Sensitivity of Influenza Antigen Tests
Rapid influenza antigen tests (RIDTs) have low to moderate sensitivity ranging from 20-70%, with significantly lower performance in adults (40-60%) compared to children (70-90%), making them unreliable for ruling out influenza when negative. 1
Key Performance Characteristics
Overall Sensitivity Range
- General population sensitivity: 20-70% compared to RT-PCR or viral culture 1
- Recent clinical studies have documented sensitivity as low as 42% in routine practice and 19% in adult clinical trial participants 1
- The Infectious Diseases Society of America reports median sensitivity of 70-75% for rapid antigen tests compared to viral culture or RT-PCR 1
Age-Dependent Performance
- Children: 70-90% sensitivity - significantly higher due to greater viral shedding 1
- Adults: 40-60% sensitivity - substantially lower performance 1
- Young children shed higher concentrations of influenza virus, which directly improves rapid test detection rates 1
Specificity
- Consistently high at >90-95% across all age groups and settings 1
- Positive results are generally reliable when community influenza activity is high 1
Critical Factors Affecting Test Performance
Timing of Specimen Collection
- Optimal window: ≤48 hours after symptom onset 1, 2
- Sensitivity decreases significantly when specimens are collected beyond 48 hours 2
- Tests perform best when collected as close to illness onset as possible 1
Specimen Type and Quality
- Nasopharyngeal specimens yield higher sensitivity than throat swabs 1, 2
- Specimen quality directly impacts test accuracy - samples must contain adequate epithelial cells 1
- Nasal aspirates, swabs, and washes are acceptable but vary by specific test 1
Viral Subtype Variations
- Pandemic H1N1: 48.7-55.9% sensitivity 3
- Seasonal H3N2: 64.0-70.5% sensitivity 3
- 2009 H1N1 virus: 11-70% sensitivity 1
- Viral load distribution differs substantially between subtypes, with pandemic H1N1 characterized by lower copy numbers affecting detection 3
Clinical Interpretation Algorithm
When Community Influenza Activity is High
- Positive RIDT: Reliable for initiating antiviral treatment 1
- Negative RIDT: Cannot rule out influenza - do not use for treatment or infection-control decisions 1
- Empirical antiviral treatment is recommended for high-risk patients despite negative rapid test 1
When Community Influenza Activity is Low (<10% prevalence)
- Positive predictive value drops below 70% 4
- Rapid tests have limited clinical utility during low-prevalence periods 4
- Consider confirmatory testing with RT-PCR for positive results 1
When Definitive Diagnosis is Required
- RT-PCR is the gold standard with >95% sensitivity and specificity 1, 5
- Confirm negative rapid test results with RT-PCR or viral culture when clinical suspicion remains high 1
- Molecular assays are preferred for hospitalized patients 1
Common Clinical Pitfalls to Avoid
Critical Errors in Test Interpretation
- Never rely on negative rapid tests to exclude influenza during active community transmission 1, 2
- Do not withhold antiviral treatment from high-risk patients based solely on negative rapid test results 1
- Avoid using rapid tests as the sole diagnostic tool when influenza prevalence is <10% 4
Specimen Collection Mistakes
- Avoid throat swabs when nasopharyngeal specimens are feasible - they have lower yields 1, 2
- Do not collect specimens beyond 48 hours of symptom onset and expect optimal sensitivity 2
- Ensure specimens contain adequate epithelial cells, not just secretions 1
Context-Dependent Interpretation Failures
- Failing to consider local influenza surveillance data when interpreting results 1
- Not accounting for patient age - adult negative results are particularly unreliable 1
- Ignoring that positive tests can detect vaccine virus strains up to 7 days post-vaccination 1
Comparison with Alternative Testing Methods
RT-PCR (Molecular Testing)
- Sensitivity: >95% - substantially superior to rapid antigen tests 1, 5
- Results available in 1-8 hours depending on platform 1
- Point-of-care PCR platforms now available with results in 15-30 minutes 1, 5
Rapid Molecular Assays
- Sensitivity: High (>95%) with results in 15-30 minutes 1
- CLIA-waived options available for point-of-care use 1
- Significantly improved antiviral prescribing compared to rapid antigen tests (82.4% vs 69.9% appropriate prescribing) 5