Differential diagnosis
Differential Diagnosis table for2009 Influenza A (H1N1) virus
| Condition | Differentiating signs/symptoms | Differentiating tests |
| Viral upper respiratory tract infection |
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| Seasonal influenza |
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Most commonly seen during the winter months.
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Generally, diarrhoea and vomiting are less commonly seen with seasonal influenza.
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Clinically, distinguishing between seasonal and 2009 H1N1 influenza is not necessary, except from an infection control or public health standpoint. Reverse transcriptase-PCR distinguishes between the 2 infections based upon positive reactivity with either the human seasonal influenza panel or the swine influenza panel.
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| Bacterial pneumonia |
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Secondary bacterial pneumonia is a known complication of influenza infection, although the incidence is unknown. Bacterial pneumonias also occur more frequently in the winter, when seasonal influenza is circulating. Cough is usually productive. Shortness of breath is more prominent.
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CXR demonstrates findings ranging from focal infiltrates to diffuse infiltrates to lobar consolidation, depending on severity and aetiology of the bacterial infection. Influenza infections can cause interstitial infiltrates, but lobar consolidation is uncommon, unless a secondary bacterial pneumonia is present.
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FBC may reveal elevated WBC count.
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Sputum or blood culture may reveal positive Gram stain.
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| SARS |
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Usually a history of travel within 10 days of onset to a location with documented or suspected recent SARS transmission, or a history of close and prolonged contact with a SARS patient.
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Early symptoms are typically non-specific (e.g., myalgia, malaise, headache) followed by fever. Cough, which is usually non-productive, develops 2 to 7 days after symptom onset. Dyspnoea is a late symptom, as is a watery diarrhoea, which occurs in the second week in up to 25% of patients.
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| Meningitis |
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Tests for influenza virus are negative. CSF analysis may show increased WBC count, elevated protein, a positive Gram stain (in bacterial meningitis). PCR may be positive for viruses other than influenza in viral meningitis.
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Last updated: Dec 30, 2011
