How to interpret pulmonary function tests

Interpreting Pulmonary Function Tests

Pulmonary function tests (PFTs) should be interpreted using a systematic, algorithmic approach that first identifies the pattern of abnormality (obstructive, restrictive, mixed, or normal) and then correlates with clinical context to determine the underlying pathology. 1

Step-by-Step Interpretation Algorithm

1. Evaluate Basic Spirometry Parameters

• FEV1/VC ratio: Primary parameter to identify obstruction

  • Below the lower limit of normal (LLN) or <70% suggests obstruction
  • Normal or increased suggests restriction or normal function

• FVC: Assess for reduction

  • Below LLN (5th percentile) in adults or <80% predicted in children suggests restriction or severe obstruction

2. Identify the Pattern

Obstructive Pattern

• Definition: Reduced FEV1/VC ratio below LLN
• Characteristics:
  • Flow-volume curve shows concave appearance
  • TLC normal or increased
  • RV often increased (air trapping)
• Severity Classification (based on FEV1 % predicted):
  • Mild: >70%
  • Moderate: 60-69%
  • Moderately severe: 50-59%
  • Severe: 35-49%
  • Very severe: <35% 1

Restrictive Pattern

• Definition: Reduced TLC below LLN with normal FEV1/VC ratio
• Characteristics:
  • Flow-volume curve shows normal shape but reduced volumes
  • FVC reduced
  • TLC reduced (definitive for restriction)
• Caution: Spirometry alone can only suggest restriction; lung volume measurements are required for confirmation 1

Mixed Pattern

• Definition: Reduced FEV1/VC ratio AND reduced TLC
• Characteristics: Features of both obstruction and restriction 1

3. Assess Bronchodilator Response (for Obstruction)

• Significant response defined as:
  • Increase in FEV1 or FVC ≥12% AND ≥0.2L from baseline 2
  • Suggests asthma or reversible component of COPD

4. Evaluate Diffusing Capacity (DLCO)

• Normal DLCO with obstruction: Suggests asthma or bronchitis
• Reduced DLCO with obstruction: Suggests emphysema
• Reduced DLCO with restriction: Suggests interstitial lung disease
• Normal DLCO with restriction: Suggests chest wall disorder or neuromuscular disease
• Severity of DLCO reduction:
  • Mild: >60% predicted but below LLN
  • Moderate: 40-60% predicted
  • Severe: <40% predicted 1

Special Considerations

Interpreting Concomitant Decreases in FEV1 and FVC

When both FEV1 and FVC are decreased with a normal FEV1/FVC ratio, consider:

• Poor test performance (incomplete exhalation)
• Small airway disease with air trapping
• Combined obstructive and restrictive disease 3

In these cases:

• Check flow-volume curve for premature termination
• Measure slow VC (may give more accurate FEV1/VC ratio)
• Perform lung volume measurements to confirm restriction
• Consider post-bronchodilator testing 1

Tracking Changes Over Time

• FEV1 is the most reliable parameter for tracking changes
• Clinically significant changes:
  • Short-term: >12% and >0.2L in FEV1
  • Year-to-year: >15% change in FEV1 1
• Consider trending multiple parameters for specific diseases:
  • ILD: VC, TLC, DLCO
  • COPD: FEV1, FVC, DLCO 1

Common Pitfalls to Avoid

1. Relying solely on FVC to diagnose restriction - TLC measurement is required for confirmation
2. Misinterpreting reduced FEV1 and FVC with normal ratio - May be due to poor effort rather than restriction
3. Tracking too many indices simultaneously - Increases risk of false-positive changes
4. Ignoring clinical context - PFT patterns must be interpreted alongside patient history, physical exam, and imaging
5. Overlooking technical quality - Poor test performance can lead to misclassification 1, 3

Correlation with Clinical Conditions

• Asthma: Obstruction with bronchodilator reversibility, normal DLCO
• COPD: Obstruction with limited reversibility, reduced DLCO in emphysema
• Interstitial lung disease: Restriction with reduced DLCO
• Neuromuscular disease: Restriction with normal DLCO
• Pulmonary vascular disease: Normal spirometry with reduced DLCO 1, 4

By following this systematic approach to PFT interpretation, clinicians can accurately identify patterns of lung dysfunction, assess disease severity, and monitor response to therapy, ultimately improving patient outcomes through appropriate management of respiratory conditions.