Is nasal swabbing using molecular assays (such as Polymerase Chain Reaction (PCR)) an appropriate diagnostic tool for Respiratory Syncytial Virus (RSV) in infants, particularly those under 6 months or with underlying health conditions?

RSV Testing in Infants: Nasal Swabbing is Appropriate and Effective

Yes, nasal swabbing using molecular assays (PCR) is an appropriate and validated diagnostic method for RSV in infants, with nasal swabs performing comparably to nasopharyngeal swabs for virus detection. 1

When to Test Infants for RSV

Testing should be selective and clinically driven, not routine:

• Test infants receiving palivizumab prophylaxis who develop bronchiolitis to determine if breakthrough RSV infection occurred, which would warrant discontinuation of further prophylaxis 2, 3
• Test febrile infants ≤60 days old being evaluated for serious bacterial infection, as a positive RSV result reduces (but does not eliminate) the risk of concurrent bacterial infection 3
• Test high-risk infants including those who are immunocompromised, transplant recipients, or those with severe underlying cardiopulmonary disease for risk stratification 3
• Do NOT routinely test otherwise healthy infants with typical bronchiolitis where management will be supportive regardless 2, 3

Optimal Sampling Technique

Nasal swabs are adequate and less invasive than nasopharyngeal swabs:

• Research demonstrates almost perfect agreement (κ > 0.8) between nasal swabs, nasopharyngeal swabs, and throat swabs for RSV detection by RT-qPCR 1
• Detection rates are not significantly different between the three methods (93-95%), regardless of clinical presentation 1
• The uncomfortable and technically demanding nasopharyngeal swab method is not mandatory for RSV detection by RT-qPCR 1
• Nasal aspirates are also highly effective, with RT-PCR detecting RSV in 56.7% of samples compared to only 33.6% by traditional immunofluorescence 4

Available Testing Platforms

Multiple FDA-approved molecular assays are available for RSV detection 2:

• Multiplex PCR panels detecting influenza A/B plus RSV (e.g., Xpert Flu/RSV XC by Cepheid) 2
• Single-target RSV assays (e.g., Alere I RSV by Abbott Laboratories) 2
• Broader respiratory panels detecting multiple viral and bacterial pathogens 2
• Most assays are approved for nasal swabs, nasal aspirates, nasal washes, and throat swabs 2

Critical Interpretation Considerations

PCR results must be interpreted cautiously in clinical context:

• RSV detected by PCR is almost always associated with active disease, unlike some other respiratory viruses 2
• Testing for RSV in infants with lower respiratory tract disease shows high concordance (92%) between upper and lower respiratory tract specimens, making upper tract sampling reliable 2
• However, in immunocompromised adults with RSV, lower respiratory tract testing is significantly more sensitive (BAL: 88.9% vs. nasal wash: 15%) 2

Common Pitfalls to Avoid

• Do not test healthy toddlers with uncomplicated URI symptoms where diagnosis is clinical and testing does not change management 5
• Do not obtain chest radiography unless respiratory distress is severe enough to warrant ICU admission or complications are suspected 2
• Do not confuse testing with treatment options: Palivizumab is prophylaxis only and has no therapeutic benefit for treating established RSV infection 3
• Avoid environmental exposure of pregnant healthcare workers to aerosolized ribavirin due to teratogenic effects 3