What are the indications and interpretations of pulmonary function tests (PFTs) in patients with respiratory symptoms or conditions?

Pulmonary Function Tests: Indications and Interpretation

Pulmonary function testing (PFT) is recommended for patients with respiratory symptoms or conditions to assist with diagnosis, monitor disease progression, and measure the effect of therapeutic interventions, with the specific tests and frequency determined by the underlying condition. 1

Key Indications for PFTs

Diagnostic Indications

• Suspected obstructive lung disease: Asthma, COPD, bronchiectasis
• Suspected restrictive lung disease: Interstitial lung disease, neuromuscular disorders, chest wall abnormalities
• Unexplained respiratory symptoms:
  • Dyspnea at rest or with exertion
  • Wheezing or increased cough
  • Chest pain disproportionate to known conditions
  • Exercise limitation compared to baseline
  • Unexplained desaturation or hypoxemia 2

Monitoring Indications

• Follow-up of known respiratory disease (accounts for 53% of all PFTs) 3
• Assessment of treatment response
• Evaluation of disease progression
• Pre-operative assessment
• Disability evaluation 4

Disease-Specific Indications

Neuromuscular Disease

• PFTs recommended every 6 months for patients with neuromuscular disease at risk of respiratory complications 2
• Tests should include:
  • Spirometry with forced or slow vital capacity (FVC/SVC)
  • Maximum inspiratory/expiratory pressures (MIP/MEP) or sniff nasal inspiratory pressure (SNIP)
  • Peak cough flow (PCF) 2

Autoimmune/Rheumatic Diseases

• Baseline PFTs recommended for all patients with Sjögren's Syndrome 2
• For patients with idiopathic inflammatory myopathies, baseline assessment should include:
  • Spirometry
  • DLCO
  • Respiratory pressures evaluation 2
• For patients with systemic autoimmune rheumatic diseases with ILD, PFTs should be performed every 3-6 months during the first year after diagnosis, then less frequently once stable 1

Sickle Cell Disease

• PFTs not recommended for routine screening in asymptomatic patients 2
• Consider diagnostic PFT when the following are present:
  • Wheezing or increased cough at rest or with exertion
  • Unexplained dyspnea
  • Chest pain at rest or with exertion
  • Abnormal 6-minute walk test
  • History of recurrent acute chest syndrome 2

Cystic Fibrosis Spectrum Disorders

• Routine PFTs not recommended for people with CRMS/CFSPID (Cystic Fibrosis-Related Metabolic Syndrome/CF Screen Positive, Inconclusive Diagnosis) 2
• PFTs should be considered if clinical concern for respiratory disease arises 2

Types of Pulmonary Function Tests

Spirometry

• Measures airflow and lung volumes
• Key parameters:
  • FEV₁ (Forced Expiratory Volume in 1 second)
  • FVC (Forced Vital Capacity)
  • FEV₁/FVC ratio
  • FEF₂₅₋₇₅% (Forced Expiratory Flow between 25% and 75% of FVC) 2

Lung Volumes

• Measures total lung capacity (TLC), residual volume (RV), functional residual capacity (FRC)
• Essential for confirming restrictive patterns 1

Diffusion Capacity (DLCO)

• Measures gas transfer across the alveolar-capillary membrane
• Important for assessment of interstitial lung disease and pulmonary vascular disease 1

Additional Tests

• Maximum voluntary ventilation (MVV)
• Respiratory muscle strength testing (MIP/MEP)
• Bronchial challenge testing
• Exercise testing 2, 1

Interpretation of PFTs

Basic Interpretation Approach

1. Use appropriate reference equations based on patient's age, height, sex, and ethnicity
2. Express results as percent predicted values, Z-scores, and lower limit of normal (LLN)
3. Identify ventilatory pattern:
   • Obstructive: Reduced FEV₁/FVC ratio below LLN
   • Restrictive: Reduced TLC below LLN with normal/increased FEV₁/FVC ratio
   • Mixed: Features of both patterns 1

Severity Classification

Obstructive Defects (based on FEV₁ % predicted)

• Mild: >70% and <LLN
• Moderate: 60-69%
• Moderately Severe: 50-59%
• Severe: 35-49%
• Very Severe: <35% 1

Restrictive Defects (based on TLC % predicted)

• Mild: >70% and <LLN
• Moderate: 60-69%
• Moderately Severe: 50-59%
• Severe: <50% 1

Diffusing Capacity Defects (based on DLCO % predicted)

• Mild: >60% and <LLN
• Moderate: 40-60%
• Severe: <40% 1

Tracking Changes Over Time

• Changes in FEV₁ >12% and >200 mL are generally significant
• Year-to-year changes in FEV₁ should exceed 15% to be considered clinically meaningful 1
• Compare with previous results when available to establish significant change over time 1

Common Pitfalls in PFT Interpretation

• Using fixed cutoffs (e.g., FEV₁/FVC <0.7) rather than LLN can lead to overdiagnosis in elderly and underdiagnosis in young patients 1
• Failing to confirm restrictive pattern with TLC measurement can lead to misdiagnosis 1
• Using inappropriate reference equations for patient population can lead to inaccurate results 1
• Overlooking the need for race/ethnic adjustment factors when specific equations are unavailable 1
• Relying solely on percent predicted values rather than using lower limit of normal (LLN) 1

Contraindications and Special Considerations

Relative Contraindications

• Recent myocardial infarction
• Pulmonary embolism
• Ascending aortic aneurysm
• Recovery from major thoracic, abdominal, or head surgery 5

Special Populations

• Children and patients with developmental delay may require special accommodations 1
• Patients with systemic autoimmune diseases may need special considerations for oxygen saturation measurement 1
• Patients with neuromuscular weakness require specialized testing protocols 2

Conclusion

PFTs are valuable tools for diagnosing and monitoring respiratory conditions, but their use should be guided by clinical indications rather than routine screening in asymptomatic individuals. The interpretation requires understanding of respiratory physiology and consideration of patient-specific factors. For most conditions requiring follow-up, PFTs should be performed at intervals appropriate to the disease progression rate, typically every 3-6 months initially and then less frequently once stable.