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Difficulty: MediumFormulating a Differential Diagnosis

A 35-year-old woman with a history of systemic lupus erythematosus (SLE) presents to the emergency department complaining of a 2-hour history of sudden-onset, left-sided pleuritic chest pain and shortness of breath. Her temperature is 37.1°C (98.8°F), blood pressure is 118/76 mmHg, heart rate is 112/min, respiratory rate is 24/min, and oxygen saturation is 91% on room air. Physical examination is notable for clear lungs bilaterally, a normal cardiac rhythm with no murmurs, and mild, asymmetric edema with tenderness in her left calf.

Based on the clinical presentation, arrange the following differential diagnoses in order from the highest pre-test probability (most likely diagnosis) to the lowest pre-test probability (least likely diagnosis).

  1. 1Acute pulmonary embolism
  2. 2Lupus pleuritis
  3. 3Community-acquired pneumonia
  4. 4Acute coronary syndrome
  5. 5Costochondritis

Answer

The correct sequence from highest to lowest pre-test probability is: Acute pulmonary embolism, Lupus pleuritis, Community-acquired pneumonia, Acute coronary syndrome, and Costochondritis.
Acute pulmonary embolism is placed first because it uniquely unifies the patient's chest pain, hypoxia, tachycardia, and calf swelling, especially given her underlying prothrombotic risk factor (SLE). Lupus pleuritis is placed second as it is a common complication of her primary disease, but it cannot explain the deep vein thrombosis symptoms. Community-acquired pneumonia and Acute coronary syndrome are placed third and fourth, respectively, as they lack core supporting features (e.g., fever, typical ischemic pain). Costochondritis is placed last because it is a benign condition that never produces hypoxemia or tachycardia.

Step-by-Step Solution

1
Analyze the primary symptom cluster and vital sign abnormalities.
The patient has sudden-onset pleuritic chest pain, dyspnea, significant hypoxia (91%), tachycardia, and signs of a deep vein thrombosis.
Objective physiological derangements dictate the severity and guide the top differential.
2
Match the clinical picture to the most unifying diagnosis.
Acute pulmonary embolism (PE) explains all findings, including the DVT symptoms, in a hypercoagulable patient.
The principle of diagnostic parsimony (Occam's razor) prioritizes the single etiology that explains both the respiratory collapse and the peripheral vascular findings.
3
Evaluate secondary diagnoses driven by the patient's underlying condition.
Lupus pleuritis is prioritized next.
Serositis is a hallmark of SLE, making it highly plausible for chest pain, though it lacks an explanation for the leg swelling.
4
Rank the remaining less likely etiologies based on missing clinical features.
Pneumonia lacks infectious signs, ACS lacks typical ischemic features, and costochondritis lacks any explanation for abnormal vitals.
Conditions are ranked lower when critical objective findings (like hypoxia or calf swelling) actively contradict their typical illness scripts.

Key Concept

Formulating a differential diagnosis requires synthesizing the entire clinical picture; the highest pre-test probability belongs to the condition that most completely explains both the subjective symptoms and objective findings (vital sign abnormalities, physical exam signs).
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Formulating a Differential Diagnosis

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