Differentials
Coronavirus disease 2019 (COVID-19)
SIGNS / SYMPTOMS
Important to consider the current COVID-19 epidemiological situation and any recent outbreaks. May give history of COVID-19 exposure.
Signs and symptoms are similar so it may be difficult to differentiate between the conditions clinically.
INVESTIGATIONS
Real-time reverse transcription polymerase chain reaction (RT-PCR): positive for SARS-CoV-2 RNA. Rapid antigen tests may also be used.
It is not possible to differentiate COVID-19 from other causes of pneumonia on chest imaging.
Community-acquired pneumonia
SIGNS / SYMPTOMS
No differentiating signs/symptoms.
INVESTIGATIONS
Diagnostic studies should be considered based on local guidance as well as microbial patterns in a particular community.
Isolation of organisms such as Streptococcus pneumoniae, Staphylococcus aureus or group A Streptococcus from sputum and blood culture, and response to typical therapy confirms diagnosis.
Chest x-ray findings for typical pneumonia are consistent with consolidation.
Positive highly pathogenic avian influenza (HPAI) A(H5N1) virus-specific tests do not exclude co-infection, although most HPAI A(H5N1) cases have not had community-acquired bacterial co-infection identified except in intubated patients with ventilator-associated pneumonia. Seasonal influenza virus infection with bacterial co-infection is more common.
Atypical pneumonia
SIGNS / SYMPTOMS
No differentiating signs/symptoms.
INVESTIGATIONS
Confirmation of infection by atypical pathogens (including atypical pneumonia pathogens such as Mycoplasma pneumoniae and Legionella pneumophila) by sputum culture, blood culture, or other specific tests.
A diagnosis of atypical pneumonia does not rule out highly pathogenic avian influenza (HPAI) A(H5N1) virus infection, but co-infection with HPAI A(H5N1) virus and atypical pneumonia pathogens has not been reported.
Seasonal influenza virus infection
SIGNS / SYMPTOMS
More common cause of severe morbidity in young children, older adults, and people with underlying chronic medical conditions (e.g., cardiopulmonary disease, immunosuppressed).
More likely to be a self-limited condition with milder symptoms among previously healthy people. Severe lower respiratory tract disease can occur among previously healthy children, young adults, pregnant women, and people with class III obesity.
INVESTIGATIONS
Diagnostic tests confirming infection by another respiratory virus does not rule out highly pathogenic avian influenza (HPAI) A(H5N1) virus infection, but co-infection with HPAI A(H5N1) virus and other respiratory viruses is uncommon.
Avian influenza A (H7N9) virus infection
SIGNS / SYMPTOMS
Epidemic has been geographically focused in China.
Most patients require hospitalisation for management of pneumonia and/or respiratory failure and often present soon after the onset of symptoms, in contrast to the late presentation often seen with A(H5N1) virus infection.
No differentiating signs/symptoms.
INVESTIGATIONS
Reverse transcription-polymerase chain reaction (RT-PCR) is positive for H7-specific viral RNA.
Endemic respiratory infections
SIGNS / SYMPTOMS
Respiratory infections due to pathogens endemic to the region where infection occurred should be considered (e.g., endemic mycotic infection, melioidosis in parts of Southeast Asia).
No differentiating signs/symptoms.
INVESTIGATIONS
Diagnostic tests confirming infection by an atypical pneumonia do not rule out highly pathogenic avian influenza (HPAI) A(H5N1) virus infection, but co-infection with HPAI A(H5N1) and endemic respiratory infections has not been reported.
Respiratory syncytial virus infection
SIGNS / SYMPTOMS
Most common cause of lower respiratory tract infection in children aged less than 1 year.
Significant and often unrecognised cause of lower respiratory tract infection in both older and immunosuppressed patients.
Gives rise to upper and lower respiratory symptoms that peak in 3-5 days and resolve within 7-10 days.
INVESTIGATIONS
Rapid assays using antigen capture technology are the mainstay of the diagnostic algorithm, as the identification by culture can take from 4 days to 2 weeks.
Diagnostic tests confirming infection by another respiratory virus does not rule out highly pathogenic avian influenza (HPAI) A(H5N1) virus infection, but co-infection with HPAI A(H5N1) virus and other respiratory viruses is uncommon.
Severe acute respiratory syndrome (SARS)
SIGNS / SYMPTOMS
No differentiating signs/symptoms.
Both can have rapid onset of fever, cough, and pneumonia.
The absence of confirmed cases since 2004 makes the diagnosis of SARS outside of re-emergence of the virus very unlikely.
INVESTIGATIONS
The diagnosis of SARS requires high clinical suspicion and should be informed by global surveillance for infections by SARS-associated coronavirus (SARS-CoV). Tests for influenza virus are negative. Real-time reverse transcription polymerase chain reaction (RT-PCR) is positive for SARS-CoV.
Middle East respiratory syndrome (MERS)
SIGNS / SYMPTOMS
Most cases are epidemiologically linked to the Arabian Peninsula. Many cases are associated with nosocomial transmission. Zoonotic transmission from dromedary camels and limited non-sustained human-to-human transmission have occurred.
No differentiating signs/symptoms. Common symptoms are acute, serious respiratory illness with fever, cough, shortness of breath, and breathing difficulties. Most patients have pneumonia, respiratory failure, and acute respiratory distress syndrome. Many also have gastro-intestinal symptoms (including diarrhoea), while others have kidney failure.
INVESTIGATIONS
Real-time reverse transcription polymerase chain reaction (RT-PCR) is positive for MERS coronavirus. The test can be found at some international public health laboratories, particularly in regions affected by MERS.
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