How do you classify asthma severity based on Pulmonary Function Tests (PFTs)?

Determining Asthma Severity Based on Pulmonary Function Tests (PFTs)

Asthma severity should be classified based on both lung function parameters from PFTs and clinical impairment, with FEV1 below 60% of predicted indicating severe persistent asthma regardless of symptom frequency. 1

Lung Function Parameters for Severity Classification

• Intermittent Asthma: Normal FEV1 ≥80% of predicted with normal FEV1/FVC ratio 1

• Mild Persistent Asthma: FEV1 ≥80% of predicted but with FEV1/FVC reduced >5% from normal 1

• Moderate Persistent Asthma: FEV1 between 60-80% of predicted with FEV1/FVC reduced >5% from normal 1

• Severe Persistent Asthma: FEV1 <60% of predicted with reduced FEV1/FVC ratio 1

Two-Domain Approach to Severity Assessment

Asthma severity classification incorporates two key domains that should be evaluated together 1, 2:

1. Impairment Domain:

   • Objective PFT measurements (FEV1 and FEV1/FVC ratio)
   • Symptom frequency and intensity
   • Nighttime awakenings
   • Short-acting beta-agonist use
   • Activity limitation 1

2. Risk Domain:

   • History of exacerbations requiring oral systemic corticosteroids
   • Patients with ≥2 exacerbations requiring oral corticosteroids in the past year should be classified as having persistent asthma, even if lung function suggests intermittent asthma 1, 2

Important Considerations When Using PFTs

• The final severity classification should be guided by the most severe category in which any feature of impairment or risk occurs 1

• Normal FEV1/FVC ratios vary by age: 85% for ages 8-19 years, 80% for ages 20-39 years, 75% for ages 40-59 years, and 70% for ages 60-80 years 1

• Bronchodilator responsiveness testing (showing ≥12% and ≥200 mL improvement in FEV1 after bronchodilator) is valuable for both diagnosis and severity assessment 1

• Variability in PEF measurements ≥20% over time is diagnostic of asthma and helps determine severity 3

Clinical Application

• Ideally, severity classification should be determined before initiating therapy, as treatment may mask the true severity 1

• For patients already on treatment, assessment of control rather than severity becomes more relevant for ongoing management 4

• The classification of asthma severity is not a static feature and may change over months or years, requiring periodic reassessment 4

Common Pitfalls to Avoid

• Relying solely on symptoms without objective PFT measurements can lead to misclassification of severity 1, 3

• Failing to consider exacerbation history when classifying severity may result in undertreating patients with frequent exacerbations despite good lung function 1, 2

• Not accounting for age-related changes in normal FEV1/FVC ratios can lead to misclassification 1

• Assessing severity during an acute exacerbation rather than during a stable period may overestimate the baseline severity 1