Is asthma classification into mild, moderate, and severe based solely on oxygen saturation (SpO2) values?

Asthma Classification: Not Based Solely on SpO2 Values

Asthma severity classification is not based on oxygen saturation (SpO2) values, but rather on a comprehensive assessment of impairment and risk domains including symptoms, lung function (primarily FEV1 and FEV1/FVC ratio), frequency of exacerbations, and medication requirements. 1, 2

Classification Framework for Asthma Severity

Two-Domain Approach

• Asthma severity is classified using two key domains: impairment and risk 1, 2
• The impairment domain assesses the "frequency and intensity of symptoms and functional limitations" 1
• The risk domain evaluates "the likelihood of asthma exacerbations, progressive decline in lung function, or risk of adverse effects from medication" 1

Severity Categories

• Asthma is categorized as intermittent or persistent (mild, moderate, or severe) 1, 2
• The classification is based on the most severe category in which any feature occurs 1, 2

Specific Parameters Used for Classification

Lung Function Parameters

• FEV1 (Forced Expiratory Volume in 1 second) is a primary objective measure:
  • Intermittent/Mild Persistent: FEV1 ≥80% of predicted 1, 2
  • Moderate Persistent: FEV1 60-80% of predicted 1, 2
  • Severe Persistent: FEV1 <60% of predicted 1, 2
• FEV1/FVC ratio is also evaluated, with normal values varying by age:
  • 8-19 years: 85%
  • 20-39 years: 80%
  • 40-59 years: 75%
  • 60-80 years: 70% 1, 2

Symptom-Based Parameters

• Frequency of daytime symptoms:
  • Intermittent: ≤2 days/week
  • Mild Persistent: >2 days/week but not daily
  • Moderate Persistent: Daily
  • Severe Persistent: Throughout the day 1
• Nighttime awakenings:
  • Intermittent: ≤2 times/month
  • Mild Persistent: 3-4 times/month
  • Moderate Persistent: >1 time/week but not nightly
  • Severe Persistent: Often 7 times/week 1
• Short-acting β2-agonist use for symptom control:
  • Intermittent: ≤2 days/week
  • Mild Persistent: >2 days/week but not daily
  • Moderate Persistent: Daily
  • Severe Persistent: Several times per day 1

Exacerbation History

• Frequency of exacerbations requiring oral systemic corticosteroids:
  • Patients with ≥2 exacerbations per year should be classified as having persistent asthma, regardless of other parameters 1, 2
• Severe exacerbations are defined as events requiring systemic corticosteroids for at least 3 days or hospitalization/ER visits requiring systemic corticosteroids 1

Role of SpO2 in Asthma Management

• SpO2 is notably absent from the formal classification criteria for asthma severity 1, 2
• SpO2 is primarily used in acute exacerbation assessment rather than for classifying baseline asthma severity 1
• During exacerbations, SpO2 may be used alongside other clinical parameters to determine the severity of the acute episode, but not the underlying chronic asthma severity 1

Common Pitfalls in Asthma Classification

• Relying solely on symptoms without objective lung function measurements can lead to misclassification of severity 2, 3
• Failing to consider exacerbation history when classifying severity may result in undertreating patients with frequent exacerbations despite good lung function 2
• Using FEV1 <80% predicted as the sole criterion for determining severity has low sensitivity (41%) in children with severe asthma 3
• Assuming parity between FEV1 and PEF measurements when expressed as percentage of predicted values can lead to misclassification in approximately 50% of cases 4
• Classifying asthma severity during an acute exacerbation rather than during a stable period may overestimate the baseline severity 2

In conclusion, while SpO2 may be monitored during acute asthma exacerbations, it is not a parameter used in the formal classification of asthma severity, which instead relies on symptoms, lung function (FEV1, FEV1/FVC), medication requirements, and exacerbation history.