Differentiating Mild from Moderate Asthma
Mild persistent asthma is characterized by symptoms >2 days/week but not daily, nighttime awakenings 3-4 times/month, and FEV₁ >80% predicted, while moderate persistent asthma presents with daily symptoms, nighttime awakenings >1 time/week but not nightly, and FEV₁ 60-80% predicted. 1, 2, 3
Key Distinguishing Features
Symptom Frequency
- Mild Persistent: Daytime symptoms occur more than 2 days per week but not every day 1, 3
- Moderate Persistent: Daily daytime symptoms 1, 2, 3
Nighttime Awakenings
- Mild Persistent: 3-4 times per month 1, 3
- Moderate Persistent: More than 1 time per week but not nightly 1, 2, 3
Short-Acting β₂-Agonist Use
- Mild Persistent: Used more than 2 days per week but not daily, and not more than once per day 1
- Moderate Persistent: Daily use of rescue inhaler 1, 2
Lung Function Parameters
- Mild Persistent: FEV₁ >80% of predicted with normal FEV₁/FVC ratio 1, 2, 3
- Moderate Persistent: FEV₁ 60-80% of predicted with FEV₁/FVC reduced >5% from normal 1, 2, 3
Activity Limitation
- Mild Persistent: Minor limitation in normal activity 1, 3
- Moderate Persistent: Some limitation in normal activity 1, 2, 3
Critical Assessment Approach
Two-Domain Classification System
Severity must be assessed using both impairment and risk domains, with the final classification determined by the most severe category in which any feature occurs 1, 2, 3
Impairment Domain includes:
- Symptom frequency (daytime and nighttime) 1, 2
- Short-acting β₂-agonist use for symptom control 1, 2
- Interference with normal activity 1, 3
- Lung function (FEV₁ and FEV₁/FVC ratio) 1, 2
Risk Domain includes:
- Exacerbations requiring oral systemic corticosteroids 1, 2
- Patients with ≥2 exacerbations requiring oral corticosteroids in the past year should be classified as having persistent asthma, even if other parameters suggest milder disease 2, 3
Age-Adjusted FEV₁/FVC Normal Values
Normal FEV₁/FVC ratios vary by age and must be considered to avoid misclassification 1, 2:
- Ages 8-19 years: 85% 1, 2
- Ages 20-39 years: 80% 1, 2
- Ages 40-59 years: 75% 1, 2
- Ages 60-80 years: 70% 1, 2
Common Pitfalls to Avoid
Do not classify severity during acute exacerbations, as this overestimates baseline severity and leads to inappropriate long-term treatment decisions 3
Do not rely on symptoms alone without objective lung function testing, as this frequently results in misclassification and either under- or overtreatment 2, 3
Do not ignore exacerbation history when lung function appears normal, as patients requiring ≥2 courses of oral corticosteroids per year need treatment for persistent asthma regardless of FEV₁ values 1, 2, 3
Do not use fixed FEV₁/FVC cutoffs across all ages, as failing to account for age-related changes leads to misclassification, particularly in older patients 2, 3
Do not assess severity after treatment has begun, as medications mask the true underlying disease severity; severity classification should ideally be determined before initiating controller therapy 2, 3
Clinical Significance
Even patients classified as having mild asthma can experience severe, potentially life-threatening exacerbations at rates of 0.12 to 0.77 per patient-year 4, 5. Severe exacerbations in mild asthma represent 30-40% of all asthma exacerbations requiring emergency consultation 4. This underscores that the term "mild asthma" can be misleading, as these patients remain at significant risk for serious outcomes 5, 6.
In clinical trials of mild and moderate asthma patients, those with moderate disease had significantly higher rates of hospitalizations, urgent care visits, and work/school absenteeism compared to mild asthma patients 1. The distinction between mild and moderate persistent asthma directly impacts treatment decisions, with mild persistent asthma typically requiring low-dose inhaled corticosteroids alone, while moderate persistent asthma requires either medium-dose inhaled corticosteroids or low-to-medium dose inhaled corticosteroids plus a long-acting β₂-agonist 3.