The App Store Disconnect

App Store, RxCalc

My frustration with the disconnect between us and our customers on the App Store continues to grow. For RxCalc we typically see ratings that have to do more with lack of features, or new features, than the features we actually have. Don’t get me wrong, that’s great! The problem is we have NO way of communicating with these users to find out what they really want. We can make an educated guess and hope we’re correct, but that’s the best we can do.

Take a look at one of our latest ratings. It’s not horrible, three stars, but this person would like something added to the app. Based on this comment we have little to no idea what they’re really after.

“I found that when using 2 levels to calculate phamacokinetics that the VD is way off from any equations I have seen. Also it does not extrapolate a Cmin and Cmax. This program is useful for calculating first doses. I hope this can be fixed in the next update since this is the only kinetic program I could find for the apple”

In this case the user is probably used to using a different equation, or method, to determine dosing. As we’ve explained here before we use Creighton vancomycin calculations, but there are many others. Different method, different results.

If Apple would allow us to make contact with our reviewers we could actually bring some of these requests to life! As it sits today, we can only hope they’ll make contact via our support page, or we can make our best guess at their request.

That’s not so good.

Hello 1.2!

App Store, Pharmacokinetics, RxCalc

It’s been a few months in the making but RxCalc 1.2 is now available in the App Store.

What’s new? Glad you asked. There are two new features to talk about.

  1. Support for dosing adjustment using single level vancomycin kinetics.
    • The calculations are performed using both patient specific information supplied by the user as well as patient population parameters.
  2. Literature supported High Dose Extended Interval (HDEI) (aka “once daily” or “pulse”) aminoglycoside dosing.
    • Calculations for gentamicin and tobramycin are based on 7mg/kg adjusted body weight.
    • Calculations for amikacin are based on 15mg/kg adjusted body weight.

This release was also unique because we finally opened the door to a Beta Tester. Our heartfelt thanks goes out to Robert B. Martin, Pharm.D. Robert provided a critical eye for Jerry’s Pharmacokinetic Calculations and his years of clinical experience were priceless. He’s given us a lot of great feedback we’ll be sure to include in future releases. That’s right, we like to hear from our users!

We hope you enjoy using 1.2 as much as we enjoyed developing it.

Yes, and No

Kinetics, RxCalc, , , ,

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.

On the flip side

RxCalc, Uncategorized,

I give that a thumbs up!

Yesterday I shared a frustration, today, I’ll share one of those happy moments. We were bumming over the bad review, and have been for a few days, when what appears in one of our mailboxes? That’s right, some positive feedback, and a feature request! Woo-hoo!

Thank you for offering such a usable app. I hope you will continue developing others as clinically useful as RxCalc.

Thanks for that! It’s nice to hear something positive after getting a bad review.

Oh, what did he ask for? Glad you asked.

I seldom order peak levels anymore except in certain cases. As you know, most of the time we can do a very good job dosing and monitoring vanco patients with trough levels only. Because RxCalc requires both peak and trough levels in order to use the “Adjust with Levels” portion of the app, I am rarely able to use this feature. Is there any way to allow the peak level input to be optional?

Ahh, just you wait.

Pitfalls of App Store Reviews

App Store, RxCalc

I believe it was Marco Arment that gave the advice “Don’t read App Store Reviews”, or something to that effect, and I can see why. For RxCalc they’ve typically been fairly poor. Some have provided great feedback that lead to changes in the applications, but recently we received a review that left us scratching our heads. Here’s the review.

This program really underdoses all my patients. It predicts troughs of 20 and 25 with normal doses. I used it just the other day and it predicted a trough of 26.4 and I got an actual trough of 14.7–no changes clinically with the patient. This program is crap. Good thing it only cost 99 cents!

Can you see the problem facing an app developer with a review like that? Actually there are many problems with it. Here are the two biggest, as I see it.

  1. It doesn’t provide any meaningful feedback – We can’t reproduce the “problem” or fix it.
  2. We have no way to contact the user to see if we can help.

