Pulmonary Function Test Interpretation

Introduction to Pulmonary Function Tests

• The European Respiratory Society and American Thoracic Society guidelines recommend interpreting Pulmonary Function Tests using a systematic approach that identifies specific ventilatory patterns (obstructive, restrictive, mixed, or normal) and grades severity based on standardized criteria 1

Test Evaluation and Quality Control

• Confirming acceptable maneuvers and reproducible tests is essential for reliable results, according to the European Respiratory Society guidelines 2
• The American Thoracic Society guidelines suggest evaluating technician performance and test reliability 1
• Special considerations are needed for patients with decreased oral aperture or systemic sclerosis, who may require a pediatric mouthpiece for PFTs, as recommended by the American College of Rheumatology 3
• Patients with Raynaud's phenomenon require ear or forehead oxygen saturation monitoring instead of finger monitoring, as recommended by the American College of Rheumatology 3
• Special considerations are needed for patients with neuromuscular weakness, systemic autoimmune diseases, or developmental delay, as recommended by the American Academy of Neurology, American College of Rheumatology, and American Thoracic Society 4, 5, 6

Patterns of Ventilatory Dysfunction

• An obstructive pattern is characterized by an FEV₁/FVC ratio < 0.7 or below the lower limit of normal (LLN), as defined by the European Respiratory Society guidelines 1
• A restrictive pattern is characterized by a total lung capacity (TLC) < 80% predicted or below the LLN, according to the American Thoracic Society guidelines 1
• A mixed pattern features both obstruction and restriction, as described by the European Respiratory Society guidelines 1
• The severity of obstructive ventilatory defects can be classified based on FEV1 (% predicted) as follows:

• The severity of restrictive ventilatory defects can be classified based on TLC (% predicted) as follows:

DLCO Impairment Classification

• The European Respiratory Society guidelines classify DLCO impairment based on the following severity categories:

Interpretation of Test Results

• The American Thoracic Society guidelines recommend considering DLCO in the context of the ventilatory pattern, and assessing bronchodilator response (significant if FEV₁ increases >12% and >200 mL) 1
• A normal spirometry with decreased DLCO may suggest pulmonary vascular disorders, early interstitial lung disease (ILD), or early emphysema, according to the European Respiratory Society guidelines 1
• Restriction with normal DLCO may indicate chest wall or neuromuscular disorders, while restriction with decreased DLCO suggests ILD, as described by the American Thoracic Society guidelines 1
• Obstruction with decreased DLCO may indicate emphysema, according to the European Respiratory Society guidelines 1
• Asthma severity can also be classified based on lung function using the following table:

Reference Values and Social Determinants

• The European Respiratory Society guidelines recommend selecting reference values from sources that used similar equipment and had a test population that included the age range, sex, and ethnic group of individuals being tested 2
• Traditional reference equations may not adequately account for racial/ethnic differences, and social determinants of health and environmental factors should be considered when interpreting lung function, as suggested by the guidelines from the American Thoracic Society and other organizations 8

Clinically Meaningful Changes

• Changes in FEV₁ >12% and >200 mL are generally significant, and year-to-year changes in FEV₁ should exceed 15% to be considered clinically meaningful, according to the European Respiratory Society guidelines 1
• Trending multiple parameters (FEV₁, FVC, DLCO) depending on suspected disease can provide a more comprehensive understanding of disease progression, as recommended by the European Respiratory Society 1
• Compare with previous results when available, and establish if there has been significant change over time, as recommended by the American College of Chest Physicians 6
• Monitor with serial spirometry for early detection of disease progression, and consider additional testing for borderline cases, as recommended by the European Respiratory Society and the American College of Chest Physicians 1, 8

Common Pitfalls in Interpretation

• Relying solely on percent predicted values without considering the lower limit of normal (LLN) is a common pitfall, as well as failing to assess quality of testing and maneuvers, according to the European Respiratory Society and American Thoracic Society guidelines 1, 2

Indications for Pulmonary Function Tests

• PFTs are recommended for patients with unexplained respiratory symptoms, such as dyspnea, wheezing, or cough, to assist with diagnosis and monitoring, as recommended by the American Thoracic Society, American College of Chest Physicians, and European Respiratory Society 9
• PFTs are essential for patients with suspected obstructive lung disease, such as asthma or COPD, to confirm diagnosis and monitor disease progression, as recommended by the American College of Chest Physicians 9
• PFTs are essential for patients with suspected restrictive lung disease, such as interstitial lung disease or neuromuscular disorders, to confirm diagnosis and assess disease severity, as suggested by the European Respiratory Society 9
• Baseline PFTs are recommended for patients with Sjögren's Syndrome or idiopathic inflammatory myopathies, including spirometry, DLCO, and respiratory pressures evaluation, as suggested by the European League Against Rheumatism and American College of Rheumatology 5
• The American College of Rheumatology recommends baseline assessment in patients with Sjögren's syndrome, even if asymptomatic, to screen for interstitial lung disease 10
• The American College of Rheumatology recommends screening in high-risk populations, such as patients with systemic autoimmune rheumatic diseases (SARDs) with risk factors for interstitial lung disease 3
• Patients with systemic sclerosis-ILD should be monitored every 3-6 months in the first year, then less frequently once stable, according to the American College of Rheumatology 3
• Patients with rheumatoid arthritis-ILD should be monitored every 3-12 months in the first year, then less frequently once stable, according to the American College of Rheumatology 3
• Asymptomatic children and adults with sickle cell disease should not undergo routine screening pulmonary function tests (PFTs), as recommended by the American Society of Hematology 9

Complete Pulmonary Function Tests

• Complete PFTs should include pre- and post-bronchodilator spirometry, lung volumes, and diffusing capacity of the lung for carbon monoxide (DLCO), as recommended by the American Thoracic Society and the American College of Rheumatology 10, 3
• Key parameters for spirometry include FEV₁, FVC, FEV₁/FVC ratio, and FEF₂₅₋₇₅%, which are essential for diagnosing obstructive and restrictive lung diseases, as recommended by the European Respiratory Society 2
• Results should be expressed as percent predicted values, Z-scores, and lower limit of normal (LLN), and should identify ventilatory pattern, including obstructive, restrictive, or mixed patterns, as suggested by the American Thoracic Society and European Respiratory Society 2, 1