Monthly Archives: July 2009

Why I wanted RxCalc

Cross-post from JerryFahrni.com. Jerry is our resident PharmD and co-creator of RxCalc.


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 onRxCalc 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.

RxCalc 1.0.1 now available

We’ve been sorting through some user feedback, it’s great stuff! We noticed a recurring theme, “Why can’t I make adjustments after my initial calculation on a New Start.” We found that strange because you can definitely make adjustments after the initial calculation’s are performed. So, we did a New Start, and it became quite obvious why people were confused. The results table fills the screen perfectly and there’s no indication you should scroll down to make your adjustment!

RxCalc results version 1.0

So, we did a quick release to fix the problem. In 1.0.1 the screen will show and scroll to the bottom, where the adjustment fields are, right away. It’s animated so you now know there’s more data above if you’re interested in viewing it. The critical data lies at the bottom of the screen; Tau and Estimated Dose, plus Desired Dose, Given Every, and Calculate. We hope this clears things up. (NOTE: The values used in the illustrations were NOT entered by a Clinical Pharmacist, or a healthcare professional. They are for illustration purposes only.)

RxCalc results version 1.0

Guidelines for Monitoring Vancomycin against Staphylococus aureus Infection

Medscape.com: “The Infectious Diseases Society of America, the American Society of Health-System Pharmacists, and the Society of Infectious Diseases Pharmacists have issued therapeutic guidelines for monitoring of vancomycin treatment for Staphylococcus aureus infection. The summary of consensus recommendations is published in the August 1 issue of Clinical Infectious Disease.

Some of the clinical recommendations include:

  • Dosing based on actual body weight, even for obese patients.
  • Measuring trough levels drawn just prior to the fourth dose.
  • Keeping trough concentrations greater than 10 mg/L and even higher, 15-20 mg/L, for complicated infections.
  • Consider alternate therapy for patients with CLcr 70-100 mL/minute and a targeted AUC/MIC > 400.

Additional clinical recommendations can be found in the guidelines, which are available for free in PDF format here.

Cockcroft-Gault remains standard for pharmacokinetic calculations, for now.

A recent article in Hospital Pharmacy presented a review of literature comparing various methods used for estimating renal function and how those equations are best used when applied to drug dosage adjustments. The article, “Drug Dosage Adjustment Using Renal Estimation equations: A Review of the Literature” discusses literature surrounding the Cockcroft-Gault (CG), the abbreviated Modification of Diet in Renal Disease equation (abbrMDRD), and to a lesser extent the original Modification of Diet in Renal Disease equation. Although promising, there simply isn’t enough literature to support the use of the abbrMDRD equation in pharmacy practice. The article concludes that “although the abbrMDRD equation has many advantages as compared with the CG equation, too little research has been completed at this time to recommend the clinical use of the abbrMDRD equation in pharmacy practice.”

RxCalc currently uses the Cockcroft-Gault equation to estimate renal function for all pharmacokinetic calculations. Apple Core Labs will continue to evaluate new and emerging data, and use this information to make changes to RxCalc when deemed appropriate.

All quiet on the western front

Since the release of RxCalc on July 4th things have been pretty quiet around the labs. We’ve been working through our list of “love to have features” and have trimmed it down to a select few.

Now comes the hard part, actually doing the work, and delivering a new release.

If you’re already using RxCalc you may be saying to yourself “Hey, I just bought a copy, what does that mean for me?” Good question! The answer is, you’ll receive a free upgrade!

Thanks for supporting RxCalc, we really do appreciate it.

Going once, going twice…

To help get the ball rolling on RxCalc we’ve decided to give away a few fully functional copies.

So, the first five healthcare professionals, or healthcare students, to e-mail their full name to us at support@applecorelabs.com, with the title “RxCalc Offer”, will receive a promotional code that will allow you to download a copy of RxCalc from iTunes.

Yes, it’s that simple.

We promise we won’t give your e-mail address, or any personal information, to anyone.

Good Luck!