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Emergentology: The Ups and Downs of Developing the MDCalc App

Walker, Graham MD

doi: 10.1097/01.EEM.0000482476.20709.dc

Dr. Walker is an emergency physician at Kaiser San Francisco. He is the developer and co-creator of MDCalc (, which also has an app (, a medical calculator for clinical scores, equations, and risk stratifications, and The NNT (, a number-needed-to-treat tool to communicate benefit and harm. Follow him @grahamwalker, and read his past columns at





Well, Mom, it's here. You can finally stop asking. The MDCalc app for Apple's iOS is now available.

I'm the creator of a couple of medical websites, including, which you've hopefully found useful and used to calculate ... well, really anything that you're calculating while working a shift. It might be someone's PE risk or another person's mortality risk from pneumonia, but we really try to have every single possible score and calculator you might use during a shift in the ED available for you.

We have had requests for years now from lots of people (including my mom) for an iPhone app. (An Android app is currently in the pipeline. We learned a ton about mobile from developing for iPhone, which will hopefully speed up our Android app timeline. I'll keep you posted.) And after a year or so in the making, you can now download it free through May from the iTunes App Store:

Everyone (again, my mom included) always asked, “What's taking so long?” and I wanted to share a number of the lessons I learned for those of you who have an app idea or have thought about combining your tech chops with medical skills. It's been quite a year of development, and it's rare to get a chance to really talk about the huge number of decisions that have to be made to go from concept to finished product on the App Store.

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The Challenges

I love the Internet and web browsers and combining medicine with technology to make my life easier and more efficient when I'm working a busy shift. Apps, I've learned, are a whole different beast. First, if you're not aware, apps are programmed with languages different from your standard web browser code. There are some exceptions, but just because you can make a website does not mean you can make an app. That would be like saying because you can print a Word document you can create a complicated Excel spreadsheet. They're just not the same.

Secondly, apps are made for small screens. (Some web browsers have small screens, but we know from that most of you visit the site on a desktop as opposed to a phone/tablet.) Really big screens make it pretty easy to display all the information you need to on a page. You can expand a lot of abbreviations, you can show all the content you want to show on the screen without scrolling, and you can even put related content on the page so users can easily find that, too.

But phones — even the big ones that blur the line between a phone and a tablet — just can't compete for screen space. You either have to make all that content tiny (and too small to read), scrunched up so close together that it's difficult to process, or spread it out on multiple screens. We have done our best to find a happy medium. We show enough content to get the point across with smart options to get more definitions with info buttons. (For example, if a score calls for “HR>100,” we assume that our users know this means “heart rate greater than 100 beats per minute,” but it might be more difficult to infer if this means “initial heart rate in triage” or “highest heart rate recorded.”)

Because there are now so many equations on the site — with more added every month — we also developed a custom search setup just for the app that lets you search and then filter your results based on disease or chief complaint, similar to how you might filter athletic shoe results by color, brand, or price on Amazon. Finally, most of today's apps require a working connection to the Internet. Having worked at several EDs without a great cell signal or any WiFi option at all, we've found a way to make the app work without a signal.

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The International Audience

It's also challenging to develop an app for international users. We have users in more than 40 countries, and we plan to develop our calculators in other languages, but we first had to deal with the fact that we're not always speaking the same language even in English. The United States uses customary units (and on the website, we actually detect what country you are in and try to default to the correct units). Everywhere else uses Systeme Intèrnationale (SI) units.

We collect each user's country in the app itself so we can do the same thing. Interestingly, most research is still in U.S. units because that is where most research originates. We do, however, allow you to switch units back and forth manually and dynamically within equations for weight, height, and temperature (nothing more annoying than having to convert kilograms back to pounds or, horror, Celsius back into Fahrenheit on the fly.)

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