What constitutes a normal Pulmonary Function Test (PFT)?

Normal Pulmonary Function Test (PFT) Parameters

A normal pulmonary function test (PFT) is defined by values that fall above the lower limit of normal (LLN), which is set at the 5th percentile of a healthy reference population, rather than using fixed percentage cutoffs. 1

Key Parameters for Normal PFTs

Spirometry

• FEV1 (Forced Expiratory Volume in 1 second): Above LLN (typically >80% predicted, but varies by age)
• FVC (Forced Vital Capacity): Above LLN (typically >80% predicted, but varies by age)
• FEV1/FVC ratio: Above LLN (not the fixed 0.70 cutoff)
• Quality: Grade A, B, or C maneuvers (at least 3 acceptable efforts with 2 repeatable ones) 1

Lung Volumes

• TLC (Total Lung Capacity): Between LLN and ULN (Upper Limit of Normal)
• RV (Residual Volume): Between LLN and ULN
• FRC (Functional Residual Capacity): Between LLN and ULN
• VC (Vital Capacity): Above LLN
• IC (Inspiratory Capacity): Above LLN

Diffusion Capacity (DLCO)

• DLCO: Above LLN (typically >75-80% predicted)
• KCO (Transfer Coefficient): Above LLN
• VA (Alveolar Volume): Above LLN
• Quality: Grade A maneuvers (VI/VC >90% or VI/VC >85% with VA within 0.200 L or 5% of VA from another acceptable maneuver) 1

Interpretation Framework

Z-scores vs. Percent Predicted

• Z-score: More accurate representation of normality than percent predicted
• Normal range is typically between -1.64 and +1.64 (5th to 95th percentile) 1
• Percent predicted: Should not be used alone to define abnormality as LLN varies with age, height, and sex 1

Reference Values

• Spirometry: Global Lung Function Initiative (GLI-2012) equations are recommended 1
• DLCO: GLI-2017 reference equations are recommended 1
• Lung volumes: No universal recommendation yet, but international projects are underway 1

Common Pitfalls in PFT Interpretation

Fixed Cutoff Errors

• Using fixed cutoffs (e.g., 80% predicted for FVC or 0.70 for FEV1/FVC) instead of LLN leads to:
  • Overdiagnosis in elderly and shorter individuals
  • Underdiagnosis in younger and taller individuals
  • Sex bias in clinical assessments 1

Quality Assessment Issues

• Failure to assess test quality can lead to erroneous interpretation
• Tests with poor quality grades (D, E, F) should be interpreted with caution or repeated 1
• For DLCO, only grade A maneuvers should be used when available 1

Reference Value Selection

• Using inappropriate reference equations for the population being tested
• Not accounting for ethnic differences in lung function 1

Clinical Application

Test Selection Algorithm

1. Start with spirometry to assess for obstructive or restrictive patterns
2. Add lung volumes if restriction is suspected (reduced FVC with normal/high FEV1/FVC)
3. Include DLCO to assess gas exchange, particularly important for:
   • Evaluating severity of obstructive disease
   • Distinguishing between asthma and emphysema
   • Detecting early interstitial lung disease
   • Identifying pulmonary vascular disease 2, 3

Special Considerations

• Isolated DLCO reduction (normal spirometry and lung volumes) may indicate:

  • Early interstitial lung disease
  • Pulmonary vascular disease
  • Anemia
  • Early emphysema 4, 5

• FVC/DLCO ratio >1.4 with reduced DLCO may suggest pulmonary vascular disease, particularly in connective tissue diseases 4, 5

• Serial measurements are more valuable than single measurements for detecting disease progression or treatment response 2

Remember that PFT results must always be interpreted in the clinical context, as values near the LLN have inherent uncertainty and should not be used alone for definitive diagnosis 1.