Spirometry Interpretation in Clinical Practice
Spirometry interpretation should focus on identifying airflow obstruction using the post-bronchodilator FEV1/FVC ratio <0.70 as the primary diagnostic criterion, with severity categorization based on the FEV1 percentage of predicted value. 1, 2
Quality Assessment of Spirometry
- Spirometry results should be evaluated for acceptability and reproducibility before interpretation, with quality grading from A (highest) to F (lowest) based on the number of acceptable tests and repeatability criteria 1
- Tests with grades A, B, or C are considered usable for clinical interpretation, while grades D and E may have limited utility 1
- Proper technician training and feedback significantly improve the quality of spirometry testing, which is essential for accurate interpretation 1
- Testing posture (sitting vs. standing) should be kept consistent and documented on repeat testing, as posture-related changes in FEV1 and FVC, although small, may impact interpretation 1
Basic Interpretation Algorithm
- Step 1: Determine if the FEV1/FVC ratio is reduced (<0.70 post-bronchodilator) to identify obstructive patterns 1, 2
- Step 2: If obstruction is present, classify severity based on post-bronchodilator FEV1 percentage of predicted value 2:
- Mild: FEV1 ≥80% predicted
- Moderate: FEV1 50-80% predicted
- Severe: FEV1 30-50% predicted
- Very severe: FEV1 <30% predicted
- Step 3: Assess for bronchodilator reversibility (increase in FEV1 >200 mL and >15% from baseline), which may suggest asthma or an asthma component in COPD 3, 4
- Step 4: If FEV1/FVC ratio is normal but FVC is reduced, consider a restrictive pattern that requires further testing with lung volumes 5, 6
Clinical Correlation
- The single best predictor of airflow obstruction is a history of >40 pack-years of smoking (positive likelihood ratio 12) 1
- The combination of >55 pack-years smoking history, wheezing on auscultation, and patient self-reported wheezing almost assures the presence of airflow obstruction (likelihood ratio 156) 1
- The absence of all three factors above practically rules out airflow obstruction (likelihood ratio 0.02) 1
- Spirometry results should always be interpreted in the context of symptoms, risk factors, and clinical presentation 2
Reference Values and Ethnic Considerations
- Use appropriate reference equations that have been developed for specific populations when available (e.g., NHANES III) 1
- For Asian Americans in North America, applying a correction factor of 0.88 to white reference values for FEV1 and FVC is reasonable until specific equations are developed 1
- Using fixed cutoff values (e.g., 80% predicted for FVC, 0.70 for FEV1/FVC) may lead to misdiagnosis, particularly in older adults and those younger than 45 years 1, 2
Longitudinal Monitoring
- Current spirometry should be compared with previous tests to evaluate changes over time 1
- Excessive decline in FEV1 should be evaluated using either a percentage decline (15% plus loss expected due to aging) or other approaches that account for testing variability 1
- Routine periodic spirometry after initiation of therapy is not recommended for monitoring disease status or modifying therapy in symptomatic COPD patients 1
- Improvements in clinical symptoms do not necessarily correlate with spirometric responses to therapy or reduction in long-term decline in FEV1 1
Common Pitfalls to Avoid
- Relying solely on fixed cutoff values (like FEV1/FVC <0.70) without considering age-related changes can lead to overdiagnosis in elderly patients and underdiagnosis in younger adults 2
- Failing to confirm that spirometry meets acceptability and reproducibility criteria before interpretation 1, 7
- Not considering ethnic differences in lung function when selecting reference values 1
- Assuming complete normalization of spirometry after bronchodilator use in true COPD (this is unusual and suggests reconsidering the diagnosis) 3
- Using spirometry alone without clinical context to diagnose respiratory conditions 2