What are the diagnostic criteria for asthma?

Diagnostic Criteria for Asthma

Asthma diagnosis requires a compatible clinical history combined with objective confirmation of variable expiratory airflow limitation through pulmonary function testing. 1 Without objective testing, asthma is frequently misdiagnosed, with studies showing that up to 33.1% of patients with physician-diagnosed asthma have no evidence of asthma on pulmonary function tests 2.

Clinical History Components

• Episodic or persistent symptoms of:
  • Wheezing
  • Dyspnea (breathlessness)
  • Chest tightness
  • Cough (particularly worse at night or early morning)
• Pattern of symptom triggers:
  • Exercise
  • Allergen exposure
  • Cold air
  • Irritants
• Diurnal variation (symptoms worse at night/early morning)
• Family history of asthma or atopic disorders

Objective Diagnostic Testing

Primary Diagnostic Methods

1. Spirometry with bronchodilator reversibility testing:

   • Demonstration of airflow obstruction (FEV₁/FVC < lower limit of normal)
   • Positive bronchodilator response: increase in FEV₁ ≥12% and ≥200 mL after bronchodilator administration 1, 2
   • For severe persistent asthma: FEV₁ <60% predicted with reduced FEV₁/FVC ratio 2

2. Peak Expiratory Flow (PEF) monitoring:

   • Excessive variability in twice-daily PEF measurements over 2 weeks
   • Diurnal variation >20% over 1-2 weeks indicates asthma 1, 2
   • Note: Less reliable than spirometry for diagnosis 2

3. Bronchial challenge testing (when spirometry is normal but clinical suspicion remains):

   • Direct challenges: methacholine, histamine
   • Indirect challenges: exercise, mannitol, hypertonic saline
   • Positive test: ≥20% fall in FEV₁ at standard dose 1, 3

4. Treatment response assessment:

   • Significant increase in lung function after 4 weeks of inhaled corticosteroid treatment 1
   • Improvement in symptoms and lung function with treatment 4

Supplementary Diagnostic Tests

1. Fractional exhaled nitric oxide (FeNO):

   • Elevated in eosinophilic airway inflammation
   • Supports diagnosis in atopic asthma
   • Values ≥45-50 ppb suggest eosinophilic inflammation 1, 2

2. Sputum eosinophil count:

   • Elevated (≥3%) in eosinophilic asthma phenotype 1, 2
   • Helps guide therapy, particularly for severe asthma

3. Allergy testing:

   • Complete blood count with differential
   • Total IgE levels
   • Allergen-specific IgE or skin prick testing 2

Diagnostic Algorithm

1. Initial assessment:

   • Document symptoms consistent with asthma
   • Perform spirometry with bronchodilator reversibility testing

2. If spirometry confirms variable airflow obstruction:

   • Diagnosis of asthma is established
   • Proceed to assessment of severity and control

3. If initial spirometry is normal but symptoms suggest asthma:

   • Option A: Repeat spirometry when symptomatic
   • Option B: Perform bronchial challenge testing
   • Option C: Monitor PEF variability over 2 weeks
   • Option D: Trial of asthma treatment with reassessment

4. For patients already on treatment:

   • Consider step-down of medication to confirm diagnosis
   • Repeat objective testing during symptomatic periods 1

Common Diagnostic Pitfalls

• Relying solely on symptoms without objective confirmation leads to misdiagnosis 2
• Single-point testing may miss asthma due to its variable nature; repeated testing may be necessary 1
• Timing of testing matters - tests performed during asymptomatic periods may be falsely negative 1
• Overlooking comorbidities that mimic or exacerbate asthma (GERD, vocal cord dysfunction, allergic bronchopulmonary aspergillosis) 2
• Failing to consider alternative diagnoses in patients with atypical presentations or poor response to therapy 3

Special Considerations

• Asthma-COPD overlap: May require additional diagnostic criteria, including:

  • Strong bronchodilator test positivity (FEV₁ >15% and >400 mL)
  • Bronchoconstrictor test positivity
  • FeNO ≥45-50 ppb and/or sputum eosinophils ≥3%
  • History of asthma 1

• Exercise-induced asthma: Requires specific bronchial provocation testing rather than empiric treatment based on symptoms alone 2

• Seasonal variation: Allergen exposure near the time of assessment may affect diagnostic outcomes, requiring consideration of seasonal patterns 1

By following this comprehensive diagnostic approach with emphasis on objective testing, clinicians can accurately diagnose asthma and minimize both over-diagnosis and under-diagnosis, ultimately improving patient outcomes related to morbidity, mortality, and quality of life.