Diagnostic Approach for Influenza

Clinical Diagnosis

The Centers for Disease Control and Prevention recommends that 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, 2
The hallmark of influenza infection is the abrupt onset of fever, cough, chills, myalgias, and malaise, with a positive predictive value of approximately 70-77% during influenza season 2, 3
The combination of fever and cough provides a positive predictive value of 71-83% in children, though lower in adults 3
Clinical diagnosis performs poorly in older adults, with a positive predictive value of only 30% in patients ≥60 years 1, 2
Among hospitalized adults with laboratory-confirmed influenza, only 44-51% presented with typical influenza-like illness symptoms 1, 2

The American Academy of Family Physicians recommends that laboratory testing should be considered when results would influence clinical management decisions, such as starting antiviral therapy or antibiotic use 4, 5
Testing is particularly valuable in hospitalized patients with suspected influenza 5, 6
During influenza season, 60-69% of practitioners report testing patients for influenza 1, 2
RT-PCR is currently the most sensitive and specific test for influenza, with results available in 2-6 hours 4, 7
Immunofluorescence Assays provide moderately high sensitivity and high specificity with results in 2-4 hours 4, 7
Rapid Diagnostic Tests detect influenza virus antigens within 10-30 minutes but have lower sensitivity compared to RT-PCR and viral culture 1, 4, 7

The Centers for Disease Control and Prevention recommends that interpretation of test results should always consider clinical and epidemiological information 1, 8
Negative rapid tests should be confirmed with viral culture or RT-PCR due to lower sensitivity 8, 5
Consider the positive and negative predictive values of tests in the context of local influenza activity 8, 5
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 8, 5

Relying solely on typical influenza-like illness symptoms in older adults or immunocompromised patients, who may present atypically 1, 2
Waiting for laboratory confirmation before initiating antiviral therapy in high-risk patients when influenza is suspected 3
Overinterpreting PCR results, as they can detect both viable and non-viable viral particles 9
Using antibiotics empirically for uncomplicated influenza without evidence of bacterial co-infection 6, 3
Dismissing the possibility of influenza outside of peak season, as sporadic cases can occur year-round 5