What laboratory tests are necessary for a 3-month-old patient diagnosed with viral bronchiolitis?

Laboratory Tests for Viral Bronchiolitis in a 3-Month-Old

Routine laboratory testing is not necessary for a 3-month-old with viral bronchiolitis—the diagnosis is clinical and labs do not alter management or improve outcomes. 1, 2

What NOT to Order

Viral Testing

• Routine virologic testing (including RSV PCR or antigen testing) is not recommended for typical bronchiolitis cases, as identifying the specific viral etiology does not change management at the individual patient level 1
• The only exception is if the infant is receiving monthly palivizumab prophylaxis—in this specific scenario, RSV testing should be performed to determine if prophylaxis should be discontinued after a breakthrough infection 1
• Viral testing may be useful solely for infection control purposes when patient cohorting is feasible, but this is an institutional decision, not a clinical management tool 1

Chest Radiography

• Chest radiographs should not be obtained routinely in children with bronchiolitis 1, 2
• Approximately 25% of hospitalized infants have radiographic atelectasis or infiltrates that are often misinterpreted as bacterial infection, leading to inappropriate antibiotic use 3
• Reserve chest radiography only for cases requiring ICU admission due to severe respiratory distress or when airway complications (such as pneumothorax) are suspected 1

Blood Tests

• Complete blood counts have not been shown to be useful in diagnosing bronchiolitis or guiding therapy 1, 4
• Routine measurement of C-reactive protein does not aid in management 4
• Blood cultures and other laboratory studies are generally unhelpful and not required 2, 5

What TO Monitor Clinically

Essential Clinical Assessments

• Pulse oximetry is the primary objective measurement needed—supplemental oxygen is indicated only if SpO2 falls persistently below 90% in previously healthy infants 3
• Count respiratory rate over a full 60 seconds, as tachypnea ≥70 breaths/minute indicates increased risk of severe disease 1, 2
• Assess work of breathing by looking for nasal flaring, grunting, and intercostal/subcostal retractions 2
• Evaluate hydration status and ability to take fluids orally—this determines need for IV fluids, not laboratory values 3

Risk Stratification Without Labs

• At 3 months of age, this infant falls into a higher-risk category (age <12 weeks) requiring closer monitoring 3, 2
• Document history of prematurity, chronic lung disease, hemodynamically significant congenital heart disease, or immunodeficiency—these are clinical risk factors, not laboratory-based 2
• Serial clinical observations over time provide more valid assessment than single examinations or laboratory values 2

When to Consider Testing for Bacterial Coinfection

Serious Bacterial Infection Risk

• The occurrence of serious bacterial infections (UTI, sepsis, meningitis) is very low in bronchiolitis—less than 1% 1, 3
• Fever alone does not justify antibiotics or bacterial workup in an infant with clear bronchiolitis 3
• Consider bacterial workup only with specific clinical indicators of coexisting bacterial infection, such as documented acute otitis media or clinical signs suggesting bacterial pneumonia that is not improving as expected 3

Critical Pitfall to Avoid

• Do not order tests simply because the infant is young (3 months old)—the diagnosis remains clinical regardless of age 2
• Routine diagnostic testing increases costs without improving outcomes and may lead to inappropriate antibiotic prescribing when radiographic findings are misinterpreted 2
• Continuous pulse oximetry may lead to less careful clinical monitoring—serial clinical assessments are more important than continuous monitoring in stable infants 3