What is the best way to diagnose asthma in a patient with respiratory symptoms and potential allergic history?

How to Diagnose Asthma

Asthma diagnosis requires three essential components: episodic respiratory symptoms (wheeze, cough, dyspnea, or chest tightness), objective demonstration of reversible airflow obstruction on spirometry, and systematic exclusion of alternative diagnoses. 1, 2

Clinical History: Key Symptom Patterns

Start by identifying specific symptom characteristics that strongly suggest asthma 1:

• Recurrent episodes of wheezing, cough (especially nocturnal), difficulty breathing, or chest tightness 1
• Symptom variability over time—diurnal variations (worse at 4 AM), seasonal patterns, or episodic nature 1
• Trigger-induced symptoms including exercise, viral infections, allergen exposure (pets, dust mites, mold, pollen), irritants (smoke, chemicals), cold air, weather changes, strong emotions, stress, or menstrual cycles 1
• Nocturnal awakening from respiratory symptoms (affects 74% of asthma patients) 1
• Family history of asthma, allergies, or atopic disorders 1, 2

Physical Examination: Focused Findings

The physical exam may be normal between episodes, but focus on 1:

• Upper respiratory tract: increased nasal secretion, mucosal swelling, nasal polyps 1
• Chest: wheezing during normal breathing or prolonged forced exhalation, hyperexpansion, accessory muscle use, hunched shoulders 1
• Skin: atopic dermatitis or eczema 1

Critical caveat: Normal physical findings do not exclude asthma due to disease variability 1

Spirometry: Mandatory Objective Testing

Spirometry is essential for diagnosis in patients ≥5 years old because history and physical examination alone are unreliable 1, 2:

• Demonstrates obstruction: Reduced FEV₁/FVC ratio 1
• Confirms reversibility: FEV₁ increase of ≥12% AND ≥200 mL from baseline after inhaled short-acting β₂-agonist 1, 2
• Some evidence suggests ≥10% of predicted FEV₁ improvement may better differentiate asthma from COPD 1

Peak flow meters are NOT recommended for diagnosis due to wide variability in devices and reference values—they are designed for monitoring, not diagnosis 1

When Spirometry is Normal but Suspicion Remains High

If spirometry shows no obstruction but clinical suspicion persists, proceed to 2:

• Bronchial challenge testing (methacholine or mannitol) to demonstrate airway hyperresponsiveness 2, 3
• Two-week peak expiratory flow rate (PEFR) monitoring to document variability 2
• FeNO testing to support diagnosis of allergic/eosinophilic asthma 2

Systematic Exclusion of Alternative Diagnoses

You must rule out other conditions that mimic asthma 1, 2:

In Adults:

• COPD: ≥10 pack-year smoking history, slowly progressive dyspnea, minimal symptom variability, poor bronchodilator response 1, 2, 4
• Vocal cord dysfunction (paradoxical vocal cord motion) 1
• Congestive heart failure 1
• Pulmonary embolism 1
• Mechanical airway obstruction (benign or malignant tumors) 1

In Children:

• Foreign body aspiration 1
• Cystic fibrosis 2
• Vascular rings or laryngeal webs 1

Additional Diagnostic Studies

Order these when clinically indicated 1, 2:

• Chest X-ray: Exclude pneumonia, heart failure, structural abnormalities 2
• Allergy testing (skin or specific IgE blood tests): Identify clinically important allergen sensitivities to guide allergen avoidance and immunotherapy decisions 1, 2
  • Do NOT order IgG testing—it has no diagnostic value for allergic asthma 1
  • Total IgE has limited diagnostic value 1

Diagnostic Algorithm Summary

1. Establish clinical probability: Detailed history of episodic symptoms, triggers, nocturnal awakening, family history 1, 2

2. Perform spirometry with pre- and post-bronchodilator testing in patients ≥5 years 1, 2

3. If spirometry shows obstruction + reversibility (≥12% and ≥200 mL FEV₁ increase): Diagnosis confirmed 1, 2

4. If spirometry is normal but suspicion high: Proceed to bronchial challenge testing or 2-week PEFR monitoring 2

5. Consider FeNO to support allergic asthma diagnosis 2

6. Systematically exclude alternatives using clinical assessment, chest X-ray, and additional testing as needed 1, 2

7. Order allergy testing if allergen identification would influence treatment decisions (immunotherapy, avoidance strategies) 1, 2

Common Diagnostic Pitfalls

• Relying on history and physical exam alone without objective spirometry—this leads to both overdiagnosis and underdiagnosis 1
• Using peak flow meters for diagnosis instead of spirometry 1
• Missing the diagnosis in children because spirometry often shows normal FEV₁/FVC ratios (only 3% of children with current asthma had FEV₁/FVC <70% in one study) 1
• Failing to time testing appropriately—asthma demonstrates strong diurnal variation with worst symptoms and lung function at 4 AM 1
• Not considering alternative diagnoses, particularly COPD in smokers and vocal cord dysfunction 1, 2