Is the FEV1 to FVC ratio or percentage predicted more useful in assessing spirometry?

FEV1/FVC Ratio vs. Percentage Predicted in Spirometry Assessment

The FEV1/FVC ratio is the primary parameter for detecting airflow obstruction, while percentage predicted values of FEV1 and FVC are more useful for assessing the severity of lung function impairment.

Key Parameters in Spirometry Interpretation

FEV1/FVC Ratio

• Used specifically to identify the presence of airflow obstruction 1
• Should be reported as a decimal fraction rather than a percentage 1
• A reduced FEV1/FVC ratio below the 5th percentile of predicted value (lower limit of normal) defines obstructive ventilatory defect 1
• Using a fixed cutoff of 0.7 (70%) can lead to misclassification, particularly in older adults 2

Percentage Predicted Values

• FEV1 % predicted is the primary parameter used to grade severity in obstructive, restrictive, and mixed pulmonary defects 1
• FVC % predicted helps assess the magnitude of restriction when the FEV1/FVC ratio is normal or increased 3
• Percentage predicted values account for age, height, sex, and ethnicity, making them more individualized 1

Clinical Application by Disorder Type

Obstructive Disorders

• Diagnosis requires a reduced FEV1/FVC ratio below the lower limit of normal 1
• Severity is graded using FEV1 % predicted (mild ≥70%, moderate 50-69%, severe <50%) 1
• In COPD, post-bronchodilator values should be used for diagnosis 1
• FEV1/SVC (slow vital capacity) ratio may be more sensitive than FEV1/FVC for detecting early obstructive disease 4

Restrictive Disorders

• Normal or increased FEV1/FVC ratio with reduced FVC suggests restriction 3
• FVC % predicted helps assess the severity of restriction 1
• Definitive diagnosis requires confirmation with reduced TLC by plethysmography 5
• Spirometry has excellent negative predictive value (>97%) but poor positive predictive value (<60%) for restriction 5

Mixed Disorders

• Characterized by both reduced FEV1/FVC ratio and reduced TLC 1
• Both ratio and percentage predicted values are needed for proper assessment 3

Bronchodilator Response Assessment

• ATS/ERS criterion: ≥12% increase over baseline AND ≥200 mL absolute change in FEV1 or FVC 6
• Alternative criterion: ≥10% absolute increase in percentage predicted FEV1 6
• The choice of criterion becomes more important in severe COPD, where percentage change from baseline may be misleading 6

Common Pitfalls and Recommendations

1. Avoid using fixed FEV1/FVC cutoffs

   • Using the lower limit of normal (typically 5th percentile) is more accurate than fixed cutoffs like 0.7, especially at age extremes 2

2. Don't rely solely on FVC for restriction diagnosis

   • A reduced FVC alone has poor positive predictive value for restriction 5
   • TLC measurement is mandatory to confirm restriction 3

3. Consider using slow vital capacity

   • FEV1/SVC ratio may detect early obstruction missed by FEV1/FVC 4
   • Particularly useful in symptomatic subjects with preserved FEV1/FVC who are <60 years old and obese 1

4. Report FEV1/FVC as decimal fraction, not percentage

   • The ATS recommends reporting the ratio as a decimal (e.g., 0.65 rather than 65%) to avoid confusion 1
   • The space for percentage predicted for FEV1/FVC ratio should be left blank to minimize miscommunication 1

In summary, both the FEV1/FVC ratio and percentage predicted values serve distinct and complementary roles in spirometry interpretation. The ratio identifies the presence of obstruction, while percentage predicted values quantify severity and are essential for proper clinical management.