Differentials
Coronavirus disease 2019 (COVID-19)
SIGNS / SYMPTOMS
Close contact with a confirmed or probable case of COVID-19, in the 14 days prior to symptom onset.
Signs and symptoms are similar so it may be difficult to differentiate between the conditions clinically. See our COVID-19 topic for further information.
INVESTIGATIONS
Real-time reverse transcription polymerase chain reaction (RT-PCR): positive for SARS-CoV-2 RNA.
It is not possible to differentiate COVID-19 from other causes of pneumonia on chest imaging.
Typical bacterial pneumonia
SIGNS / SYMPTOMS
Acute onset with a chill followed by a high fever and pleuritic chest pain suggests pneumococcal pneumonia.
INVESTIGATIONS
Sputum cultures and blood cultures may be positive for Streptococcus pneumoniae or other bacterial pathogens.
Viral pneumonia
SIGNS / SYMPTOMS
Symptoms of dry cough, fever, myalgia, and malaise, which are clinically difficult to differentiate from atypical bacterial pneumonia.
INVESTIGATIONS
Influenza and RSV PCR may be positive. Relative lymphocytosis on FBC may be detected.
Tuberculosis
SIGNS / SYMPTOMS
A history of immunosuppression or prolonged course that is not responding to antibacterial therapy suggests tuberculosis.
INVESTIGATIONS
Sputum cultures and acid fast bacilli stains positive. A cavity on the chest x-ray may be observed.
Fungal pneumonia
SIGNS / SYMPTOMS
Travel or exposure in endemic area might suggest histoplasmosis-, blastomycosis-, or coccidiomycosis-related pneumonia.
There may be extrapulmonary symptoms (e.g., rheumatological).
INVESTIGATIONS
Sputum culture and stain may demonstrate hyphae or yeasts.
Antigen detection assays or polymerase chain reaction may identify specific mycoses (e.g., aspergillosis).
Pneumocystis jirovecii pneumonia
SIGNS / SYMPTOMS
History of HIV or risk factors should raise suspicion.
INVESTIGATIONS
Special stain of sputum or bronchoalveolar lavage will be positive for Pneumocystis jirovecii.
Pulmonary embolism
SIGNS / SYMPTOMS
Absence of fever and/or lack of response to antimicrobial therapy support a non-infectious cause for a patient’s diagnosis of pulmonary embolism.
INVESTIGATIONS
The ventilation-perfusion scan will be positive in pulmonary embolism.
Inhalation/occupational lung injury
SIGNS / SYMPTOMS
A history of exposure to chemicals or special work conditions should raise suspicion of inhalation/occupational lung injury.
INVESTIGATIONS
Cultures will be negative. There may be diffuse disease on the chest x-ray.
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