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Horses, Zebras, Ninjas

Ben_3Ben Bryner -- I was reading this story the other day, in which a camp counselor was mistaken for a ninja, which then prompted a school lockdown. It reminded me of the old medical adage, "when you hear hoofbeats, think horses, not zebras." Briefly, if you’re in an area where horses are more common, when you hear hoofbeats outside, it's much more likely to be the sound of horses, not zebras. The idea is that when a patient presents with symptoms that are consistent with a common disease, but are also consistent with a much less common disease, you work under the assumption that it is the more common disease until you can confirm it. In other words, if you’re in New Jersey, a person dressed in a ninja getup is more likely to be a regular person who’s just into karate or dress-up than an actual ninja.

The saying is usually used to correct a student or resident’s differential diagnosis. When you’re on rounds and presenting a new patient with an unknown or not-quite-certain diagnosis, when you get to the end of your presentation, your attending will generally expect you to list the “horses” (the more common diseases) first and the “zebras” second. If you don’t, the attending may request that you do so by saying “Horses, not zebras,” or by the less-conventional technique of whinnying while slapping his or her legs to simulate hoofbeats.

So you should follow what I like to call the “Family Feud” strategy of presentations, based on the game show of the same name. (If you are wondering whether I think all of medical school can be reduced to elements of game shows, the answer is: No, only 80%.) The point of this show was to guess the most popular answers to open-ended questions, with one team trying to list off the top answers to build up points, and the other team waiting for their chance to pounce and steal the points by giving an answer the other team neglected. In a presentation, if you go through the most common possible diagnoses and then get down to the more obscure ones, it’s less likely that someone else on your team will steal the diagnosis you’re waiting to reveal, or that you’ll get interrupted before listing the most important diagnosis. Also, if you’re on a surgical rotation, you should look around after listing off more than two or three potential diagnoses, as your team has probably already moved on to the next patient.

This is not to say that you can ignore the zebras. You try to confirm the presence of a horse before moving on to investigate the presence of a zebra. And you do this mindful of the setting. If you are in feudal-era Japan and you see a shadowy masked figure running around outside, then “Ninja!” is a pretty good theory. By the same token, identical symptoms in a newborn, a teenager, and an adult may prompt very different diagnoses.

As long as you’re not in a true emergency situation, in which you have to try to rule out even uncommon diagnoses if they could cause death rapidly, taking the Family Feud approach to diagnosis has its advantages. Less money is wasted on low-yield tests and evidence-based medicine has a better chance of being followed. When you jump straight to the weird diagnoses, patients get scared, easy fixes get missed and everyone gets confused. Think of the kids in that school who are probably less likely to take a future lockdown seriously. They’ll laugh -- “What is it this time, a pirate in the cafeteria?” And then when ninjas really do attack, they won’t be ready.


And if you’re not ready for a ninja, you don’t have a chance.

July 8, 2008 in Ben Bryner | Permalink


exactly...but it is also wrong to just consider the HORSES and totally ignore the probability of it being a ZEBRA...no matter how small the probability is...if after careful evaluation and lab work ups, it still doesn't add up... just a month ago, as an emergency doctor,i saw a patient being treated as preeclampsia by an attending but it so happened that the patient actually has AORTIC ANEURYSM... sad to know, after 2 days, the baby died then the mother w/ the rupture of the anuerysm...

Posted by: ninette | Jul 8, 2008 5:35:25 PM

I just want to say that you made me laugh hysterically. A ninja.....really?! :)

Posted by: | Jul 9, 2008 3:41:21 PM

when you hear hoofbeats you should' t think of horses imho, you should think of hoofs, because 7 out of the 10 cases you end up treating the sound, not the animal, pathophysiology rules by the way:)

Posted by: zeynep | Jul 10, 2008 9:17:25 AM

firstly, think of common diagnosis and you will be right most of the time.
secondly, rare diagnosis are rare because physicians rarely think about them.

Posted by: Kitr | Jul 12, 2008 5:32:37 AM

How good you wrote this...funny and relevant both!

Posted by: Mini | Jul 12, 2008 6:24:56 PM

You've got a great sense of humor, which you apply to embellish that parallelisms you draw between medical carrier and your Phytonian imagination, with brilliant results, indeed.

I do also use to draw this sort of parallelisms but my mates never seem to laugh, which can mean a) Mine are not funny. b) They are not funny people. Probably a.

By the way, talking about medicine and ninjas, take a look at this comics I've found...


Greetings from Barcelona, and please be so kind to excuse my obbiously not Shakespearean mastering of the english language.

All the best.

Posted by: Omar | Jul 16, 2008 10:18:06 AM

As a health professional who turned out be a Zebra...I encourage horse first BUT Zebras second, My personal experience was horse first and nothing next. As a private practitioner on per diem...I about went Bankrupt (credit ruined). I finally figured it out and insisted on Zebra testing. (I had an ill defined Lupus like autoimmune disorder, ultimately needed chemo as it had progressed...). Then we all got on track. Problem was...my PCP/HMO...wouldn't consider Zebras.

Posted by: Melissa | Jul 17, 2008 7:42:18 PM

Thanks too much for this advisement. We should really concentrate on the most important and brilliant choices then come the next ones, on sequence.

Posted by: Soma | Jul 21, 2008 10:11:48 PM

I probably wouldnt be able to count the number of times I've been stopped for jumping to the Zebras every oppotunity I get.

Most of the time its because the Zebras fascinate me more than the horses! And also becuz more often than not - I want it to be a Zebra and not a horse. I've seen enough horses already, where are all the zebras hiding?!!

Posted by: Kareema | Jul 22, 2008 11:24:57 PM

I have to laugh because as a practitioner of Ninpo (I've been in Bujinkan, Genbukan and now Jinenkan) and as a medical student I have to agree with the author.

I was in medicine in Japan 17 years ago, came back to the US and went into technology for several years. Now I'm getting back into medicine and I know the old saying is true however I would caution both physicians and students with the following adage, "When you hear hoof-beats, you have to ask yourself why in the heck the horses are running in the first place?"



Posted by: Nicholas Donovan | Jul 24, 2008 11:51:27 AM

Or perhaps ninjas on horseback should be considered...

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Posted by: Soft Cialis | Nov 26, 2009 7:21:55 AM

And if you’re not ready for a ninja, you don’t have a chance.

Posted by: hairy pussy | May 17, 2010 5:43:22 AM

If an African man fucks an American man then he fucks another American man, an american MD may think the "zebra" question may be unlikely, but he would be completely wrong. And, this goes just as well for heterosexuals who happen to be adventursome or just free and open minded about sex. Am MD should be professional enought to rise above embarement and cultural norms to ask all questions that may be relavent, no matter how remountly possible, that may be diagnostic.

Posted by: S E | Jan 15, 2012 11:07:24 PM

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