Occam’s razor, also known as parsimony, is the scientific and medical principle that the simplest explanation that addresses all the data or symptoms is usually the most likely diagnosis. The principle of parsimony is meant to ease diagnostics by leading clinicians to follow observable symptoms toward a single uniting diagnosis. As 13th-century Franciscan friar and theologist William of Ockham explained when writing about his Aristotelian-based theory, “Plurality is not to be assumed without necessity [...] it is futile to do with many what can be done with few.”
But does that line of reasoning always work? As Dr. James Kelly writes in “The Diagnostic Approach in Complex Patients: Parsimony or Plenitude?”: “Atomic theory was initially rejected as it inferred the existence of invisible particles that had not yet been detected.” With that in mind, what diagnoses have been missed due to a clinical preference for Occam’s razor?
As healthcare workers, it’s crucial to know when to use Occam’s razor when diagnosing patients and when to pursue more complex leads based on your knowledge and intuition.
The appeal and limitations of Occam’s razor
Why have doctors so long relied on the principle of parsimony? The appeal of using Occam’s razor in diagnostics is clear: a single, uniting cause that explains all symptoms in one fell swoop and results in their treatment is an appealing outcome. If that doesn’t identify the underlying illness, using Occam’s razor to test diagnoses can still help clinicians rule out the more obvious potential causes of a patients’ symptoms.
However, despite the appeal of a single diagnosis, that is not always what patients are facing. What about cases with multiple comorbidities? Or when the disease being diagnosed is a rare condition? Or one where symptoms are less visible?
Complex and rare cases can present challenges when seen through the lens of parsimony, and seeking the simplest explanation that covers all the symptoms in such situations may lead to serious misdiagnoses. Dr. Kelly continues,
“Diagnostic parsimony may be associated with under-ascertainment of secondary diagnoses in patients with chronic diseases. [A study evaluating] how often older patients with an established chronic disease were likely to be treated for a second unrelated prespecified diagnosis in comparison with matched patients without the chronic disease [...] showed that treatment for a second diagnosis occurred 30%-60% less often in the chronic disease group. It seems likely that under-diagnosis was a factor here. [...] The law of parsimony is gradually replaced by the law of plenitude as age, frailty, and comorbidities increase[.] Those involved in the care of patients with polymorbidity would do well to heed the words of Francis Crick, who observed that ‘Occam's razor … can be a very dangerous implement in biology.’
What happens when Occam’s razor fails?
Patients depend on clinicians’ accuracy and insight for their health, so stakes can be very high in diagnostic situations — literally life and death.
While working in an emergency department in Wollongong, Australia as a medical student, Fahad Farooq encountered what he was told was a simple case of a woman experiencing anxiety over a rash on her foot. He explained in “A Close Cut: Occam’s Razor and Developing Intuition” that the patient “muttered something quickly” about discomfort in her shoulders while Farooq inspected her swollen foot, the latter of which the attending nurse and resident focused on as the sole cause of her issues. Initially he agreed with his coworkers, opting to assume the correctness of Occam’s razor in this case.
