Interpreting Spirometry Charts for Diagnosing Obstructive and Restrictive Lung Diseases
The most effective approach to interpreting spirometry charts is to first examine the FEV1/FVC ratio to identify obstruction (ratio <0.7 or below the 5th percentile), then assess FVC for restriction (below 5th percentile or <80% predicted), and finally evaluate post-bronchodilator response to confirm COPD diagnosis and rule out asthma. 1
Key Spirometric Parameters
- FEV1 (Forced Expiratory Volume in 1 second): Volume exhaled during the first second of a forced expiratory maneuver 1
- FVC (Forced Vital Capacity): Total volume exhaled during a forced expiratory maneuver 1
- FEV1/FVC ratio: The primary parameter for identifying airflow obstruction 1
- TLC (Total Lung Capacity): Total volume of air in the lungs after maximal inspiration; required to confirm restrictive disease 1
Step-by-Step Interpretation Algorithm
Step 1: Assess FEV1/FVC Ratio for Obstruction
- If FEV1/FVC < 0.7 (or below 5th percentile): Obstructive pattern present 1
- If FEV1/FVC normal or increased: No obstruction; consider restriction or normal 1
Step 2: Assess FVC for Restriction
- If FVC < 5th percentile (adults) or <80% predicted (children): Possible restrictive pattern 2, 3
- If FVC normal: Normal lung volumes 1
Step 3: Determine Pattern Type
- Low FEV1/FVC + Normal FVC: Pure obstruction 1
- Normal FEV1/FVC + Low FVC: Possible restriction (requires TLC confirmation) 1
- Low FEV1/FVC + Low FVC: Possible mixed defect or severe obstruction with air trapping 1, 4
Step 4: Assess Bronchodilator Response
- Significant response: Improvement in FEV1 or FVC ≥12% and ≥200mL in adults (≥12% in children) 2, 3
- Positive response: Suggests asthma or partially reversible COPD 1
- Minimal/no response: Consistent with fixed airway obstruction (COPD) 1
Interpreting Flow-Volume Loops
- Obstructive pattern: Concave expiratory flow curve, normal or increased TLC 1
- Example values: FEV1 38%, FEV1/VC 46%, TLC 101% 1
- Restrictive pattern: Convex or normal-shaped curve with reduced volumes, low TLC 1
- Example values: FEV1 66%, FEV1/VC 80%, TLC 62% 1
- Mixed pattern: Features of both obstruction and restriction 1, 4
- Example values: FEV1 64%, FEV1/VC 64%, TLC 72% 1
Special Considerations
Volume and Flow Responders
- Volume responders: Pre-BD FEV1/FVC ≥0.7 but post-BD FEV1/FVC <0.7 due to greater improvement in FVC than FEV1; may indicate gas trapping 1
- Flow responders: Pre-BD FEV1/FVC <0.7 but post-BD FEV1/FVC ≥0.7 due to greater improvement in FEV1 than FVC; require monitoring as they may develop persistent obstruction 1
Common Pitfalls to Avoid
- Misinterpreting reduced FVC: A reduced FVC alone doesn't confirm restriction; TLC measurement is required 1
- Overlooking air trapping: When FEV1 and FVC are both reduced with normal FEV1/FVC ratio, consider air trapping or incomplete exhalation 1, 5
- Using only pre-bronchodilator values: GOLD 2025 recommends using pre-BD spirometry to rule out COPD and post-BD measurements to confirm diagnosis 1
- Relying solely on fixed ratio: Using FEV1/FVC <0.7 may overestimate COPD in elderly and underestimate in younger patients; consider using lower limit of normal (LLN) 1, 6
- Misclassifying severity in mixed defects: In patients with both obstruction and restriction, the decrease in FEV1 may overestimate obstruction severity; consider adjusting FEV1 for the decrease in TLC 4
Severity Classification
Obstruction severity (based on FEV1 % predicted):
Restriction severity (based on TLC % predicted):
By following this systematic approach to spirometry interpretation, clinicians can accurately diagnose obstructive and restrictive lung diseases, assess their severity, and guide appropriate treatment decisions to improve patient outcomes 1.