How do you interpret and manage Pulmonary Function Test (PFT) results in patients with lung diseases?

Pulmonary Function Test Interpretation and Management

Systematic Interpretation Algorithm

Interpret PFTs using a standardized, stepwise approach: first assess test quality, then compare results to reference values using z-scores and lower limits of normal (LLN), identify physiological patterns (obstructive, restrictive, or mixed), assess severity, and finally integrate findings with clinical context to guide treatment decisions. 1

Step 1: Quality Assessment

• Review technical quality before relying on numerical results by ensuring proper technique was used and results meet acceptability and reproducibility criteria 1
• Verify that the technician obtained acceptable maneuvers and reproducible tests, as quality control with feedback to technicians is critical for high-quality data 2
• Check for adequate patient effort, proper calibration, and absence of technical artifacts that could invalidate results 2

Step 2: Compare to Reference Values

• Use appropriate reference equations based on age, sex, height, and ethnicity from studies of healthy subjects with similar anthropometric characteristics 2
• Display measured values before predicted values and report displacement from predicted in standard deviation units (z-scores) to better understand abnormality 2
• Apply the lower limit of normal (LLN, defined as the 5th percentile) rather than fixed percentage cutoffs to avoid misclassification, particularly at extremes of age 1, 3

Step 3: Identify Physiological Patterns

For spirometry, routinely report only FVC, FEV₁, and FEV₁/FVC as most other calculated parameters do not add clinical utility 2

Obstructive Pattern

• FEV₁/FVC < LLN indicates obstruction (not the fixed 70% cutoff, which misclassifies patients at age extremes) 3
• Measure slow VC and calculate FEV₁/VC as a useful adjunct in suspected airflow obstruction 2
• Consider measuring lung volumes to detect air trapping (elevated RV/TLC) which confirms obstruction even when spirometry shows pseudo-restriction 4

Restrictive Pattern

• Reduced FVC with normal or elevated FEV₁/FVC ratio suggests restriction on spirometry alone 1
• Always measure lung volumes (TLC) to confirm true restriction (TLC < 80% predicted), as reduced FVC alone can occur with air trapping in obstructive disease 1, 4
• A common pitfall is failing to measure lung volumes when restrictive pattern is suspected based on spirometry alone 1

Mixed Pattern

• Both FEV₁/FVC < LLN and TLC < 80% predicted 5

Step 4: Assess Severity

Base severity classification primarily on FEV₁ % predicted for obstructive, restrictive, and mixed defects 1:

• Mild: >70%
• Moderate: 60-69%
• Moderately severe: 50-59%
• Severe: 35-49%
• Very severe: <35%

For diffusing capacity (DLCO), values <60% predicted are associated with higher mortality and pulmonary morbidity 1

• Always adjust DLCO for hemoglobin and carboxyhemoglobin, especially when monitoring for toxicity 1
• The LLN should be the 5th percentile of the reference population 1

Step 5: Evaluate Bronchodilator Response (if performed)

• Significant response is defined as improvement in FEV₁ and/or FVC of ≥12% AND ≥200 mL 4
• Bronchodilator responsiveness in patients with apparent restrictive pattern may indicate a variant of obstructive disease with early airway closure and air trapping 4
• In symptomatic patients with reversible restrictive pattern, a therapeutic trial of bronchodilators may be beneficial 4

Race and Ethnicity Considerations

Recent evidence suggests race-neutral reference equations (GLI-Other) may better predict clinical outcomes including mortality, morbidity, and quality of life compared to race-specific equations 2

• Race-specific equations may mask relationships between lung function and important outcomes in Black individuals, as the same z-score reflects lower absolute FVC values 2
• Studies show that race-neutral equations more accurately reflect clinically relevant outcomes including COPD Assessment Test scores, CT findings, and mortality than race-specific equations 2
• For borderline cases, the impact of race/ethnicity on interpretation requires additional thoughtfulness, though this is less significant in clearly normal or abnormal results 2
• Consider that adjusting for body proportions and socioeconomic status attenuates observed differences between racial groups 2

Clinical Application to Guide Treatment

COPD Management

• FEV₁ correlates with symptom severity and prognosis in COPD 1
• Use FEV₁ % predicted (not FEV₁/FVC ratio) to determine severity of obstruction 1

Interstitial Lung Disease

• VC may be only slightly impaired despite marked loss of DLCO in diffuse interstitial diseases 1
• Both FEV₁ and DLCO should be measured systematically in these patients 1

Preoperative Evaluation for Lung Resection

• Reduced DLCO (<60%) indicates higher risk with 25% mortality rate and 40% pulmonary morbidity in patients undergoing lung resection 1
• Both FEV₁ and DLCO should be measured systematically in preoperative evaluation of lung cancer patients 1

Upper Airway Obstruction

• Disproportionately reduced PEF compared to FEV₁ (with normal FEV₁/FVC) suggests upper airway pathology such as vocal cord dysfunction, tracheal stenosis, or laryngeal issues 6
• Evaluate flow-volume loop for evidence of upper airway obstruction 6
• Recognize that upper airway obstruction may be life-threatening despite being classified as only mildly reduced by FEV₁ % predicted 1

Critical Pitfalls to Avoid

• Never rely solely on computer interpretations without reviewing test quality 1
• Do not use FEV₁/FVC ratio to determine severity of obstruction; use FEV₁ % predicted instead 1
• Do not diagnose restriction based on spirometry alone; always measure lung volumes (TLC) for confirmation 1
• Avoid using fixed 70% cutoff for FEV₁/FVC, as this misclassifies 16% of subjects >74 years of age compared to LLN method 3
• Do not forget to adjust DLCO for hemoglobin and carboxyhemoglobin 1
• Recognize that PFTs are only one tool and must be interpreted in clinical context with other available information 2

Standardized Reporting

• Use uniform format with clinical application included, showing measured values before reference values 2
• Report FEV₁/FVC as a percentage of predicted value for this ratio to minimize miscommunication 2
• Include quality grading to communicate test reliability 2
• Maintain consistent interpretation procedures within your laboratory to avoid inferring patient changes when differences result from interpretation approach changes 2

Borderline Cases

• In borderline or "gray zone" cases, additional testing or assessment may be necessary to establish diagnosis 2
• Consider repeat testing, additional pulmonary function measurements, or other diagnostic modalities when results are equivocal 2