I really wish this user would’ve contacted us via our support e-mail address. We’re very confident our math is correct, we spend a lot of time verifying the results, it’s what makes the application useful. I’d venture to guess this is a configuration issue, but alas, we’ll never know.

We’re grateful for our users and we want to make their experience the best they’ve ever had. RxCalc should help you do your job, not hinder it. We like the feedback, good or bad, and love when it leads to improvements in the overall usability, and performance, of the application.

It’s disappointing when we see a review in the App Store like this because we can’t help this user solve their problem.

Introducing RxCalc 1.1

Pharmacokinetics, Pharmacy, RxCalc

RxCalc 1.1When we released RxCalc 1.0 we felt the need to focus most of our effort on making sure our math was correct. We felt if the standard user interface was good enough for Apple, it was good enough for us. Since that time we’ve collected a bit of feedback, some great, some not so great, and a couple of really horrible comments that made us cringe. Fear not! We picked ourselves up off the ground, regrouped, and went to work on RxCalc 1.1. We hope you enjoy using it as much as we enjoyed developing it.

What’s New in 1.1?

Options

  • User configurable Units of Measure
    1. Height
    2. Weight
    3. Serum Creatnine
  • Configurable Volume of Distribution
    1. Aminoglycoside
    2. Vancomycin
  • Less rigid scrolling
  • Less rigid data entry

The big addition is the user configurable Units of Measure and adjustable Volume of Distribution. These should help folks outside the United States and gives flexibility to those that would like to modify the default Volume of Distribution values. We’re also really happy with the new navigation experience. Version 1.0 was a bit rigid, we’ve changed that. You can now freely move between entry fields and scroll top to bottom with complete freedom, while the keyboard is showing.

Oh, yeah, it also has a great new icon! Courtesy of our good friend, Mr. Layne Lev.

Cockcroft-Gault equation remains an effective way to estimate GFR

aminoglycosides, Pharmacokinetics, ,

From The Annals of Pharmacotherapy Vol. 44, No. 6, pp. 1030-1037:

Evaluation of Aminoglycoside Clearance Using the Modification of Diet in Renal Disease Equation Versus the Cockcroft-Gault Equation as a Marker of Glomerular Filtration Rate

BACKGROUND: Accurate estimation of kidney function is essential for safe administration of renally cleared drugs. Current practice recommends adjusting renally eliminated drugs according to the Cockcroft-Gault (CG) equation as an estimation of glomerular filtration rate. Few data exist regarding the utility of the Modification of Diet in Renal Disease (MDRD) equation in drug dosing.

OBJECTIVE: To evaluate glomerular filtration rate based on creatinine clearance (CrCl) derived from the MDRD or the CG equation compared with patient-specific CrCl calculated from aminoglycoside peak and trough concentrations.

METHODS: Medical records of patients who received aminoglycoside antibiotics were reviewed over 1 year. Patients who received aminoglycosides via conventional dosing with peak and trough concentrations at steady state were included. Calculations based on standard pharmacokinetic equations were used to estimate CrCl from aminoglycoside serum concentrations. Patient-specific CrCl estimated from aminoglycoside concentrations was compared with estimated CrCl from the CG or MDRD equation.

RESULTS: Fifty-five patients were included in the final analysis. The primary outcome showed concordance between estimated and actual aminoglycoside clearance was 0.53 (95% CI 0.18 to 0.88) for the CG equation and 0.41 (95% CI 0.04 to 0.78) for the MDRD equation. Subgroup analysis also favored CG as a better predictor of CrCl. This signified a stronger correlation between the CG equation and aminoglycoside clearance.

CONCLUSIONS: Compared with the MDRD equation, the CG equation provided better correlation of estimated glomerular filtration rate for aminoglycoside antibiotics. Institutions should continue to use the CG equation as the standard of practice to safely adjust aminoglycoside doses in patients with renal dysfunction.

It appears that the Cockcroft-Gault (CG) equation remains an effective way to estimate GFR for aminoglycoside PK calculations. I’ve been using the CG equation since my pharmacy school days and have no immediate plans to make a change.