Stop hunting for zebras in Texas: end the diagnostic culture of “rule-out”BMJ 2014; 348 doi: https://doi.org/10.1136/bmj.g2625 (Published 07 April 2014) Cite this as: BMJ 2014;348:g2625
- Saurabh Jha, assistant professor of radiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA
“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 …