Diagnostic Testing for Influenza
For a patient presenting with fever, chills, and flu-like symptoms, PCR (RT-PCR) of a nasopharyngeal swab is the most likely test to confirm the diagnosis of influenza, as it provides the highest sensitivity and specificity among available diagnostic options. 1, 2, 3
Why PCR is Superior
RT-PCR is the gold standard diagnostic test with sensitivity of 86-100% and specificity >95%, making it far more reliable than other available options for confirming influenza. 2, 3 The test provides results within 2-6 hours and can distinguish between influenza A subtypes, which is valuable for treatment decisions and surveillance. 2
Comparison of Diagnostic Options
Rapid antigen tests have unacceptably low sensitivity (10-70%) and should not be used to guide treatment decisions or definitively diagnose influenza. 1, 3 While rapid antigen tests can detect influenza virus antigens within 10-30 minutes, their poor sensitivity means negative results cannot exclude influenza, requiring confirmation with more sensitive methods like RT-PCR or viral culture. 1
Serology testing requires paired acute and convalescent sera, is not widely available, and is explicitly not recommended for clinical diagnosis except for epidemiologic investigations and research purposes. 1 Antibody testing is only useful retrospectively to confirm diagnosis after the acute illness has resolved, making it clinically impractical for immediate diagnostic decisions. 4
Clinical Context Matters
The accuracy of clinical diagnosis alone is limited, with positive predictive values ranging from only 30% in older adults (≥60 years) to 79-88% in healthy younger adults during influenza season. 1, 2, 5 This highlights why laboratory confirmation is valuable when results would influence management decisions. 2, 3
Testing is particularly important in:
- Hospitalized patients with suspected influenza 3, 6
- High-risk individuals where confirmation would guide antiviral therapy 3
- Situations where infection control measures depend on confirmed diagnosis 3
Specimen Collection
Nasopharyngeal specimens provide higher diagnostic yields than throat swabs for both viral isolation and rapid detection of influenza viruses. 1 Among specimen types, nasopharyngeal swabs detected influenza in 60% of cases compared to only 36% for throat swabs in one study. 7
Common Pitfalls to Avoid
Do not rely on negative rapid antigen tests to exclude influenza during periods of high community influenza activity—the sensitivity is too low and requires confirmation with RT-PCR or viral culture. 1, 5
Do not wait for test results before initiating antiviral therapy in high-risk patients presenting within 48 hours of symptom onset, as treatment benefit is greatest when started within 24 hours. 3, 6
Do not use serology for acute diagnosis—it requires paired samples weeks apart and provides no value for immediate clinical decision-making. 1, 4