How do I interpret a pulmonary function test (PFT)?

How to Read a Pulmonary Function Test (PFT)

To properly interpret a pulmonary function test, you must systematically evaluate key parameters including FEV1, VC, FEV1/VC ratio, and TLC to identify normal function, obstructive patterns, restrictive patterns, or mixed defects. 1

Basic Parameters and Their Interpretation

Key Measurements to Evaluate

• Forced Expiratory Volume in 1 second (FEV1): Volume exhaled in the first second
• Vital Capacity (VC): Largest available volume measurement (whether obtained via inspiration, slow expiration, or forced expiration)
• FEV1/VC ratio: Primary indicator for obstruction
• Total Lung Capacity (TLC): Essential for confirming restrictive defects
• Diffusing Capacity (DLCO): Evaluates gas exchange at the alveolar-capillary membrane

Normal vs. Abnormal Values

• Use the 5th percentile of predicted value (Lower Limit of Normal or LLN) rather than fixed cutoffs 1
• Avoid using fixed ratios like 0.7 for FEV1/FVC, which can overestimate defects in older individuals 1

Systematic Approach to PFT Interpretation

Step 1: Evaluate for Obstruction

• Definition: FEV1/VC ratio below the 5th percentile of predicted value 1
• Severity Classification (based on FEV1 % predicted):
  • Mild: FEV1 ≥ 70% predicted
  • Moderate: FEV1 60-69% predicted
  • Moderately severe: FEV1 50-59% predicted
  • Severe: FEV1 35-49% predicted
  • Very severe: FEV1 < 35% predicted

Step 2: Evaluate for Restriction

• Definition: TLC below the 5th percentile of predicted value 1, 2
• Important: A reduced VC alone does not confirm restriction - TLC measurement is required 1, 2
• Pattern: Typically shows normal or increased FEV1/VC ratio with reduced VC

Step 3: Identify Mixed Defects

• Definition: Both FEV1/VC ratio and TLC below their respective 5th percentiles 1
• Represents combination of obstructive and restrictive processes

Step 4: Assess Bronchodilator Response (when performed)

• Significant response typically defined as:
  • Increase in FEV1 ≥ 12% and ≥ 200 mL from baseline 1
• Note: Lack of response in laboratory testing does not preclude clinical response to bronchodilator therapy 1

Interpreting Common PFT Patterns

Obstructive Pattern

• Key findings: Reduced FEV1/VC ratio, normal or increased TLC
• Additional features: May show increased Residual Volume (RV), air trapping
• Common causes: Asthma, COPD, bronchiectasis
• Caution: A concomitant decrease in FEV1 and VC is most commonly caused by poor effort, but may rarely reflect airflow obstruction 1

Restrictive Pattern

• Key findings: Reduced TLC, normal or increased FEV1/VC ratio
• Additional features: Typically shows reduced VC, preserved RV/TLC ratio
• Common causes: Pulmonary fibrosis, pneumonia, pleural disease, neuromuscular disorders, thoracic cage abnormalities 2
• Important: Approximately 58% of patients with restrictive pattern on spirometry have confirmed restriction when TLC is measured 2

Mixed Pattern

• Key findings: Reduced FEV1/VC ratio and reduced TLC
• Common causes: Combination of obstructive and restrictive diseases (e.g., COPD with pulmonary fibrosis)

Common Pitfalls in PFT Interpretation

1. Relying solely on FVC instead of VC: Use the largest available VC measurement for accurate interpretation 1

2. Not measuring TLC when restriction is suspected: A reduced VC or FVC alone is insufficient to confirm restriction 1, 2

3. Using fixed cutoff values: Using the 5th percentile of predicted values rather than fixed cutoffs (like 0.7 for FEV1/FVC) prevents overdiagnosis in older individuals 1

4. Examining too many parameters simultaneously: Focus primarily on key parameters (FEV1, VC, FEV1/VC, TLC) to avoid overdiagnosis of abnormalities 1

5. Interpreting without previous results: Since most PFTs are performed to follow disease progress or treatment response, comparing with previous results is essential for meaningful interpretation 3

6. Ignoring clinical context: PFT interpretation should always consider the clinical situation and reason for testing 4

By following this systematic approach, you can effectively interpret pulmonary function tests and identify the underlying ventilatory defects to guide clinical management.