Determining Asthma Severity Based on Pulmonary Function Tests (PFTs)

Lung Function Parameters for Severity Classification

The American Academy of Family Physicians recommends that 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, 2
Intermittent Asthma is characterized by normal FEV1 ≥80% of predicted with normal FEV1/FVC ratio 1, 2
Mild Persistent Asthma is defined as FEV1 ≥80% of predicted but with FEV1/FVC reduced >5% from normal 1, 3
Moderate Persistent Asthma is characterized by FEV1 between 60-80% of predicted with FEV1/FVC reduced >5% from normal 1, 2
Severe Persistent Asthma is defined as FEV1 <60% of predicted with reduced FEV1/FVC ratio 1, 3

The American Academy of Family Physicians recommends that asthma severity classification incorporates two key domains that should be evaluated together: Impairment Domain and Risk Domain 2
The Impairment Domain includes objective PFT measurements (FEV1 and FEV1/FVC ratio) 3, nighttime awakenings 3, short-acting beta-agonist use 3, and activity limitation 2, 4
The Risk Domain includes history of exacerbations requiring oral systemic corticosteroids 1
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

The final severity classification should be guided by the most severe category in which any feature of impairment or risk occurs 1, 2
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 3
Bronchodilator responsiveness testing is valuable for both diagnosis and severity assessment 5

Ideally, severity classification should be determined before initiating therapy, as treatment may mask the true severity 2, 4
Relying solely on symptoms without objective PFT measurements can lead to misclassification of severity 5
Failing to consider exacerbation history when classifying severity may result in undertreating patients with frequent exacerbations despite good lung function 1
Not accounting for age-related changes in normal FEV1/FVC ratios can lead to misclassification 3
Assessing severity during an acute exacerbation rather than during a stable period may overestimate the baseline severity 2, 4

The American College of Physicians and Mayo Clinic guidelines classify asthma severity based on two key domains: impairment and risk, which assess frequency and intensity of symptoms, functional limitations, lung function, likelihood of exacerbations, lung function decline, and medication side effects 6, 7, 8
Asthma severity is categorized as intermittent or persistent (mild, moderate, severe), with patients experiencing ≥2 exacerbations requiring oral corticosteroids per year classified as having persistent asthma, regardless of their impairment domain status 6, 9, 8

A higher number of reported triggers correlates with more severe disease, with the American Thoracic Society suggesting that trigger burden is a significant factor in asthma severity 10, 11, 12
Patients with high trigger burden experience more frequent and severe asthma attacks, hospitalizations, and have a greater need for oral corticosteroids, with the American Academy of Allergy, Asthma, and Immunology recommending trigger avoidance strategies 10, 11

Allergen exposure, such as sensitization to specific allergens (e.g. Alternaria, cockroaches), has been linked to increased risk of severe asthma and mortality, according to the American Journal of Respiratory and Critical Care Medicine 10, 11, 12
Respiratory infections are documented in approximately 50% of adult asthma exacerbations and up to 80-85% of exacerbations in school-aged children, with the Centers for Disease Control and Prevention recommending vaccination against influenza 10, 11
Occupational exposures, such as workplace triggers, can lead to progressive disease, with removal from exposure often improving symptoms, as suggested by the National Institute for Occupational Safety and Health 10, 11
Environmental factors, such as exposure to cigarette smoke, air pollution, and other irritants, can worsen asthma control, with the Environmental Protection Agency recommending reduction of exposure to these factors 12

The concept of "multi-symptom asthma" provides an epidemiological marker for severity, with patients experiencing lower FEV1 values and higher exhaled nitric oxide levels, according to the Mayo Clinic Proceedings 6

Patients with multiple triggers may require more aggressive therapy and trigger avoidance strategies, with the American College of Allergy, Asthma, and Immunology recommending comprehensive trigger identification and management 9