What tests are recommended to assess lung function and how do FEV1 (Forced Expiratory Volume in 1 second) values correlate with disease severity?

Standard Lung Function Tests and FEV1 Correlation with Disease Severity

The most important tests for assessing lung function are spirometry (FEV1, FVC, FEV1/FVC ratio), lung volume measurements, and diffusing capacity, with FEV1 values below 70% of predicted indicating mild disease, 50-59% indicating moderately severe disease, 35-49% indicating severe disease, and below 35% indicating very severe disease. 1

Core Lung Function Tests

1. Spirometry

• Essential measurements: 1

  • Forced Expiratory Volume in 1 second (FEV1)
  • Forced Vital Capacity (FVC)
  • FEV1/FVC ratio
  • Slow Vital Capacity (SVC) when appropriate
  • Forced Expiratory Time (FET) for quality assessment

• Additional parameters: 1

  • Peak Expiratory Flow (PEF) - helpful for diagnosing extrathoracic airway obstruction
  • Mid-expiratory flow (MEF 25-75%) - may suggest airway obstruction in borderline cases

2. Lung Volume Measurements 1

• Total Lung Capacity (TLC)
• Functional Residual Capacity (FRC)
• Residual Volume (RV)
• Vital Capacity (VC)
• Inspiratory Capacity (IC)

3. Diffusing Capacity 1

• DLCO (Diffusing capacity for carbon monoxide)
• KCO (Transfer coefficient - DLCO/VA)
• VA (Alveolar Volume)

4. Additional Tests

• Maximal Voluntary Ventilation (MVV) - helpful in neuromuscular disorders and upper airway obstruction 1
• Pre- and post-bronchodilator testing - essential for confirming COPD diagnosis 1

FEV1 Correlation with Disease Severity

The European Respiratory Society Task Force provides the following classification of disease severity based on FEV1 percentage of predicted value: 1

Severity	FEV1 % predicted
Mild	>70%
Moderate	60-69%
Moderately severe	50-59%
Severe	35-49%
Very severe	<35%

This classification system has been validated by multiple studies showing strong correlation with:

• Mortality risk 2
• Respiratory symptoms 1
• Exercise capacity 1
• Quality of life 1

Pre- vs Post-Bronchodilator Testing

• Post-bronchodilator spirometry is required to confirm COPD diagnosis 1
• A single normal pre-bronchodilator measurement can exclude COPD 1
• Both pre- and post-bronchodilator measurements offer similar prediction for mortality 1
• Significant reversibility (increase in FEV1 ≥12% and ≥200mL) suggests asthma or asthma-COPD overlap 3

Interpretation Approach

1. First assess FEV1/FVC ratio to determine presence of obstruction 1

   • Obstruction defined as FEV1/VC ratio below the lower limit of normal (LLN) 3
   • Fixed ratio of 0.70 commonly used but may lead to false positives in elderly 3

2. If obstruction present, use FEV1 % predicted to classify severity 1

3. If FEV1 and FVC both reduced with normal ratio, consider restrictive pattern 3

   • Confirm with lung volume measurements (TLC) 3

Recent Developments in Severity Classification

Recent research suggests that FEV1/FVC ratio may be a more robust measure for severity classification than FEV1% predicted alone, particularly for:

• Better discrimination between absence of airflow obstruction and mild disease 4
• More uniform gradation of disease severity 4
• Less sensitivity to race/ethnicity 5

Common Pitfalls to Avoid

• Relying solely on FEV1 without considering FEV1/FVC ratio 3
• Using fixed FEV1/FVC ratio (0.70) in elderly patients (risk of overdiagnosis) 3
• Overlooking volume responses (FVC improvement) after bronchodilator administration 3
• Not measuring lung volumes when spirometry suggests restriction 3
• Including too many parameters in interpretation, which increases false positives 1

By following this standardized approach to lung function testing and interpretation, clinicians can accurately assess respiratory impairment and guide appropriate management decisions.