How do you interpret Pulmonary Function Tests (PFTs) results?

Pulmonary Function Test Interpretation

The interpretation of Pulmonary Function Tests (PFTs) should be clear, concise, and informative, going beyond merely stating which values are normal or low to provide clinically meaningful insights that impact patient outcomes. 1

Step 1: Assess Test Quality

• Verify test quality and reproducibility before interpreting numerical results
• Ensure proper technique was used (complete exhalation, good effort)
• Check for acceptability criteria (rapid start, smooth continuous exhalation, adequate duration)
• Common pitfall: Relying solely on numerical results without quality review 2

Step 2: Identify the Pattern

Obstructive Pattern

• Defined by reduced FEV1/FVC ratio below the 5th percentile of predicted value 1
• Key findings:
  • Reduced FEV1/FVC ratio (<70-80% in adults, <90% in children) 2
  • Disproportionate reduction in maximal airflow relative to maximal volume
  • Concave shape on flow-volume curve (indicates small airway involvement) 1
  • Proportionally greater reduction in FEF75% or FEF25-75% than in FEV1 1

Restrictive Pattern

• Characterized by reduced lung volumes with normal or increased FEV1/FVC ratio
• Requires confirmation with total lung capacity (TLC) measurement
• TLC <80% predicted confirms restriction 2

Mixed Pattern

• Features of both obstruction and restriction
• Reduced FEV1/FVC ratio with reduced TLC

Normal Pattern

• All parameters within normal limits using appropriate reference values

Step 3: Assess Severity

• Use z-scores rather than percentages of predicted values for more accurate interpretation 2
• For obstructive diseases, severity classification based on FEV1:
  • Mild: FEV1 ≥80% predicted
  • Moderate: FEV1 60-80% predicted
  • Severe: FEV1 ≤60% predicted 2

Step 4: Evaluate Bronchodilator Response

• Significant bronchodilator reversibility: increase in FEV1 ≥12% and ≥200ml 2
• Important for asthma diagnosis and distinguishing between asthma and COPD

Step 5: Consider Diffusion Capacity

• DLCO <80% or >120% predicted indicates abnormal gas transfer 2
• Low DLCO suggests:
  • Emphysema (when combined with obstruction)
  • Pulmonary vascular disease
  • Interstitial lung disease (when combined with restriction)
• High DLCO suggests:
  • Pulmonary hemorrhage
  • Polycythemia
  • Left-to-right shunt

Step 6: Integrate Clinical Context

• PFT interpretations are most meaningful when considering:
  • Clinical symptoms (cough, phlegm, wheezing, dyspnoea)
  • Smoking status and recent bronchodilator use
  • Relevant clinical diagnoses
  • Chest radiograph findings
  • Recent hemoglobin values
  • Suspicion of neuromuscular disease or upper airway obstruction 1

Special Considerations

Reference Values

• Use the most current reference standards such as Global Lung Initiative (GLI) reference equations 2
• Consider the effect of race and ethnicity on PFT interpretation with caution 1
• The American Thoracic Society recommends avoiding race-specific reference equations 2

Longitudinal Monitoring

• Compare with previous results when available
• Establish if there has been significant change over time 2
• For patients with systemic autoimmune rheumatic diseases, perform PFTs every 3-6 months during the first year after ILD diagnosis, then less frequently once stable 2

Borderline Results

• Consider additional testing for borderline cases 2
• The concept of a "gray zone" may signal the need for additional assessment 1
• Borderline results should be interpreted in the context of pre-test probability of disease 2

Common Pitfalls to Avoid

1. Relying solely on percent predicted values instead of using lower limit of normal (LLN) 2
2. Using inappropriate reference equations leading to systematic errors 2
3. Ignoring pre-test probability and clinical context 2
4. Over-interpreting small changes in PFT results that may represent normal variation
5. Misinterpreting mixed patterns as purely obstructive or restrictive
6. Failing to recognize that computer-generated interpretations may not account for clinical context 3

By following this systematic approach to PFT interpretation, clinicians can more accurately diagnose respiratory conditions, monitor disease progression, and evaluate treatment response, ultimately improving patient outcomes.