Differentiating Influenza from Other Respiratory Illnesses
Influenza is characterized by the abrupt onset of fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis—but clinical symptoms alone cannot reliably distinguish influenza from other respiratory pathogens, making laboratory confirmation essential when treatment decisions depend on accurate diagnosis. 1
Key Clinical Features of Influenza
Hallmark Presentation
- Sudden onset of constitutional and respiratory symptoms is the defining characteristic 1
- Fever with chills or rigors appears abruptly 1, 2
- Diffuse myalgia and headache are prominent systemic features 1
- Nonproductive cough with sore throat and rhinitis 1
- Malaise that is often severe and debilitating 1, 3
The typical incubation period is 1-4 days (average 2 days), followed by abrupt symptom onset 1.
Age-Specific Variations
In children:
- Otitis media, nausea, and vomiting occur commonly alongside respiratory symptoms 1
- May present initially as high fever mimicking bacterial sepsis 1
- Febrile seizures occur in 6-20% of hospitalized children with influenza 1
- Less likely to report typical influenza symptoms; may appear as simple febrile illness 1
- Can be infectious for ≥10 days after symptom onset 1
In elderly patients:
- May present with less prominent respiratory symptoms 4
- Fever, lassitude, and confusion may be the only manifestations 4
- Lower positive predictive value of clinical criteria (only 30% in patients ≥60 years with fever, cough, and acute onset) 5
Critical Limitations of Clinical Diagnosis
Poor Diagnostic Accuracy
The accuracy of clinical diagnosis based on symptoms alone is severely limited because symptoms overlap considerably with other respiratory pathogens. 1, 5
- In healthy adults, acute onset of cough and fever has a positive predictive value of only 79-88% for laboratory-confirmed influenza 5
- In patients ≥60 years, fever, cough, and acute onset has only 30% positive predictive value 5
- In hospitalized elderly patients with chronic cardiopulmonary disease, fever, cough, and illness <7 days has only 53% positive predictive value 5
When to Suspect Alternative Diagnoses
The presence of rash, lymphadenopathy, or central nervous system symptoms makes influenza less likely and suggests alternative viral infections. 6
- Enterovirus: respiratory symptoms with vesicular rash around mouth and trunk 6
- Adenovirus: respiratory symptoms potentially accompanied by rash 6
- Respiratory syncytial virus: intense respiratory symptoms with marked cough but rash is rare 6
Diagnostic Approach
Clinical Diagnosis
Do not rely on "sudden onset" alone to diagnose influenza—consider the diagnosis in any patient with respiratory symptoms or fever during influenza season, regardless of whether onset appears sudden or gradual. 5
The diagnosis should be made clinically in most outpatient settings, and treatment decisions should not be delayed for laboratory confirmation 3, 2.
Laboratory Testing Indications
Laboratory confirmation is useful in specific circumstances 1, 3:
- Hospitalized patients with suspected influenza 1
- When confirmed diagnosis will change treatment decisions 1, 3
- During periods of uncertain community influenza activity 1
Available Diagnostic Tests
Rapid molecular assays are the preferred diagnostic tests because they can be performed at point of care, are highly accurate, and provide fast results. 3
Other available tests include 1, 5:
- Viral culture 1
- Serology 1
- Reverse transcriptase-polymerase chain reaction (RT-PCR) 1, 5
- Immunofluorescence assays 1, 5
Important caveat: Rapid diagnostic tests have high specificity (>90%) but low to moderate sensitivity (20-70%), meaning negative results should not exclude influenza when community activity is high 5.
Clinical Course and Complications
Typical Resolution
- Uncomplicated influenza typically resolves after 3-7 days in most persons 1, 6
- Cough and malaise can persist for >2 weeks 1
- Adults are contagious from 1 day before symptoms through 5-10 days after onset 1
Complications to Monitor
- Primary influenza viral pneumonia 1, 7
- Secondary bacterial pneumonia (particularly Staphylococcus aureus, including MRSA) 1, 7
- Exacerbation of underlying cardiopulmonary conditions 1
- Otitis media and sinusitis 1
- Rare complications: encephalopathy, myositis, myocarditis, Reye syndrome 1
Common Pitfalls
Do not assume absence of fever rules out influenza: Mild illness without fever occurs in 6-33% of persons infected with influenza virus 1.
Do not wait for laboratory confirmation to initiate antiviral therapy in high-risk patients presenting within 48 hours of symptom onset 3, 2.
Do not dismiss influenza in elderly patients with atypical presentations lacking prominent respiratory symptoms 4.