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Stop hunting for zebras in Texas: end the diagnostic culture of “rule-out”

BMJ 2014; 348 doi: (Published 07 April 2014) Cite this as: BMJ 2014;348:g2625
  1. Saurabh Jha, assistant professor of radiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA
  1. saurabh.jha{at}

We need to make the most of clinical context rather than order every investigation, writes Saurabh Jha

“The patient’s skull was struck by a baseball bat. He has a perfectly legitimate reason for subarachnoid hemorrhage. He already had a CT [computed tomogram] of the head showing the bleed in good detail. Why another?” I remonstrated with Watson, the neurosurgeon.

“But you don’t know that there is no intracranial aneurysm. You can’t rule that out. He needs a CT angiogram of the brain immediately,” protested Watson.

Hit by a hard object (cause) and blood in brain (effect) is deductive reasoning at its simplest. But Watson was correct: I could not rule out cerebral artery aneurysm without a CT angiogram. I could not, for that matter, rule out bleeding brain metastases from lung cancer. Perhaps the patient needed a CT of the chest, I suggested facetiously.

The diagnostic permutations in medicine are innumerable, and what prevents doctors from descending into parody is the application of conditional probability: Bayes’ theorem. Without clinical context we stare into the abyss.

The likelihood that someone with cerebral aneurysm hit by a bat develops subarachnoid hemorrhage (near certainty) is not the same as the likelihood that someone who develops subarachnoid hemorrhage after high impact trauma has an aneurysm, hitherto undisclosed (very low). In fact, the likelihood of the latter is less than the likelihood of finding a cerebral artery aneurysm in a person picked randomly off the street.

No statute protects patients with cerebral aneurysms from being hit by a bat. It’s the law of parsimony summed up by the popular adage: if you hear hoof beats in Texas think of horses not zebras. However improbable, it was not impossible. That was Watson’s point. He was practicing a form of diagnostic medicine designed to catch zebras in Texas: “rule-out” medicine.

Watson’s rationale for fishing for rarities—“can’t be ruled out”—is unfalsifiable. This phrase cannot be disproved. It smashes Bayes’ theorem and Occam’s razor to smithereens. It is kryptonite to clinical acumen.

Before I am asked to get off my high horse I should confess that I too use the phrase. My interpretations of medical imaging often contain such truisms as “small pulmonary clots can’t be entirely ruled out” and “the possibility of infection can’t be excluded with absolute certainty.”

Recently I reported on a CT of a college student tackled fiercely in soccer. He had flank ecchymosis, a couple of fractured ribs, and blood in the adrenal gland. I asked that he receive follow-up magnetic resonance imaging (MRI) to exclude the possibility of an underlying adrenal mass.

“Is being kicked in the flank not enough to explain adrenal hemorrhage?” the trauma surgeon asked rhetorically at the collapse of my critical faculties. “I’ve been burnt before,” I sheepishly defended. And I had, or rather the patient had, but that was a different body part. With a safety culture it is easier to extrapolate, to see similarities between patients, than to differentiate. Discernment requires effort.

How did we arrive here? How did medicine change from confirmation to refutation, from “rule-in” to “rule-out”? Two common explanations for the high costs of healthcare—fee for service and defensive medicine—do not readily help. Watson had no monetary gain from the CT angiogram. Nor do I think Watson had a premonition of a subpoena for missing an aneurysm.

Watson ordered the study because he could; because CT is available. George Mallory quipped, when asked why he wanted to climb Everest: “Because it is there.” Watson ordered the study because he feared being wrong.

Of course, if the CT was negative for aneurysm, which it was, Watson would be wrong. But there is wrong: falsely declaring disease in a healthy person—a false positive. And there is (really) wrong: falsely declaring health in a diseased person—a false negative.

Since the Institute of Medicine’s report on medical errors, To Err is Human, many doctors have chosen being wrong over being really wrong. Between a false positive and false negative, I’ll take the false positive. If I must err then I will err on the side of caution.

The choice has made diagnosis more sensitive: the chances of missing cancer in someone with cancer when cancer is suspected are exceedingly low. But you can’t have your sensitivity and eat it, too. The trade off is specificity: there is a chance of declaring health in someone who actually is healthy.

Watson is a fine product of medical education. He is well read. He is thorough. He is non-judgmental. He presents long lists of possibilities. Nothing can be elementary for Watson. He knows too much; so much so that in every horse he sees an opportunity for a zebra. Zebras are intellectually exciting.

With this change in medical culture, the adage “common things are common” now disclaims “yes but rare things are also common.” Students used to be praised for arriving at a paucity of diagnoses after targeted patient assessment; they are now lauded for thinking of the atypical instead of the obvious.

The dialectic between an imperfect science and our unyielding quest for accuracy has led to our implacable intolerance of uncertainty, which drives the use of diagnostic services. Conquering uncertainty is impossible. Rule-out begets more rule-outs, more tests, and more uncertainty. Chest radiography to rule out pneumonia leads to “pneumonia not excluded but questionable lung mass, CT recommended,” leading to “no mass but questionable aortic dissection, CT angiogram recommended,” leading to “no aortic tear but questionable adrenal cancer, MRI recommended.”

But who pays for this treasure hunt? Shuttled around the hospital with lines and tubes attached, hauled from bed to CT scan to bed, with a large bore cannula so that iodinated contrast can be injected at 5.5 ml a second, indulging Watson’s boundless intellectual curiosity is not a trivial task for his patients. And neither is it for the healthcare system that must heal both Watson and his patients.


Cite this as: BMJ 2014;348:g2625


  • Competing interests: I have read and understood the BMJ Group policy on declaration of interests and have no relevant interests to declare.

  • Patient consent not required (patient anonymised, dead, or hypothetical)

  • Provenance and peer review: Not commissioned; not externally peer reviewed.

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