What is the interpretation of spirometry results in respiratory medicine?

Interpretation of Spirometry Results in Respiratory Medicine

Spirometry interpretation requires assessment of post-bronchodilator FEV1/FVC ratio <0.7 to diagnose COPD, while pre-bronchodilator values are essential for asthma evaluation, with results compared to predicted values based on age, sex, height, and ethnicity. 1, 2

Key Parameters for Interpretation

Essential Measurements

• FEV1 (Forced Expiratory Volume in 1 second): Volume exhaled in the first second
• FVC (Forced Vital Capacity): Total volume exhaled during forced expiration
• FEV1/FVC ratio: Primary parameter for detecting airflow obstruction
• Post-bronchodilator values: Required for COPD diagnosis
• Pre-bronchodilator values: Important for asthma assessment and monitoring

Interpretation Algorithm

1. Assess test quality and validity:

   • Minimum of 3 acceptable maneuvers
   • Reproducibility within 150 mL for FEV1 and FVC
   • Adequate expiratory time (≥6 seconds)
   • Proper effort and technique

2. Determine ventilatory pattern:

   • Obstructive pattern: FEV1/FVC ratio < 0.7 (or below LLN)
   • Restrictive pattern: Normal/increased FEV1/FVC ratio with reduced FVC (<80% predicted or below LLN)
   • Mixed pattern: Features of both obstruction and restriction

3. Assess severity (based on FEV1% predicted):

   • Mild: FEV1 ≥80% predicted
   • Moderate: FEV1 50-79% predicted
   • Severe: FEV1 30-49% predicted
   • Very severe: FEV1 <30% predicted

4. Evaluate bronchodilator response (when applicable):

   • Positive response: Increase in FEV1 or FVC ≥12% AND ≥200 mL from baseline
   • Important for differentiating between asthma and COPD, though not definitive 1, 3

Disease-Specific Interpretation

COPD

• Diagnostic criterion: Post-bronchodilator FEV1/FVC ratio <0.7
• Key feature: Persistent airflow limitation that is not fully reversible
• Classification: Based on post-bronchodilator FEV1% predicted
• Note: The GOLD 2025 report acknowledges that post-bronchodilator testing is more time-consuming and may impede clinical practice efficiency, potentially contributing to underutilization of spirometry 1

Asthma

• Key feature: Variable airflow limitation
• Diagnostic indicators:
  • Pre-bronchodilator obstruction that reverses with bronchodilator
  • Normal spirometry with positive bronchoprovocation testing
• Monitoring: Pre-bronchodilator FEV1 is a fundamental objective measure of asthma control 1

Special Considerations

Fixed Ratio vs. Lower Limit of Normal (LLN)

• Fixed ratio (0.7): More commonly used in clinical practice, especially for COPD
• LLN approach: More statistically accurate, especially in older adults
• Caveat: Fixed ratio may overdiagnose elderly and underdiagnose younger patients 2

Ethnic Variations

• Use appropriate reference equations for the patient's ethnic background
• For Asian Americans, applying a correction factor of 0.88 to white reference values is reasonable when specific equations aren't available 1

Longitudinal Monitoring

• Compare current results with previous tests
• Excessive FEV1 decline over time (>15% plus expected age-related loss) warrants further evaluation 1
• Spirometry typically provides little new information more frequently than every 1-2 years in stable COPD patients 2

Common Pitfalls to Avoid

1. Using pre-bronchodilator values for COPD diagnosis: This can substantially overestimate COPD prevalence 2

2. Relying solely on FEV1/FVC ratio: Consider additional parameters like inspiratory capacity for evaluating hyperinflation

3. Misinterpreting borderline results: Patients with high clinical suspicion of COPD but normal FEV1/FVC should undergo additional evaluation 2

4. Inconsistent testing posture: Maintain consistent posture (sitting or standing) for serial measurements 1

5. Overlooking quality control: Poor technique can lead to misdiagnosis; ensure proper test performance and interpretation 4

By following this structured approach to spirometry interpretation, clinicians can accurately diagnose and monitor respiratory conditions, leading to improved patient outcomes through appropriate management strategies.