How do you interpret Pulmonary Function Test (PFT) results to guide treatment for lung diseases?

Pulmonary Function Test Interpretation for Guiding Treatment of Lung Diseases

Proper interpretation of Pulmonary Function Tests (PFTs) requires a systematic approach that begins with quality assessment, followed by pattern recognition of physiological abnormalities, and comparison with appropriate reference values to guide treatment decisions for optimal patient outcomes.

Quality Assessment First

• Always begin by assessing test quality before interpreting numerical results 1
• Tests with suboptimal quality may still contain useful information but require identification of potential errors
• Relying solely on numerical results without quality review is a common mistake, especially when using computer interpretations 1

Systematic Interpretation Approach

1. Compare results with appropriate reference values

   • Use Global Lung Initiative (GLI) reference equations as the current standard 2
   • Avoid race-specific reference equations as recommended by recent ATS guidelines 1
   • Consider z-scores rather than percentages of predicted values 2

2. Identify the primary pattern of abnormality

   • Obstructive pattern: Reduced FEV1/FVC ratio (<70-80% in adults, <90% in children)

     • Characteristic concavity of expiratory flow-volume loop 2
     • Assess bronchodilator reversibility (increase in FEV1 ≥12% and ≥200ml) 2

   • Restrictive pattern: Reduced FVC with normal or increased FEV1/FVC ratio

     • Confirm with lung volume measurements (reduced TLC)

   • Mixed pattern: Features of both obstruction and restriction

   • Normal pattern: All parameters within normal limits

3. Assess severity of abnormality

   • For asthma classification 2:

     Classification	Lung Function
     Mild Intermittent	FEV1 ≥80% predicted
     Mild Persistent	FEV1 ≥80% predicted
     Moderate Persistent	FEV1 60-80% predicted
     Severe Persistent	FEV1 ≤60% predicted

4. Compare with previous results when available

   • Particularly important as 53% of PFTs are performed for follow-up of known respiratory disease 3
   • Establish if there has been significant change over time

Specific Disease Considerations

Asthma

• Look for reduced FEV1/FVC ratio with significant bronchodilator reversibility 2
• Normal spirometry does not exclude asthma as patients may have normal PFTs between exacerbations 2
• Consider bronchial challenge testing for patients with normal spirometry but suspected asthma 2
• Low FEV1 is a strong predictor of future exacerbations 2

COPD

• Look for reduced FEV1/FVC ratio with limited bronchodilator reversibility
• Assess for lung hyperinflation (increased RV, RV/TLC)
• Monitor FEV1 over time to assess disease progression 4

Interstitial Lung Disease

• Look for restrictive pattern (reduced FVC, normal or increased FEV1/FVC)
• Assess diffusion capacity (DLCO) which is typically reduced
• For patients with systemic autoimmune diseases, perform PFTs every 3-6 months during the first year after ILD diagnosis, then less frequently once stable 2

Post-HSCT Bronchiolitis Obliterans

• Pre-HSCT PFTs are essential to appropriately interpret post-HSCT PFT data 1
• Look for new airflow obstruction pattern
• Monitor with serial spirometry for early detection 1

Common Pitfalls to Avoid

1. Relying solely on percent predicted values

   • Use lower limit of normal (LLN) rather than fixed cutoffs 1, 5
   • Only 3 of 17 laboratories in a survey reported using LLN to define abnormality 5

2. Using inappropriate reference equations

   • Substantial variation exists in reference equations used across laboratories 5
   • Use the most current reference standards (GLI) 2

3. Ignoring pre-test probability

   • Interpret PFTs in light of clinical context, not in isolation 6, 7
   • PFTs cannot distinguish among potential causes of abnormalities by themselves 6

4. Overlooking borderline results

   • Patients with results near thresholds of abnormality are at greatest risk of misclassification 1
   • Consider additional testing for borderline cases 1

5. Failing to consider technical factors

   • Poor patient effort can affect results
   • Equipment calibration issues can lead to systematic errors

Special Considerations

• Children and patients with developmental delay: May be unable to perform some or all PFT maneuvers 1
• Patients with systemic autoimmune diseases: May require special accommodations (e.g., pediatric mouthpieces for patients with systemic sclerosis due to decreased oral aperture) 2
• Patients with Sjögren's Syndrome: Baseline chest radiograph and full PFTs are recommended for all patients 1, 2

By following this systematic approach to PFT interpretation, clinicians can more accurately diagnose respiratory conditions, monitor disease progression, and guide treatment decisions to improve patient outcomes.