How is the predicted best Forced Expiratory Volume in 1 second (FEV1) to Forced Vital Capacity (FVC) ratio determined?

Determining the Predicted Best FEV1/FVC Ratio

The predicted best FEV1/FVC ratio should be determined using reference equations derived from measurements in a representative sample of healthy subjects with similar anthropometric characteristics (sex, age, height) and ethnic background as the patient being tested.

Key Factors Affecting Reference Values

Anthropometric Factors

• Sex: Males and females have different lung function parameters
• Age: FEV1/FVC ratio naturally declines with age
• Height: Taller individuals typically have different values than shorter individuals
• Weight: Should be measured at time of testing, though less influential than other factors

Ethnic Considerations

• Patient should self-identify their race/ethnic group
• Race/ethnic-specific reference equations should be used when available
• When specific equations aren't available, adjustment factors may be applied:
  • 12% reduction for Black subjects for FEV1 and FVC (but not for FEV1/FVC ratio)
  • 6-7% reduction for Asian Americans 1

Recommended Approach

1. Select appropriate reference equations:

   • Use equations derived from a population similar to the patient
   • All parameters (FEV1, FVC, FEV1/FVC) should come from the same reference source 1
   • Consider using Global Lung Function Initiative (GLI) equations when available

2. Avoid fixed ratio cutoffs:

   • Using a fixed FEV1/FVC ratio of 0.70 leads to misclassification
   • Overdiagnoses obstruction in elderly patients
   • Underdiagnoses obstruction in younger patients 2

3. Use Lower Limit of Normal (LLN):

   • The European Respiratory Society recommends defining obstruction as FEV1/VC ratio below the 5th percentile of normal distribution 1, 3
   • This is more accurate than fixed ratios across all age groups

4. Consider measurement technique:

   • Use of slow vital capacity (SVC) instead of FVC may better detect airflow obstruction 3
   • Ensure proper measurement technique to avoid false results

Common Pitfalls to Avoid

1. Using inappropriate reference equations:

   • Using equations derived from populations with different characteristics
   • Extrapolating beyond the size and age limits of reference population 1

2. Applying fixed cutoff values:

   • The 0.70 fixed ratio for FEV1/FVC leads to significant misclassification
   • Up to 16% of subjects over 74 years of age may have discordant results when comparing fixed ratio to LLN 2

3. Ignoring ethnic differences:

   • Reference values vary significantly between populations
   • Using GLI normal values may result in overdiagnosis in certain populations like Western Indians 4

4. Failing to report quality control metrics:

   • Number of acceptable maneuvers
   • Repeatability of measurements
   • Technical quality of the test 1

Reporting Standards

The standardized pulmonary function report should include:

• FEV1 (L)
• FVC (L)
• FEV1/FVC ratio (as decimal fraction)
• Lower limit of normal (LLN)
• Z-score
• Percent predicted
• Reference source used 1

By following these guidelines, clinicians can accurately determine the predicted best FEV1/FVC ratio, which is essential for proper diagnosis and management of respiratory conditions.