How do you interpret a pulmonary function test (PFT)?

Pulmonary Function Test Interpretation

To interpret a pulmonary function test (PFT), follow a systematic pattern-recognition approach that identifies obstructive, restrictive, or mixed ventilatory defects by comparing key parameters to appropriate reference values, with special attention to FEV1/FVC ratio, lung volumes, and diffusing capacity. 1

Step 1: Assess Test Quality

• Verify acceptable technique and reproducibility of efforts
• Check for at least 3 acceptable maneuvers with consistent results
• Ensure proper calibration of equipment
• Review flow-volume and volume-time curves for evidence of:
  • Good initial effort (sharp peak)
  • Complete exhalation (plateau on volume-time curve)
  • Absence of coughing, glottis closure, or early termination

Step 2: Compare Results with Reference Values

• Use appropriate reference equations based on:
  • Patient's age, height, sex, and ethnicity
  • Global Lung Initiative (GLI) reference equations are preferred 1
• Express results as:
  • Percent predicted values
  • Z-scores (standard deviations from mean)
  • Lower limit of normal (LLN) - 5th percentile of reference population 2

Step 3: Identify Ventilatory Pattern

Obstructive Pattern

• Key finding: Reduced FEV1/FVC ratio below LLN (5th percentile) 2
• Additional findings:
  • Concave flow-volume curve
  • Increased RV, FRC, and TLC
  • Normal or reduced FVC
  • Disproportionate reduction in expiratory flow rates

Restrictive Pattern

• Key finding: Reduced TLC below LLN with normal/increased FEV1/FVC ratio 2
• Additional findings:
  • Reduced FVC, FEV1, and lung volumes
  • Normal or increased FEV1/FVC ratio
  • Symmetrically reduced flow-volume loop
  • Requires lung volume measurement to confirm

Mixed Pattern

• Key finding: Both reduced FEV1/FVC ratio and reduced TLC 2
• Suggests coexisting obstructive and restrictive processes

Normal Pattern

• All parameters within normal limits (above LLN)

Step 4: Assess Severity

For Obstructive Defects

Severity	FEV1 (% predicted)
Mild	>70% and <LLN
Moderate	60-69%
Moderately Severe	50-59%
Severe	35-49%
Very Severe	<35%

For Restrictive Defects

Severity	TLC (% predicted)
Mild	>70% and <LLN
Moderate	60-69%
Moderately Severe	50-59%
Severe	<50%

For Diffusing Capacity (DLCO)

Severity	DLCO (% predicted)
Mild	>60% and <LLN
Moderate	40-60%
Severe	<40%

Step 5: Evaluate Bronchodilator Response

• Significant response defined as:
  • Increase in FEV1 and/or FVC ≥12% AND ≥200 mL from baseline 1
• Suggests asthma but can occur in other conditions
• Absence of response does not exclude asthma

Step 6: Interpret Diffusing Capacity

• Reduced DLCO with normal spirometry suggests:
  • Pulmonary vascular disease
  • Early interstitial lung disease
  • Emphysema
• Reduced DLCO with restriction suggests:
  • Interstitial lung disease
  • Pulmonary edema
• Elevated DLCO suggests:
  • Polycythemia
  • Pulmonary hemorrhage
  • Left-to-right shunt

Step 7: Evaluate Additional Tests When Available

Lung Volumes

• Help distinguish between true restriction and air trapping
• Elevated RV/TLC ratio suggests air trapping in obstructive disease

Maximum Voluntary Ventilation (MVV)

• Disproportionately reduced compared to FEV1 suggests:
  • Neuromuscular weakness
  • Poor effort
  • Upper airway obstruction

Flow-Volume Loop Abnormalities

• Fixed upper airway obstruction: flattened inspiratory and expiratory limbs
• Variable extrathoracic obstruction: flattened inspiratory limb
• Variable intrathoracic obstruction: flattened expiratory limb

Common Pitfalls to Avoid

1. Using fixed cutoffs (e.g., FEV1/FVC <0.7) rather than LLN, which can lead to overdiagnosis in elderly and underdiagnosis in young patients 2, 1
2. Relying solely on percent predicted values without considering LLN 1
3. Failing to confirm restrictive pattern with TLC measurement 2
4. Interpreting PFTs in isolation without clinical context 1
5. Using inappropriate reference equations for patient population 2
6. Overlooking the need for race/ethnic adjustment factors when specific equations are unavailable 2

Longitudinal Interpretation

• For tracking change over time:
  • Changes in FEV1 >12% and >200 mL are generally significant 2
  • Year-to-year changes in FEV1 should exceed 15% to be considered clinically meaningful 2
  • Consider trending multiple parameters (FEV1, FVC, DLCO) depending on suspected disease 2

Remember that PFT interpretation should always be performed in the context of clinical information, including symptoms, physical examination findings, and imaging results to provide the most accurate and clinically useful interpretation 1, 3.