What is the diagnostic approach for influenza?

Diagnostic Approach for Influenza

The diagnosis of influenza should be made clinically during influenza season based on symptoms and signs, with laboratory testing reserved for specific situations where confirmation would change management decisions or for surveillance purposes. 1

Clinical Diagnosis

• The hallmark of influenza infection is the abrupt onset of fever (>38°C), cough, chills, myalgias, and malaise 1, 2
• During influenza season, clinical diagnosis based on symptoms alone has a positive predictive value of approximately 70-77%, comparable to some rapid diagnostic tests 2, 3
• The combination of fever and cough provides a positive predictive value of 71-83% in children, though lower in adults 2
• Clinical diagnosis is most accurate when influenza is known to be circulating in the community and typical symptoms are present 3
• Clinical diagnosis performs poorly in older adults - in patients ≥60 years, the presence of fever, cough, and acute onset had a positive predictive value of only 30% 1
• Among hospitalized adults with laboratory-confirmed influenza, only 44-51% presented with typical influenza-like illness symptoms 1

Laboratory Testing

When to Consider Testing

• Laboratory testing should be considered when results would influence clinical management decisions, such as starting antiviral therapy or antibiotic use 1
• Testing is particularly valuable in hospitalized patients with suspected influenza 1
• During influenza season, 60-69% of practitioners report testing patients for influenza 1

Preferred Diagnostic Methods (in order of priority)

1. RT-PCR (Reverse Transcription Polymerase Chain Reaction): Currently the most sensitive and specific test for influenza, with results available in 2-6 hours; highly recommended and can differentiate between influenza types and subtypes 1

2. Immunofluorescence Assays: Direct or indirect fluorescent antibody staining provides moderately high sensitivity and high specificity with results in 2-4 hours; can detect and distinguish between influenza A and B 1

3. Rapid Diagnostic Tests (RIDTs): Commercial rapid tests detect influenza virus antigens within 10-30 minutes but have lower sensitivity (70-90% in children, 40-60% in adults) compared to RT-PCR and viral culture 1

Specimen Collection

• Nasopharyngeal specimens generally yield better results than throat swab specimens 1
• Specimens should be collected as close to illness onset as possible, ideally within 48 hours 1
• For patients with lower respiratory tract illness, especially later in the course, endotracheal aspirate or bronchoalveolar lavage specimens have higher yields 1

Special Considerations

• Interpretation of test results should always consider clinical and epidemiological information 1
• Negative rapid tests should be confirmed with viral culture or RT-PCR due to lower sensitivity 1
• Consider the positive and negative predictive values of tests in the context of local influenza activity 1
• Viral culture remains important for surveillance purposes and monitoring of antiviral resistance, but is not useful for immediate clinical decision-making due to longer turnaround time 1

Common Pitfalls to Avoid

• Relying solely on typical influenza-like illness symptoms in older adults or immunocompromised patients, who may present atypically 1
• Waiting for laboratory confirmation before initiating antiviral therapy in high-risk patients when influenza is suspected 2
• Overinterpreting PCR results, as they can detect both viable and non-viable viral particles 4
• Using antibiotics empirically for uncomplicated influenza without evidence of bacterial co-infection 1, 2
• Dismissing the possibility of influenza outside of peak season, as sporadic cases can occur year-round 1

Diagnostic Algorithm

1. During influenza season:

   • If typical symptoms (sudden onset fever, cough, myalgia) in an otherwise healthy individual → clinical diagnosis is sufficient 2
   • If high-risk patient (pregnant, immunocompromised, elderly, chronic conditions) → consider rapid testing and start empiric antiviral treatment while awaiting results 1, 2

2. Outside influenza season or atypical presentation:

   • Consider laboratory testing with RT-PCR or immunofluorescence 1
   • Evaluate for alternative diagnoses 1

3. For hospitalized patients:

   • Obtain specimens for RT-PCR testing 1
   • Consider point-of-care C-reactive protein testing to help differentiate viral from bacterial infection 1
   • Assess severity using tools like the CRB65 score for pneumonia risk assessment 1