is primarily cleared by glomerular filtration I don’t distinguish between creatinine clearance and vancomycin clearance.
    RxCalc uses the following for vancomycin calculations:

    Cockcroft and Gault equation using IBW for creatinine clearance (use ABW if less than IBW)
    Elimination rate constant: ke = (0.00083 * CrCl) + 0.0044
    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 that it is difficult to please everyone. We’ve received some pretty negative feedback regarding RxCalc, and for the most part I feel that it is undeserved. Everyone seems to have their own way of doing vancomycin pharmacokinetics, which leads to a lot of confusion among users of the application. I was trained by Mike Winters at UCSF, but found I didn’t like his methods after maturing in my practice a bit. It’s all personal taste. The equations used in RxCalc are those that I’ve found successful in my practice over the years.

    Just a note, but I’ve never really used a kinetics calculator for initial vancomycin dosing as the therapeutic window is so large and the dosing so simple. RxCalc seems to come in handy when making adjustments, for me anyway. I typically just use 15 mg/kg actual body weight and round up to the nearest 250mg, i.e. 1250mg, 1500mg, etc. Better to be a little too aggressive than the other way around.

    I hope that answers your question. If not feel free to contact me as I would welcome the opportunity to talk about kinetics. Thanks for stopping by and taking the time to comment. Always appreciate the feedback.

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