How do you interpret pulmonary function tests (PFTs) in patients with lung disease?

Interpretation of Pulmonary Function Tests

Interpret PFTs using a systematic algorithm that first assesses test quality, then compares results to reference values using the 5th percentile as the lower limit of normal, identifies physiological patterns (obstructive, restrictive, mixed, or normal), and finally correlates findings with clinical context to guide diagnosis and treatment decisions. 1

Step 1: Verify Test Quality Before Interpretation

• Review technical aspects and ensure proper technique was used before relying on numerical results, confirming that results meet acceptability and reproducibility criteria 1
• Poor effort commonly causes concomitant decreases in both FEV1 and VC with a normal FEV1/VC ratio, which can mimic restrictive disease 2

Step 2: Compare Results to Reference Values

• Use the 5th percentile of predicted values as the lower limit of normal (LLN) rather than fixed cutoffs 2
• Compare patient results with predicted values based on age, sex, height, and ethnicity using appropriate reference equations 1
• The key parameters for interpretation are VC (vital capacity), FEV1, FEV1/VC ratio, and TLC (total lung capacity) 2
• Use the largest available VC whether obtained on inspiration (IVC), slow expiration (SVC), or forced expiration (FVC), as FVC is usually reduced more than IVC or SVC in airflow obstruction 2

Step 3: Identify the Physiological Pattern

Obstructive Pattern

• Obstruction is defined by FEV1/VC < 5th percentile of predicted 2
• The FEV1/VC ratio should be compared to VC (not just FVC) because VC more accurately identifies obstructive patterns 2
• Measurement of lung volumes is not mandatory to identify obstruction but may help disclose underlying disease 2
• An increase in TLC, RV, or RV/TLC ratio above upper limits may suggest emphysema, bronchial asthma, or other obstructive diseases and the degree of lung hyperinflation 2

Restrictive Pattern

• Restriction is defined by TLC < 5th percentile of predicted with normal FEV1/VC 2
• A reduced VC alone does not prove restriction—it is associated with low TLC only about half the time 2
• A low TLC from single-breath tests (such as VA from DLCO testing) should not be interpreted as demonstrating restriction, as these systematically underestimate TLC, especially in severe obstruction (by as much as 3 L) 2

Mixed Pattern

• Mixed defect is characterized by both FEV1/VC and TLC < 5th percentile of predicted 2

Step 4: Assess Severity Using FEV1 % Predicted

• Severity classification for obstructive, restrictive, and mixed defects:
  • Mild: >70% predicted
  • Moderate: 60-69% predicted
  • Moderately severe: 50-59% predicted
  • Severe: 35-49% predicted
  • Very severe: <35% predicted 1

Step 5: Interpret DLCO in Context

DLCO Severity Grading

• Mild: >60% and < LLN
• Moderate: 40-60% predicted
• Severe: <40% predicted 2

Clinical Patterns with DLCO

• Normal spirometry and lung volumes with decreased DLCO: suggests anemia, pulmonary vascular disorders, early interstitial lung disease, or early emphysema 2
• Restriction with normal DLCO: consistent with chest wall or neuromuscular disorders 2
• Restriction with decreased DLCO: suggests interstitial lung diseases 2
• Obstruction with decreased DLCO: suggests emphysema, though also seen in lymphangioleiomyomatosis 2
• High DLCO: associated with asthma, obesity, and intrapulmonary hemorrhage 2

DLCO Adjustments

• Adjust DLCO for hemoglobin and carboxyhemoglobin changes, especially when monitoring for drug toxicity or in patients with large hemoglobin shifts (e.g., chemotherapy) 2
• The relationship between DLCO and lung volume is not linear, so DLCO/VA or DLCO/TLC do not provide appropriate normalization for lung volume 2

Step 6: Apply Clinical Context for Treatment Decisions

• In COPD, FEV1 correlates with symptom severity and prognosis 1
• In preoperative evaluation for lung cancer, both FEV1 and DLCO should be measured systematically 1
• Reduced DLCO <60% indicates higher risk (25% mortality rate and 40% pulmonary morbidity) in patients undergoing lung resection 1
• VC may be only slightly impaired in diffuse interstitial diseases despite marked loss of diffusing capacity 1

Critical Pitfalls to Avoid

• Do not rely solely on computer interpretations without reviewing test quality 1
• Do not use FEV1/VC ratio to determine severity of obstruction—use FEV1 % predicted instead 1
• Do not diagnose restriction based on spirometry alone without measuring lung volumes 1
• Do not use fixed cutoff of 0.7 for FEV1/FVC, as this overestimates obstruction in older adults 2
• Recognize that upper airway obstruction may be life-threatening despite being classified as only mildly reduced by FEV1 % predicted 1
• When TLC is at the LLN and coexists with a disease expected to cause restriction (e.g., lung resection), the restrictive defect may be difficult to prove if TLC remains above the 5th percentile 2

Additional Testing When Indicated

• When major spirometric parameters lie near their upper or lower limits of normal, additional studies should be performed if indicated by the clinical problem, including bronchodilator response, DLCO, gas-exchange evaluation, respiratory muscle strength measurement, or exercise testing 2