Interpretation of Pulmonary Function Tests (PFTs)
Pulmonary Function Tests should be interpreted using a systematic approach that identifies specific ventilatory patterns (obstructive, restrictive, mixed, or normal) and grades severity based on standardized criteria from the European Respiratory Society and American Thoracic Society guidelines. 1, 2
Key Components of PFT Interpretation
1. Essential PFT Components
- Complete PFTs should include:
- Pre- and post-bronchodilator spirometry
- Lung volumes
- Diffusing capacity of the lung for carbon monoxide (DLCO) 2
2. Systematic Interpretation Algorithm
Step 1: Assess Quality of Testing
- Evaluate technician performance and test reliability
- Confirm acceptable maneuvers and reproducible tests 1
Step 2: Identify Ventilatory Pattern
- Obstructive Pattern: FEV₁/FVC ratio < 0.7 or below LLN
- Restrictive Pattern: TLC < 80% predicted or below LLN
- Mixed Pattern: Features of both obstruction and restriction
- Normal Pattern: All parameters within normal limits 1, 2
Step 3: Grade Severity
For Obstructive Defects (based on FEV₁ % predicted):
| Severity | FEV₁ (% predicted) |
|---|---|
| Mild | >70% and <LLN |
| Moderate | 60-69% |
| Moderately Severe | 50-59% |
| Severe | 35-49% |
| Very Severe | <35% |
For Restrictive Defects (based on TLC % predicted):
| Severity | TLC (% predicted) |
|---|---|
| Mild | >70% and <LLN |
| Moderate | 60-69% |
| Moderately Severe | 50-59% |
| Severe | <50% |
For DLCO Impairment:
| Severity | DLCO (% predicted) |
|---|---|
| Mild | >60% and <LLN |
| Moderate | 40-60% |
| Severe | <40% |
Step 4: Evaluate Additional Parameters
- Assess bronchodilator response (significant if FEV₁ increases >12% and >200 mL)
- Evaluate flow-volume loop morphology for characteristic patterns
- Consider DLCO in context of ventilatory pattern 1, 2
Clinical Correlation and Pattern Recognition
Obstructive Pattern
- Characterized by decreased expiratory flow rates
- Common in asthma, COPD, bronchiectasis
- Evaluate bronchodilator response to differentiate between asthma (typically reversible) and COPD (typically less reversible) 2
Restrictive Pattern
- Characterized by reduced lung volumes with normal or increased flow rates
- Consider:
Mixed Pattern
- Features of both obstruction and restriction
- Common in advanced COPD with air trapping and reduced lung compliance 2
DLCO Interpretation
- Normal spirometry with decreased DLCO: Consider pulmonary vascular disorders, early ILD, or early emphysema
- Restriction with normal DLCO: Consider chest wall or neuromuscular disorders
- Restriction with decreased DLCO: Suggests ILD
- Obstruction with decreased DLCO: Suggests emphysema 1
Important Considerations
Reference Values
- Select 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
- Use the same source for all spirometric indices 1
Racial and Ethnic Considerations
- Traditional reference equations may not adequately account for racial/ethnic differences
- Consider social determinants of health and environmental factors that may affect lung function 1
Longitudinal Monitoring
- Changes in FEV₁ >12% and >200 mL are generally significant
- Year-to-year changes in FEV₁ should exceed 15% to be considered clinically meaningful
- Multiple measurements over time are more reliable indicators of change than two isolated measurements 1, 2
Common Pitfalls to Avoid
- Relying solely on percent predicted values without considering LLN
- Failing to assess quality of testing and maneuvers
- Not adjusting DLCO for hemoglobin and carboxyhemoglobin levels
- Over-interpreting small changes in lung function parameters
- Not considering the clinical context when interpreting results 1
By following this systematic approach, clinicians can effectively interpret PFTs to aid in diagnosis, assess disease severity, monitor disease progression, and evaluate response to treatment.