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PATIENT 
Ms. L is a 22-year-old woman who comes to your office in November complaining of cough and fever.
What is the differential diagnosis of acute cough and congestion? How would you frame the differential?
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CONSTRUCTING A DIFFERENTIAL DIAGNOSIS
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The differential diagnosis of acute cough and congestion ranges from trivial self-limited upper respiratory viral infections to serious, imminently life-threatening pneumonia. Importantly, there are many causes of pneumonia that need to be identified in order to make an accurate diagnosis and provide appropriate treatment.
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PATIENT
Ms. L reports that she was in her usual state of health until 5 days ago when a cough developed. There was no associated sore throat, rhinitis, myalgias or headache. Two days ago, a low-grade fever (37.8°C) developed, which increased last night to 38.8°C. She reports that her sputum is yellow and that she has no chest pain or shortness of breath.
How reliable is the history and physical exam for detecting pneumonia?
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On physical exam, Ms. L is in no acute distress. Vital signs are RR, 18 breaths per minute; BP, 110/72 mm Hg; pulse, 92 bpm; temperature, 38.8°C. Pharynx is unremarkable; lung exam reveals normal breath sounds without crackles, dullness, bronchophony, or egophony.
At this point, what is the leading hypothesis, what are the active alternatives, and is there a must not miss diagnosis? Given this differential diagnosis, what tests should be ordered?
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RANKING THE DIFFERENTIAL DIAGNOSIS
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As noted above, the initial differential diagnosis of acute cough and fever includes acute bronchitis, influenza, and pneumonia. Like many real-life patients, this patient’s clinical picture is not typical for any of these conditions. Acute bronchitis may cause a cough and low-grade fever but 38.8°C is unusual. Pneumonia could clearly cause a cough and fever but is often associated with an abnormal lung exam. Influenza often causes cough and fever, (and a normal lung exam) but the subacute onset of fever and lack of other upper respiratory symptoms is unusual. It is also early in the season for influenza. Nonetheless, you decide that influenza is the leading hypothesis with acute bronchitis and pneumonia being active alternatives. Given the clinical uncertainty, a chest film and nasopharyngeal swab for influenza are ordered. Table 10-2 lists the differential diagnosis.
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