Formulation is not a one-time event. The initial formulation is a hypothesis; the patient's unfolding course is the experiment that confirms, refines, or rejects it. Reformulation over time is what distinguishes the clinician who is still working the case from the clinician who has settled into a frozen first impression.
Triggers for reformulation include: unexpected treatment response (patient doing better than expected — was the diagnosis right? Better than expected response sometimes suggests the original problem was less severe than thought, or that the right treatment for a different problem coincidentally helped). Unexpected non-response (the most common trigger — failure of expected response should prompt diagnostic reassessment, not just medication escalation). New symptoms emerging during treatment. New information surfacing — collateral, records, the patient's later disclosure of something not initially shared. New context — life event, medical illness, substance use change.
The most consequential reformulation is often unipolar to bipolar. The patient diagnosed with recurrent depression who experiences a clearer hypomanic or manic episode now has a different illness, different prognosis, different treatment. Catching this reformulation matters substantially — antidepressant monotherapy is contraindicated in bipolar, and mood stabilizers become essential.
Document the reformulation explicitly. "Reformulation 6 weeks in: initial impression was unipolar MDD, but mood lability and brief hypomanic episode in week 4 prompts revision to bipolar II. Treatment plan revised: discontinuing sertraline, adding lamotrigine maintenance, monitoring for further mood episodes." The chart shows the diagnostic evolution; the next reader sees the thinking.
Share major shifts with the patient. "I'm revising what I think is going on. Originally I thought X; now it looks more like Y. Here's what changes about our plan." Transparency about diagnostic evolution maintains alliance better than silent shifts in treatment that the patient doesn't understand.
The patient teaches you their illness over time. The first visit produces a working understanding; the next year fills in the picture; the long-term trajectory reveals what couldn't be seen at the start. Stay open to learning what the patient has to teach.