How do you interpret Forced Expiratory Volume in 1 second (FEV1) and Forced Vital Capacity (FVC) values in lung function tests?

How to Interpret Lung Function Tests: FEV1 and FVC

Interpreting lung function tests requires a systematic approach that evaluates both FEV1 and FVC values against predicted norms, calculates the FEV1/FVC ratio to distinguish obstructive from restrictive patterns, and integrates these findings with clinical context—never relying on a single parameter alone. 1

Step 1: Compare Measured Values to Predicted Values

• Calculate percent predicted for both FEV1 and FVC by comparing the patient's actual measurements to reference values matched for age, height, sex, and ethnicity 1
• Use the lower limit of normal (LLN), defined as the 5th percentile of the normal distribution, rather than arbitrary fixed percentages when possible 1
• A common pitfall: Using a fixed FEV1/FVC ratio of 0.70 will overdiagnose obstruction in elderly patients and underdiagnose it in younger patients compared to the LLN method 1, 2

Step 2: Calculate and Interpret the FEV1/FVC Ratio

• The FEV1/FVC (or FEV1/VC) ratio is the critical first step in determining whether a pattern is obstructive, restrictive, or normal 1
• Obstructive pattern: FEV1/FVC ratio below the LLN (or <0.70 if using fixed ratio) indicates airflow obstruction 1
• Normal or elevated ratio with low FVC: Suggests possible restrictive pattern, but requires lung volume measurements (TLC) for confirmation 3
• Critical warning: The FEV1/FVC ratio should NOT be used to grade severity of obstruction—only to identify its presence 1

Step 3: Grade Severity Using FEV1 Percent Predicted

Once obstruction is confirmed by a reduced FEV1/FVC ratio, use FEV1 % predicted to classify severity:

European Respiratory Society Classification 1:

• 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

GOLD Classification 1:

• Mild: FEV1 ≥80% predicted (with FEV1/FVC <0.70)
• Moderate: FEV1 50-79% predicted
• Severe: FEV1 30-49% predicted
• Very severe: FEV1 <30% predicted

Note the discrepancy: Different guidelines use different thresholds, which can lead to confusion in severity classification 4

Step 4: Recognize Patterns and Their Limitations

Obstructive Pattern

• Confirmed by: FEV1/FVC below LLN AND reduced FEV1 1
• Examples: An FEV1 of 72% with reduced FEV1/FVC represents mild obstruction 5; an FEV1 of 46% represents moderate-to-severe obstruction 4
• Clinical correlation: FEV1 correlates with mortality risk—patients with FEV1 >2 SD below average are 12 times more likely to die of COPD over 20 years 1

Restrictive Pattern on Spirometry

• Suggested by: Low FVC with normal or elevated FEV1/FVC ratio 3
• Major limitation: Only 58% of patients with this spirometric pattern have true restriction confirmed by TLC measurement 3
• Action required: Measure lung volumes (TLC) to confirm restriction—spirometry alone cannot definitively diagnose restrictive disease 3

Normal Spirometry

• High negative predictive value: If FVC is normal, probability of restrictive defect is <3%, and lung volume measurement can usually be avoided unless clinical suspicion is high 3

Step 5: Assess for Additional Parameters

Beyond basic spirometry, consider:

• Bronchodilator testing: Changes >12% and >200 mL in FEV1 suggest reversibility 1, 5
• Inspiratory capacity (IC): Reduced IC indicates hyperinflation, which correlates with dyspnea and mortality in COPD better than FEV1 alone 1
• Diffusing capacity (DLCO): Important predictor of mortality in general population and surgical candidates 1
• Flow-volume loops: Essential for detecting upper airway obstruction, which may be life-threatening despite only mild FEV1 reduction 1

Step 6: Interpret Changes Over Time

When tracking individual patients:

• Short-term (within a day): Changes >5% in FEV1 or FVC are significant 1
• Week-to-week: Changes >12% in FEV1 or >11% in FVC suggest real change 1
• Year-to-year: Changes >15% in FEV1 indicate clinically meaningful progression 1
• Use coefficient of repeatability (CR) from your own laboratory rather than generic values when possible 1

Critical Pitfalls to Avoid

1. Never diagnose obstruction based on FEV1 alone—always evaluate the FEV1/FVC ratio first 1, 5
2. Never confirm restrictive disease without measuring TLC—spirometry has poor positive predictive value for restriction 3
3. Never use FEV1/FVC ratio to grade severity—use FEV1 % predicted instead 1
4. Avoid fixed ratio of 0.70 at extremes of age—16% of patients >74 years will be misclassified 2
5. Don't assume FEV1 predicts symptoms—correlations are poor for individual patients 1
6. In patients with concomitant decreases in both FEV1 and FVC, spirometry interpretation has weak agreement with clinical diagnosis (kappa 0.35), requiring integration of additional clinical findings 6