The Differential Diagnosis

One of the main things I'm learning is how to make a differential diagnosis. This is entirely different than making a diagnosis on a multiple choice exam. We've had several clinical scenarios that have clear-cut diagnoses. Yet as I listen to residents think these cases through, I realize that despite knowing the answer, they can still generate a rich library of alternate possibilities. This is an amazing ability to me. I'm too used to jumping directly to the one answer, convinced it cannot be anything else.

For example, we had a young woman present with altered mental status, fever, hypotension, and acute renal failure several days after she was diagnosed with a kidney stone at another hospital. She also had a low white blood cell and platelet count. This felt strongly of sepsis to me, perhaps compounded by DIC, and sepsis was the right diagnosis. But at resident report, I heard a wealth of fascinating diagnoses: she met all the criteria for TTP/HUS and someone threw out neuroleptic malignant syndrome since she got phenergan.

Another case: I admitted a young woman with cystic fibrosis presenting with a few months of productive cough and hemoptysis (coughing up blood). This felt like a clear-cut case of CF exacerbation. But my resident brought up the idea that given this woman's travel history, she might have TB. Once he said it, it made sense. The answer was CF. But to forget TB (especially in San Francisco) would be a mistake.

It is important to think in broad strokes for several reasons. Cases can be sold in almost any way. I can take a case of a pulmonary embolism and make it sound like acute coronary syndrome or a pneumothorax or a pneumonia. I've seen this done. The ER can pawn off a non-cardiac case to cardiology simply by making the story sound right; no lying is involved, it's just putting the right emphasis on things. And how cases are presented colors our perception subconsciously. So given any case, I need to remember to keep an open mind and a broad differential (at least in medicine; the opposite occurs in surgery).