What Tests Are Needed to Diagnose Asthma in Babies?

Understanding Asthma Diagnosis in Infants and Young Children

Diagnosing asthma in babies and young children can be challenging due to their limited ability to communicate symptoms and the similarity of respiratory issues with other common childhood conditions. However, early and accurate diagnosis is crucial for effective management. A comprehensive evaluation typically involves a combination of clinical assessments and specialized tests tailored to the child's age and condition.

Key Diagnostic Tests for Pediatric Asthma

To confirm asthma or rule out other respiratory disorders, healthcare providers rely on a series of diagnostic tools. These tests help assess airway function, inflammation levels, allergic triggers, and structural abnormalities in the lungs.

1. Complete Blood Count (CBC) with Eosinophil Analysis

A peripheral blood test, commonly known as a complete blood count (CBC), is often one of the first steps in evaluating a child suspected of having asthma. Elevated levels of eosinophils—a type of white blood cell associated with allergic reactions—are a strong indicator. In asthmatic infants, eosinophil percentages may rise above 6%, and in those with atopic (allergic) predisposition, levels can reach 20% to 30%. Absolute eosinophil counts typically range from (0.4–0.6) × 10⁹/L but may spike to (1.0–2.0) × 10⁹/L during flare-ups.

2. Sputum (Phlegm) Examination for Airway Inflammation

Sputum cytology is another valuable tool, especially when the child can produce a sample. The presence of more than 2.5% eosinophils in sputum suggests ongoing airway inflammation typical of asthma. Additionally, microscopic analysis may reveal eosinophil degranulation—where these cells release inflammatory substances—further supporting an asthma diagnosis.

3. Pulmonary Function Testing Based on Age

Lung function tests are essential in assessing airflow limitation, reversibility, and airway hyperresponsiveness—the hallmark features of asthma. However, testing methods vary significantly depending on the child's age:

Children aged 6 years and older can usually perform forced expiratory maneuvers (spirometry), which measure how much and how quickly air is exhaled.
For toddlers between 3 and 6 years old, impulse oscillometry (IOS) is preferred. This non-invasive method evaluates respiratory resistance and reactance without requiring active cooperation.
Infants under 3 may undergo tidal breathing flow-volume loop analysis, which assesses lung function during normal breathing patterns.

4. Bronchodilator Reversibility Test

This test follows baseline pulmonary function measurements. After initial spirometry or IOS readings, the child inhales a short-acting bronchodilator (such as albuterol). A significant improvement in airflow—typically defined as a ≥12% increase in FEV₁ (Forced Expiratory Volume in 1 second)—indicates reversible airway obstruction, strongly suggesting asthma.

5. Allergen-Specific Testing to Identify Triggers

Since allergies play a major role in pediatric asthma, identifying specific allergens is critical. Two main approaches are used:

Skin prick tests: Small amounts of common allergens (like dust mites, pollen, pet dander) are introduced into the skin to observe immediate allergic reactions.
Specific IgE blood tests: These measure the level of immunoglobulin E antibodies in the bloodstream, indicating sensitization to particular allergens.

Combining both methods enhances diagnostic accuracy and helps develop personalized avoidance and treatment strategies.

6. Non-Invasive Biomarkers of Airway Inflammation

Emerging techniques allow doctors to monitor airway inflammation without invasive procedures. One such method is measuring fractional exhaled nitric oxide (FeNO), a gas that increases when airways are inflamed. Elevated FeNO levels correlate with eosinophilic inflammation and can guide decisions about corticosteroid therapy, especially in children over 5 years old.

7. Imaging Studies to Rule Out Other Conditions

While asthma cannot be confirmed through imaging alone, chest X-rays or occasionally CT scans are used to exclude alternative diagnoses such as pneumonia, foreign body aspiration, congenital anomalies, or chronic lung diseases. These imaging tools ensure that asthma is not misdiagnosed when structural or infectious causes are present.

Conclusion: A Multifaceted Approach Ensures Accuracy

Diagnosing asthma in babies requires a careful, multi-step process that combines clinical observation with targeted testing. From blood work and lung function assessments to allergy screening and imaging, each component provides vital clues. Early detection and proper classification enable timely intervention, helping young patients breathe easier and lead healthier lives.