I know dichotomous keys are often used to identify flora and fauna.
I was wondering if they are also used in medicine for identifying diseases -- are they? If not, why not?
For histopathology they use them all the time to describe cells and tissues, very much as they are used to describe biological entities. Characteristics like size, cytoplasm to nucleus ratio, grouping, staining characteristics, immunohistochemistry and now molecular attributes are all used to categorize a cell of interest. For cancer, the identity of the malignant cell is crucial to diagnosis and subsequent effective treatment - based on successful treatments used in the past for people with that type of cancer.
Here is a histopathology dichotomous key I found at Noelways.com
There are several doctors that post on this site, so hopefully one of them can give a better answer. Anyways, several years ago I took a certification program as an emergency medical responder (basically the lowest level of EMT in my area). Various dichotomous keys are used to assess patients. These were not diagnostic but rather used for determining correct patient management. For example, here is a key for airway management. Obviously, these would be committed to memory.
I think the equivalent concept in medicine is the differential diagnosis.
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Sadly, no. In plants, you know exactly the number of sepals, the number of petals, the shape of leaves, the type of pollination and other variables used in 'travelling' through the key. Equivalently, a dichotomous key in medicine would require that a patient with a certain disease display all of the signs and symptoms so-and-so (eg. belly pain, vomiting) and none of the signs and symptoms this-and-that (eg fever and cough) for any given disease. Very very sadly, this is not the case.
As an example, appendicitis: in the typical clinical picture, a patient presents with dull pain around the navel for a few hours with loss of appetite, before progressing to sharp right lower quadrant pain with a low fever and nausea. Simple? No. This presentation appears in about 30% of patients with proven appendicitis, the rest may or may not have had pain around the navel, may or may not have lost their appetite, may or may not run a fever that may or may not be suitably low-grade, they may or may not have been nauseous, they may or may not have vomited, they may or may not have accompanying malaise, some even show up without the basis for diagnosis (sharp right lower quadrant pain)... To make things even more complicated, there's a significant number of patients who DO present with the classical signs of appendicitis (including the obvious appy symptom of sharp lower right quadrant pain) or dangerously similar, but don't have it. What do they have? Your guess is as good as mine, the differential diagnosis is loooooong, and includes things ranging from "OR RIGHT NOW BUT IN A DIFFERENT SPECIALTY!!!!" to "Don't you dare take that patient to surgery, it could kill them!"
And that's just a 'simple' appy. There's a reason they call it the art of medicine.