What are the diagnostic steps and treatment options for viral influenza?

Diagnostic Approach for Influenza

RT-PCR is the most sensitive and specific test for diagnosing influenza, with results available in 4-6 hours, and should be the primary diagnostic method when laboratory confirmation is needed. 1

Clinical Diagnosis

• During influenza season, a clinical diagnosis based on symptoms and signs is often sufficient, particularly when the patient presents with the hallmark symptoms of abrupt onset of fever, cough, chills, myalgias, and malaise 1, 2
• The combination of fever and cough provides a positive predictive value of 71-83% in children, though lower in adults 1, 3
• Clinical diagnosis performs poorly in older adults, with a positive predictive value of only 30% in patients ≥60 years 1
• Among hospitalized patients with laboratory-confirmed influenza, only 44-51% present with typical influenza-like illness symptoms 1

When to Consider Laboratory 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
• Testing should be prioritized for high-risk patients who may benefit from antiviral therapy 4, 1

Recommended Diagnostic Tests (in order of priority)

1. RT-PCR (Reverse Transcription Polymerase Chain Reaction)

   • Currently the most sensitive and specific test for influenza 4, 1
   • Results available in 4-6 hours 1
   • Can differentiate between influenza types and subtypes 4
   • Test of choice for patients with exposure to animals with possible influenza 4

2. Immunofluorescence Assays

   • Moderately high sensitivity and specificity 4, 1
   • Results available in 2-4 hours 1
   • Performance depends on laboratory expertise and sample quality 4
   • Samples must contain respiratory epithelial cells 4

3. Rapid Diagnostic Tests

   • Provide results in 10-30 minutes 4
   • Lower sensitivity (70-90% in children, 40-60% in adults) compared to RT-PCR and viral culture 4, 1, 5
   • Different tests can detect: only influenza A; both A and B without distinguishing; or both A and B with distinction 4
   • Most sensitive when performed within 48 hours of symptom onset 5

4. Viral Culture

   • Not useful for immediate clinical decision-making due to longer turnaround time (3-10 days) 4
   • Important for surveillance, confirming other test results, and monitoring antiviral resistance 4
   • Highest specificity among all tests 4

Specimen Collection

• Nasopharyngeal specimens are typically more effective than throat swab specimens for viral isolation or rapid detection 4, 1
• Proper specimen collection technique is crucial for accurate results 5
• Specimens should be collected within 5 days of symptom onset, ideally within 48 hours 4, 5

Interpretation of Results

• Test results should always be evaluated in the context of clinical information and local influenza activity 1
• Consider confirming negative rapid test results with RT-PCR or viral culture due to lower sensitivity of rapid tests 4
• The positive and negative predictive values of tests vary with the prevalence of influenza in the community 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 1, 3
• Testing too early or too late in the course of illness, as viral shedding patterns affect test sensitivity 5
• Using inappropriate specimen collection techniques that may reduce test sensitivity 5
• Dismissing the possibility of influenza outside of peak season, as sporadic cases can occur year-round 1

Treatment Considerations

• Antiviral treatment should be considered if the patient presents within 48 hours of symptom onset, particularly for high-risk patients 3, 6
• The benefit of treatment is greatest when antiviral therapy is started within 24 hours of symptom onset 3, 2
• Neuraminidase inhibitors (oseltamivir, zanamivir) can reduce the duration of uncomplicated influenza by approximately 1 day compared with placebo 6, 7