Hoofbeats... Think ANY hooved animal...

If you have had two days in a hospital, you can come across the phrase,

When you hear hoofbeats, thinks horses not zebras.

This is supposed to belay the information that common things are common - therefore you are more likely to see common things rather than rare things.

I hate this phrase.

I think this is lazy thinking and stupid medicine. It is dumb. It strips away all diagnostic thought process and teaching people to assume common things are common. However, this is nonsense and ludicrous. Actually, common things are common however if you flip heads 10 times in a row, the next time you flip the coin there is still a 50-50 odds that it will be heads for that individual coin toss. Therefore, it makes more sense (to me, at least) that you should approach your differential diagnosis in a systemic and thoughtful way - not whether things are "common" or "not common".

-End rant-

Okay, here is my thought process on how to approach differential diagnoses in a systematic way.

The thought process begins before you walk into the room.

Before you walk into the room to gather the history of present illness (HPI), everyone will have a Chief Complaint. The Chief complaint is a short, two or three word description of why the patient presented.

Take the chief complaint and write down three or four organ systems that could be contributing or causing the chief complaint.

Your goal is not to identify the "most likely" differential diagnosis - just to identify any three or four organ systems that could possibly contribute.

After you identify 3-4 organ systems, break it down by organ system. Come up with minimum 3-4 differential diagnoses for each organ system.

Why would we do this before seeing the patient?

This circumvents the issue of “experiential learning” - it does not rely on the clinician’s individual experiences and personal medical knowledge but instead now is dependent only upon how creative one can be in generating a thorough differential diagnosis.

It allows the learner to extract information more efficiently. When you hear the patient tell their story, using open-ended questions, oftentimes it is not synthesized. That is okay - patients are not supposed to synthesis their story. Physicians are the ones who take the data and turn it into a narrative that makes sense.

EXAMPLE.

Chief complaint: Shortness of breath

Consider relevant organ systems:

1. Pulmonary

2. Cardiovascular

3. Hematology

4. Musculoskeletal

Consider Differential Diagnoses for each system:

1. Pulmonary - Pneumonia, Pneumothorax, Pleural effusion, asthma, carbon monoxide poisoning, Diabetic ketoacidosis with respiratory failure/Kussmaul's breathing, etc...

2. Cardiovascular - Congestive Heart Failure, Myocardial Infarction, Pericardial effusion, etc...

3. Hematology - Pulmonary Embolus, Anemia, Polycythemia vera...

4. Musculoskeletal - Muscule strain, Guillan-Barre, rib fracture, post-surgical pain leading to atelectasis, etc...

Do you see how all of these scenarios would present extremely differently? The physician's job is to hear the patient's story and then ask follow up questions that allow you to determine which organ system is likely causing the problem. If someone presents with SOB (shortness of breath) and you haven't even thought about pulmonary embolus on your differential diagnosis, then you will not ask about risk factors such as prolonged immobility, recent travel, etc. You are relying on the patient to tell you risk factors - but that is our job in a thorough history!

This way of approaching the HPI requires some initial effort, but is an easy way to incorporate it into your practice. Start now - and see if that helps guide your history taking prowess!