Common things are common, except when the diagnosis is rare


During my training at Hennepin County Medical Center (HCMC) in Minneapolis, MN, my mentor would use the following teaching pearl during rounds: “When you hear hoofbeats, think of horses not zebras!” He would also frequently use: “If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.” What did my mentor, the Sage of HCMC, mean by this?

From Wikipedia: “Zebra is a medical slang term for a surprising  diagnosis. Although rare diseases are, in general, surprising when they are encountered, other [common] diseases can be surprising in a particular person and time, and so “zebra” is the broader concept.”

Therefore, when evaluating a patient, a physician must consider common diagnoses before rare ones because “common things are common.” So when a physician “hears” about symptoms that can be explained by a common diagnosis, the common diagnosis is usually the correct one — not the rare diagnosis.

Wikipedia defines the “duck test” as: a humorous term for a form of inductive reasoning. Inductive reasoning… is a kind of reasoning that allows for the possibility that the conclusion is false even where all of the premises are true.

Clinicians are taught that common things are common. However, sometimes, the rare diagnosis is the right one. So if it quacks like a duck, it probably is a duck — unless it is an American coot which is not a duck.

Occam’s Razor is defined as: the simplest explanation is usually the correct one. In other words, if one diagnosis can explain a constellation of symptoms, the clinician should give the patient one diagnosis, not more. If the clinician increases the number of diagnoses, beyond what is “necessary”, the clinician is making more assumptions than necessary.

In  medicine, parsimony is preference for the least complex explanation for an observation. So the thinking is that if one medical diagnosis can explain all of a person’s signs and symptoms, then it must be the correct diagnosis. (I find that Occam’s Razor and parsimony are used interchangeably in the medical community.)

During an evaluation of a patient, the physician evaluates the signs and symptoms of a particular patient’s problem, tries to put all of the information together, and creates a  differential diagnosis (a list of the possibilities describing what a person’s diagnosis may be). Then, the most likely possibilities determine which tests are ordered and which treatments are prescribed.

So which way of thinking is right here? Do we choose the diagnosis that is rare so that we can have one “label” to explain everything, or do we choose more likely diagnoses that are more common even though that may make the situation more complex?

Let me answer the question with a question. Did you know that a patient is more likely to have many common diagnoses than one uncommon/rare one?

For example, a patient with protein in the urine,  high blood pressure, chronic kidney disease and frequent urination is more likely to have diabetes than  Fabry’s disease. Nephrologists need to know about Fabry’s disease because there is a therapy for this  genetic disorder. However, the patient I describe is more likely to have diabetes in addition to high blood pressure as causes of her  kidney failure than Fabry’s (the zebra possibility in this case). One could argue that if we use Occam’s Razor, the Fabry’s would put everything together and simplify the situation — but it’s not the right diagnosis.

Fabry’s disease is an example of a “must not miss” diagnosis that kidney specialists must know about. However, if we are always “ruling out” the zebras, this way of thinking can become expensive and inappropriate.

Search engines can cause confusion for patients because they surf the web and find these zebra diagnoses. Many times I’ve been asked to order extremely expensive, unnecessary tests because WebMD has an article discussing a zebra diagnosis. I sometimes try to redirect the patient’s passion for garnering information from the internet from searching for zebras to learning more about the diseases they actually have.

So remember that if you hear hoofbeats, you are more likely to see a horse … unless you are at Omaha’s Henry Doorly Zoo’s Hoofstock exhibit — then it may be a zebra!

Michael Aaronson is a nephrologist who blogs at his self-titled blog,
Michael L. Aaronson M.D.

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