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Respiratory Syncytial Virus: Diagnosis and Evaluation

Respiratory Syncytial Virus: Diagnosis and Evaluation

The diagnostic approach to RSV combines clinical acumen with targeted laboratory investigation. Because RSV shares a clinical phenotype with other major respiratory viruses such as influenza, human metapneumovirus, and parainfluenza, distinguishing the etiology is crucial for infection control and prognostic counseling. However, for the otherwise healthy infant with typical bronchiolitis, international guidelines emphasize that diagnosis can often be made on clinical grounds alone to avoid unnecessary testing. At Liv Hospital, the evaluation strategy is risk-stratified: identifying the pathogen in high-risk or hospitalized patients while minimizing invasive procedures in stable outpatients.

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Clinical Diagnosis and Risk Assessment

Clinical Diagnosis and Risk Assessment

In the outpatient setting, the diagnosis of RSV bronchiolitis is primarily clinical. Clinicians evaluate the constellation of symptoms of rhinorrhea, cough, and tachypnea in the context of the season. The physical examination is focused on assessing the severity of respiratory compromise to guide admission decisions.

  • Auscultation: The hallmark findings are diffuse wheezing and scattered crackles across all lung fields. Focal silence may indicate severe obstruction (atelectasis) or consolidation.
  • Hydration Assessment: Examination of mucous membranes, fontanelle tension, and capillary refill time is vital, as dehydration is a key driver for admission.
  • Work of Breathing: The use of accessory muscles is graded. Head bobbing in infants is an ominous sign of impending respiratory failure.
  • History of Prematurity: Assessing gestational age is critical, as preterm infants have less physiological reserve and deteriorate more rapidly.
  • Feeding History: Quantifying the reduction in oral intake helps determine the need for fluid support.
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Pulse Oximetry and Gas Exchange Analysis

Pulse Oximetry and Gas Exchange Analysis

Pulse oximetry is the “fifth vital sign” in RSV evaluation. It provides a non-invasive measure of oxygen saturation (SpO2). However, clinical guidelines warn against over-reliance on a single number. SpO2 can fluctuate with coughing, sleep, and agitation. Persistent hypoxemia (SpO2 < 90-92%) is a strong indicator for hospitalization. In severe cases, blood gas analysis is performed.

  • Capillary Blood Gas: Often preferred in infants to avoid arterial puncture. It provides pH and PCO2 levels.
  • Hypercarbia: Elevated carbon dioxide levels indicate respiratory fatigue and alveolar hypoventilation, signaling the potential need for mechanical support.
  • Acidosis: Respiratory acidosis (low pH, high CO2) is a sign of uncompensated failure.
  • Oximetry Trends: Continuous monitoring is valuable in the acute phase but can prolong hospitalization if “transient desaturations” delay discharge in an otherwise stable child.
  • Oxygen Dissociation: Understanding that fetal hemoglobin (in young infants) has different oxygen-binding properties.

 

Molecular Diagnostics: PCR and NAAT

Nucleic Acid Amplification Tests (NAAT), specifically Reverse Transcription Polymerase Chain Reaction (RT-PCR), represent the gold standard for RSV diagnosis. These tests amplify viral genetic material, offering exceptional sensitivity and specificity.

  • Multiplex Panels: Modern PCR platforms often test for RSV, Influenza A/B, and SARS-CoV-2 (COVID-19) in a single nasopharyngeal swab.
  • Viral Load: While PCR is qualitative, the cycle threshold (Ct) value can roughly correlate with viral load, though this is rarely used clinically.
  • Sensitivity: PCR can detect the virus even days after symptoms have begun to improve, which is essential for isolation protocols.
  • Turnaround Time: Rapid PCR tests can provide results in under an hour, enabling hospitals to make immediate bed management decisions.
  • Utility: Essential for immunocompromised patients where detection of any pathogen alters management.

Rapid Antigen Detection Tests (RADT)

Rapid antigen tests are immunoassays that detect viral proteins in nasopharyngeal secretions. They are widely used in point-of-care settings due to their speed and lower cost compared to PCR.

  • Sensitivity Profile: RADTs generally have lower sensitivity than PCR. They perform well in young children who shed high quantities of virus but may yield false negatives in older children or adults.
  • Specificity: Generally high; a positive result is reliable and usually confirms the diagnosis.
  • Sample Quality: Accuracy depends heavily on obtaining a high-quality nasopharyngeal sample with adequate cells.
  • Clinical Role: Useful for quick screening during peak season, but a negative result often requires PCR confirmation in high-risk patients.
  • Interference: Recent use of nasal saline or sprays can theoretically dilute the sample, affecting antigen detection.

Chest Radiography Indications

Chest Radiography Indications

Routine chest X-rays are discouraged for typical bronchiolitis because the findings often lead to unnecessary antibiotic use. However, imaging is indicated in specific scenarios: severe distress, high fever, asymmetric lung sounds, or worsening clinical course.

  • Hyperinflation: The classic RSV X-ray shows flattened diaphragms and increased retrosternal air space due to air trapping.
  • Peribronchial Cuffing: Thickening of the bronchial walls appears as “doughnuts” or parallel lines (tram-tracking) on the image.
  • Patchy Atelectasis: Areas of collapsed lung due to mucus plugs are common and are often misread as bacterial pneumonia.
  • Lobar Consolidation: A dense, white area confined to a lung lobe is more suggestive of secondary bacterial pneumonia.
  • Pneumomediastinum: Rarely, severe coughing or obstruction can cause air to leak into the chest cavity, which is visible on X-ray.

Laboratory Blood Evaluation

Blood work is not routine for uncomplicated RSV, but is used to evaluate systemic toxicity or dehydration.

  • Complete Blood Count (CBC): Usually shows a normal or mildly elevated white blood cell count. A very high count with a “left shift” (immature cells) may point to bacterial superinfection.
  • Inflammatory Markers: C-reactive protein (CRP) and Procalcitonin are typically low in viral bronchiolitis. Elevated Procalcitonin is a specific marker used to rule in bacterial sepsis or pneumonia.
  • Electrolyte Panel: Essential for infants with poor feeding to screen for hyponatremia (low sodium) or metabolic acidosis due to dehydration.
  • Blood Culture: Performed only in febrile infants who appear toxic or are younger than 60 days to rule out bacteremia.
  • Transaminases: Liver enzymes may be checked in severe cases to assess for systemic involvement.

Urine Analysis

Urinary tract infections (UTIs) are the most common severe bacterial infection co-occurring with RSV in febrile infants. Because the symptoms of a UTI are non-specific, a catheterized urinalysis and culture are recommended for RSV-positive infants who have high fevers, particularly if they are uncircumcised males or females under 24 months. This ensures that a treatable bacterial infection is not missed under the assumption that the fever is solely due to RSV.

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FREQUENTLY ASKED QUESTIONS

How is the RSV swab done?

A healthcare provider inserts a thin, flexible swab gently into the nose, reaching the back of the nasal cavity (nasopharynx). It may be uncomfortable and cause sneezing/tearing, but it is swift.

X-rays usually don’t change the treatment for typical RSV and expose the baby to radiation. Doctors can generally tell what is happening in the lungs by listening with a stethoscope.

No, standard blood tests cannot diagnose RSV. Only a nasal swab can confirm the virus. Blood tests are used to check for dehydration or other infections.

If the rapid test is negative but symptoms strongly suggest RSV, the doctor might send the sample for a more sensitive PCR test to be sure, or they may treat based on symptoms.

Yes, but it is just one piece of the puzzle. Doctors look at how hard the baby is working to breathe, not just the number on the monitor. A baby can have a decent number but still be exhausted.

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