What is the best approach to diagnose asthma in a patient with recurrent respiratory symptoms?

How to Diagnose Asthma

Diagnose asthma by confirming episodic respiratory symptoms (wheezing, dyspnea, cough) plus objective evidence of reversible airflow obstruction on spirometry, while systematically excluding alternative diagnoses. 1

Essential Diagnostic Components

Clinical History - Key Indicators

The diagnosis begins with identifying specific symptom patterns that strongly suggest asthma 1:

• Recurrent episodes of wheezing, cough, difficulty breathing, or chest tightness 1
• Symptom variability including diurnal variations (often worse at night), seasonal patterns, or episodic nature 1
• Trigger-induced symptoms such as exercise, cold air, allergen exposure, or irritants 1
• Family history of asthma, allergies, or atopic disorders (eczema, allergic rhinitis) 1

Physical Examination Focus

Concentrate your examination on three specific areas 1:

• Upper respiratory tract - assess for rhinitis or nasal polyps 1
• Chest - listen for wheezing (though absence doesn't exclude asthma) 1
• Skin - examine for atopic dermatitis or other allergic manifestations 1

Objective Testing - The Critical Step

Spirometry (Age ≥5 Years)

Spirometry is mandatory for diagnosis in patients aged 5 years and older 1:

• Demonstrate airflow obstruction: FEV₁/FVC ratio below the lower limit of normal 2, 3
• Prove reversibility: Significant bronchodilator response (≥12% and ≥200 mL increase in FEV₁ after short-acting β₂-agonist) 1, 2, 3
• Timing matters: Normal spirometry is common in mild asthma when patients are asymptomatic, so testing during symptoms increases diagnostic yield 3

When Spirometry is Normal

If spirometry is normal but clinical suspicion remains high 3:

• Bronchial challenge testing with methacholine or histamine confirms airway hyperresponsiveness 1, 3
• Exercise challenge or mannitol testing can demonstrate exercise-induced bronchoconstriction 1
• Peak flow variability measured twice daily for 2 weeks showing >20% diurnal variation supports the diagnosis 1

Fractional Exhaled Nitric Oxide (FeNO)

Elevated FeNO (>50 ppb in adults) increases probability of allergic asthma and helps differentiate from COPD 1, 3:

• High FeNO makes allergic asthma more likely 3
• Useful when spirometry is equivocal 1

Critical Differential Diagnoses to Exclude

In Adults

The most important alternative diagnoses to systematically rule out 4, 2:

• COPD: Consider in patients with ≥10 pack-year smoking history, slowly progressive dyspnea, minimal symptom variability, and poor bronchodilator response 4

  • Key differentiator: Low diffusing capacity (DLCO) makes COPD more likely and asthma much less likely 3
  • Post-bronchodilator spirometry showing persistent obstruction suggests COPD 4

• Vocal cord dysfunction: Suspect when symptoms are unresponsive to standard asthma therapy 4

  • Flow-volume loops showing flattening of inspiratory limb help identify this 5

• Cardiac asthma (heart failure): Consider in patients with dyspnea and wheezing, especially with cardiovascular risk factors 4

• Drug-induced cough: ACE inhibitor-related cough can mimic asthma 4

In Children (Ages 5-16)

The European Respiratory Society guidelines emphasize 1:

• Doctor diagnosis must be supported by at least one objective test: spirometry with bronchodilator response, FeNO, challenge testing, or 2-week PEFR variability 1
• Foreign body aspiration: Acute onset of symptoms in young children 4
• Cystic fibrosis: Chronic respiratory symptoms in younger patients 4

Age-Specific Diagnostic Approach

Children Under 5 Years

Spirometry is not feasible in this age group 1:

• Diagnosis relies more heavily on clinical history and symptom patterns 1
• Trial of asthma therapy may be considered, but response doesn't definitively confirm diagnosis 1
• Consider referral to specialist for additional testing 1

Patients ≥5 Years

Spirometry is the cornerstone 1:

• Must demonstrate both obstruction and reversibility for definitive diagnosis 1
• If spirometry normal, proceed to bronchial challenge testing 1, 3

Additional Diagnostic Studies

When to Order Chest Radiography

Obtain chest X-ray to exclude alternative diagnoses when 1:

• Atypical presentation or symptoms
• Concern for pneumonia, heart failure, or structural abnormalities
• Poor response to initial asthma therapy

Allergy Testing

Identifying clinically important allergen sensitivities is useful 2:

• Helps guide environmental control measures 1
• Informs decisions about allergen immunotherapy 1
• However, allergy testing alone cannot diagnose asthma 1

Common Diagnostic Pitfalls

Avoid these critical errors:

• Don't rely on clinical assessment alone - physicians' subjective assessments of airway obstruction are often inaccurate 6
• Don't diagnose based on bronchodilator response alone - patients with poorly controlled asthma may lack substantial reversibility 3
• Don't assume normal spirometry excludes asthma - it's common in mild, well-controlled asthma 3
• Don't confuse asthma with COPD in smokers - use DLCO and post-bronchodilator spirometry to differentiate 3
• Don't skip objective testing in children ≥5 years - doctor diagnosis must be supported by at least one abnormal objective test 1

Diagnostic Algorithm Summary

1. Establish clinical probability through detailed history of episodic symptoms, triggers, and family history 1
2. Perform spirometry (age ≥5) with pre- and post-bronchodilator testing 1
3. If spirometry shows obstruction + reversibility → Diagnosis confirmed 1
4. If spirometry normal but suspicion high → Proceed to bronchial challenge testing or 2-week peak flow monitoring 1, 3
5. Consider FeNO to support diagnosis of allergic asthma 1, 3
6. Systematically exclude alternatives using DLCO (for COPD), flow-volume loops (for vocal cord dysfunction), and clinical assessment 4, 3
7. Order additional studies (chest X-ray, allergy testing) as clinically indicated 1, 2