What tests can be done in a pediatric ER to rule out influenza and Respiratory Syncytial Virus (RSV)?

Diagnostic Testing for Influenza and RSV in the Pediatric Emergency Department

In a pediatric ER, use rapid molecular assays (nucleic acid amplification tests) as the first-line diagnostic test for both influenza and RSV, as these provide results in 15-30 minutes with high sensitivity (86-100%) and high specificity (>95%), far superior to rapid antigen tests which have unacceptably low sensitivity (10-70% for influenza, as low as 28% for influenza B). 1

Recommended Testing Algorithm

First-Line Testing: Rapid Molecular Assays

• Obtain a nasopharyngeal swab or nasopharyngeal aspirate for both influenza and RSV testing, as these specimen types provide optimal sensitivity 1
• Use rapid molecular assays (nucleic acid amplification tests) that detect influenza A, influenza B, and RSV viral RNA with results available in 15-30 minutes 1
• Multiplex molecular assays can simultaneously detect influenza A, B, RSV, and other respiratory pathogens (adenovirus, rhinovirus, parainfluenza 1-3) in 1-2 hours with high sensitivity and specificity 1

When Rapid Molecular Testing is Unavailable

• Direct or indirect immunofluorescence assays provide moderate sensitivity with results in 1-4 hours for both influenza and RSV 1
• Standard RT-PCR (if rapid molecular unavailable) provides high sensitivity (86-100%) with results in 1-8 hours 1

Critical Timing Considerations

• Collect respiratory specimens as close to illness onset as possible, ideally within the first 48 hours when viral shedding is highest and test sensitivity is optimal 1, 2
• Do not delay antiviral treatment while waiting for test results in high-risk patients or those with severe illness 1, 2

Tests to AVOID or Use with Extreme Caution

Rapid Antigen Tests (Rapid Influenza Diagnostic Tests)

• Rapid antigen tests have unacceptably low sensitivity (10-70% for influenza A, as low as 28% for influenza B) despite high specificity (>95%) 1, 3
• Negative rapid antigen test results cannot rule out influenza or RSV due to high false-negative rates 1, 2
• If rapid antigen testing is the only available option, negative results in hospitalized patients or those with high clinical suspicion should be confirmed with molecular testing (RT-PCR) 1, 2
• Rapid antigen tests may be used to "rule in" infection when positive (high specificity 97%), but never to "rule out" when negative 1

Specimen Collection Best Practices

Optimal Specimen Types (in order of preference)

1. Nasopharyngeal aspirate or washing - highest yield 1
2. Nasopharyngeal swab - acceptable alternative, easier to obtain 1
3. Throat swab combined with nasal swab - if nasopharyngeal collection not feasible 1
4. Lower respiratory specimens (BAL fluid) - for patients with evidence of lower respiratory tract infection 1

Collection Timing

• Collect within first 2 days of illness when sensitivity is highest (85% for RSV) 4
• Sensitivity decreases significantly after 48 hours of symptom onset 2, 4

Additional Testing for Hospitalized or Severely Ill Children

Laboratory Studies

• Complete blood count with differential - influenza commonly causes leukopenia and lymphopenia (lymphocyte count <1.0); thrombocytopenia occurs in 5-7% of influenza A cases 1, 5
• Urea, creatinine, and electrolytes - assess for dehydration and renal function 1
• Liver enzymes - elevated in 27% of influenza A cases 1
• Blood culture - if bacterial superinfection suspected 1

Imaging and Monitoring

• Pulse oximetry - perform in every child being assessed for admission with pneumonia or respiratory distress 1
• Chest radiograph - only for children who are hypoxic (oxygen saturation <92%), have severe illness, or are deteriorating despite treatment 1

Common Pitfalls to Avoid

• Never rely on negative rapid antigen tests to exclude influenza or RSV - sensitivity is too low (10-70%), especially for influenza B 1, 2, 3
• Do not delay antiviral treatment while awaiting test results in high-risk patients (age <2 years, immunocompromised, chronic medical conditions) 1, 2
• Do not use sputum collection in young children - unreliable except possibly in children >12 years of age 1
• Avoid urinary pneumococcal antigen tests in children - lack both sensitivity and specificity in pediatric populations 1
• Do not perform routine chest radiography in well-appearing children with uncomplicated respiratory illness - it does not affect clinical outcomes 1

Performance Characteristics by Age Group

RSV Testing Performance

• Children <6 months: Optimal sensitivity 98%, specificity 96% with rapid molecular assays 6
• Children ≥2 years: Sensitivity 87%, specificity 98% with rapid molecular assays 6
• Overall pediatric population: Sensitivity 78.6-93%, specificity 93.9-96% for second-generation rapid tests 4, 6

Influenza Testing Performance

• Rapid molecular assays: Sensitivity 80.6-100%, specificity 99.3% across all pediatric age groups 1, 4
• Rapid antigen tests: Sensitivity as low as 47.7% for influenza A and 20.3% for influenza B in real-world pediatric settings 3