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, 2

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 3, 4

Special Considerations

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

  • Early interstitial lung disease
  • Pulmonary vascular disease
  • Anemia
  • Early emphysema 5, 6

• FVC/DLCO ratio >1.4 with reduced DLCO may suggest pulmonary vascular disease, particularly in connective tissue diseases 5, 6

• Serial measurements are more valuable than single measurements for detecting disease progression or treatment response 3

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, 2.