Originally posted 28 September, 2009
Dr. Jerome Groopman, whose writing I generally enjoy, put out a book a couple of years ago called How Doctors Think. It examined, well, how doctors think, how they think they think, and what the future holds for diagnosing disease. It’s a good book, but with some faulty assumptions. I’m not the guy to write about how decisions are made—I don’t know enough about the field, a field which needs much more research. But most doctors do not, as is sometimes posited, make diagnoses via algorithm. Nor are we slavishly bound to statistical likelihood, as the use of likelihood ratios and, er, the like has some problems. What we do teach formally is the process of differential diagnosis.
Differential diagnosis (DDx) is the fun part of medical thinking, and hopefully the lessons learned about the process endure. When a resident or medical student presents a case to me, they often have an immediate feeling for what is wrong with the patient. This feeling may or may not coincide with reality. Getting a gestalt feeling for a case is important, but it is only a starting point. One of the gestalts I like my residents to get a feel for is whether a patient is really sick—I don’t mean whether or not they have a cold or whatever, but do they appear seriously ill. There are parameters which can help determine this, but when someone comes in with a vague picture and you don’t have access to sophisticated diagnostic equipment, it’s good to be able to make that judgment.
One of the luxuries of being a teaching physician is being able to take the time to break down a case in a more formal manner and to develop a traditional differential diagnosis.
One way to do this is to present very little information to start with. For example, if a student tells me a patient is anemic (has a low red blood cell count) I may stop them and ask them what, very broadly, can cause anemia.
- Loss of blood
- Decreased production of blood
- Destruction of red blood cells
- Sequestration of red blood cells
Then we can look at the evidence to make some guesses as to what may be causing anemia in this patient. Are they bleeding? Do they have a big spleen? In addition to these broad categories, we can look at different categories of cause, or “etiology”. What disease categories can cause anemia?
- Neoplasia (cancer)
- Immune-mediated disease
This process often takes place implicitly, but I prefer to make it explicit as often as possible. Once we’ve laid out the thought process, we can look at the evidence presented by the patient herself and start to cross out possibilities. For example, if the patient isn’t obviously bleeding, her stool is tested negative for occult blood, she no longer menstruates, and has no evidence of internal bleeding, then acute blood loss seems unlikely as an explanation. If the labs show normal red
blood cell morphology, but decreased kidney function, then decreased of red cell production due to kidney failure seems more likely. If she has poor kidney function, her red cells appear deformed, and her platelet count is also low, this opens up another door.
Differential diagnosis stands in stark contrast to fake medical thinking. If you look at how some alternative practitioners think, you see the evil, goatee’d twin of differential diagnosis, in which someone ignores biology and comes up with elaborate explanations for disease which bear little resemblance to reality.
Differential takes the implicit and makes it explicit. It forces us to think of all possibilities and winnow them down to the most likely answers. It’s also a helluva lot of fun. Perhaps we will have a few morning report cases here where we follow this process. If you want to submit a case, go ahead, and I’ll turn it into a post so we can all play. If you do submit a case, please remember that privacy is a must, both legally and ethically.