Diagnosis: Clinical Problem Identification and Differential Diagnosis
Build the differential deliberately
Generate your list in three tiers: the most likely diagnosis, the must-not-miss diagnoses (aortic dissection, ectopic pregnancy, meningitis, testicular torsion, necrotizing fasciitis, PE), and the look-alikes that change management. Then use the history and exam to move probability up or down. Pattern recognition is fast and often right; it fails when the presentation is atypical, so switch to analytic reasoning when a patient does not fit or does not improve.
Criteria you should be able to recite
Many questions hinge on a validated rule or threshold: Centor/McIsaac for streptococcal pharyngitis, Wells for DVT/PE, CURB-65 for pneumonia severity, Ottawa rules for ankle/knee imaging, HbA1c ≥6.5% for diabetes, spirometry FEV1/FVC <0.70 post-bronchodilator for airflow obstruction, and DSM-5-TR time criteria for depression (2 weeks) versus mania (1 week, or any duration if hospitalized).
Common look-alikes
- Cellulitis vs stasis dermatitis (bilateral, chronic, afebrile, no leukocytosis favors stasis; a large share of "cellulitis" admissions are misdiagnosed).
- Asthma vs COPD vs heart failure: reversibility, smoking history, BNP, and examination findings.
- Gout vs CPPD: negatively birefringent needle-shaped crystals vs positively birefringent rhomboid crystals.
- Delirium vs dementia vs depression: acuity and fluctuation separate delirium.
Traps
- Anchoring and premature closure - the first plausible label is kept after new data arrive.
- Treating a number, not a person: positive ANA, low-positive troponin, or asymptomatic bacteriuria in a nonpregnant adult.
- Making a diagnosis of exclusion without excluding the dangerous alternatives.
Rule of thumb
If a finding is bilateral and symmetric, favor a systemic or venous cause over a focal infection. If the patient is not improving on appropriate therapy, reopen the differential rather than escalating the same treatment.