A while back, I posted three “rules” of presenting on a consult service. I’d now like to add a fourth rule.

Rule Number 4: In patients with chronic disorders, consider more than simply a “disease flare” in your differential diagnosis.

This rule follows up on Rule Number 3. The rule is most relevant to patients with underlying chronic disorders (e.g., inflammatory bowel disease, emphysema, ischemic cardiomyopathy) and is important for fleshing out a broad framework for a differential diagnosis.

After you have gone through a detailed presentation of a patient with a chronic disorder, the path of least resistance when a patient presents with similar signs, symptoms, and findings, is to diagnose a “disease flare.” However this knee-jerk reaction excludes a number of other broad options that may be going on.

Here is the framework I suggest in considering your differential diagnosis in a patient with a chronic underlying disorder. The presentation could be caused by:

1. The disease;
2. A complication of the disease;
3. A complication of the treatment of the disease; and/or
4. A completely unrelated disease

In virtually any patient, this general schema can be helpful to make sure that you do not anchor your diagnostic possibilities on the chronic disorder.

As an illustration of how I use the framework on teaching rounds, I’ll describe a typical (made-up) case we might see on the GI consult service, a 25-year old man with Crohn’s disease and a possible flare. He was diagnosed 6 months prior with inflammatory ileocecal Crohn’s disease after presenting with right lower quadrant abdominal pain and watery diarrhea. The colonoscopy at the time revealed severe inflammation in the cecum and terminal ileum. He has been treated with steroids and infliximab, and was brought into remission within 3 months. Now, he presents with 3 days of acute watery diarrhea and recurrent abdominal pain. Without giving any more details, here is how I might break down my thinking:

1. The disease: Sure, it is easy to say that this is a “Crohn’s flare,” but then you’d have to ask yourself, “Why is the disease flaring?” Could the medications no longer be working? Has the patient been adhering the treatment regimen? Are the medication dosages too low? Nevertheless, this is an easy place to stop unless you consider the next 3 broad possibilities.
2. A complication of the disease: Crohn disease can cause at least 2 complications that can lead to similar presentations: fistulae and strictures. Of course, you could argue that these are the disease itself, but I would refute the argument because the treatment of these complications can be different from treating the underlying inflammatory process itself.
3. A complication of the treatment of the disease: As much as physicians don’t like to admit it, our therapies can definitely play a role in our patients’ worsening. Surgeons are quite attuned to looking for complications of their surgeries while their patients are recovering in the hospital, but medical therapies also have complications that should be considered, especially in the outpatient setting. In this case, the patient is on immunosuppressive agents. Could the treatment have led to an infectious disease, such as CMV colitis?
4. A completely unrelated disease: Importantly, this element, sometimes known as “true, true, and unrelated,” is how we are taught to think when we are creating differential diagnoses in medical school and residency training, but can often get neglected in the presence of a chronic disorder. Does he have a young child in preschool, who could have contracted a Rotavirus infection and transmitted it to your patient? Could the patient have taken an antibiotic for a sinus infection and developed C difficile colitis? Here the differential diagnosis can be quite broad, but should certainly not be overlooked when the patient has a chronic disorder.

Acknowledgement: Dr. Arvey Rogers, my first clinical mentor, deserves the credit for teaching me this framework. He is a wonderful clinician, a thoughtful educator, and a gem of a person.