What is the step-by-step approach to interpreting Pulmonary Function Tests (PFTs)?

Step-by-Step Interpretation of Pulmonary Function Tests

Interpret PFTs using a systematic five-step algorithm: (1) assess test quality, (2) compare results to reference values, (3) identify physiological patterns, (4) grade severity, and (5) answer the clinical question. 1, 2

Step 1: Assess Test Quality First

• Review technical quality before relying on any numerical results—this is the most common mistake in PFT interpretation. 1, 2
• Verify that spirometry meets acceptability and reproducibility criteria per ATS/ERS standards. 1
• Check that the patient performed maximal inspiratory and expiratory efforts; submaximal efforts commonly cause a pattern of reduced FEV1 and FVC with normal FEV1/FVC ratio that mimics restriction but is actually poor technique. 1
• Examine the flow-volume curve morphology for quality issues and characteristic patterns (concave shape suggests obstruction, convex suggests restriction). 1

Step 2: Compare to Reference Values

• Use predicted values based on age, sex, and height from validated reference equations; for pediatrics, use Global Lung Initiative (GLI) equations. 1
• Do not use race-specific normative equations per recent ATS guidance. 1
• Define abnormal as values below the 5th percentile (lower limit of normal) of the predicted value, not arbitrary cutoffs like 80% predicted. 1
• Measure height and weight at each visit, as these affect predicted values. 1

Step 3: Identify Physiological Pattern

Obstructive Pattern

• Obstruction is defined by FEV1/FVC ratio below the 5th percentile of predicted. 1
• Look for concave shape on the flow-volume curve indicating slowed terminal expiratory flow. 1
• Lung volume measurement is not mandatory to diagnose obstruction but helps identify hyperinflation (increased TLC, RV, or RV/TLC ratio). 1
• Consider bronchodilator testing if baseline shows obstruction to assess reversibility. 1

Restrictive Pattern

• Restriction requires TLC below the 5th percentile AND normal FEV1/FVC ratio. 1, 2
• Reduced VC alone does NOT prove restriction—it is associated with low TLC only about 50% of the time. 1
• Flow-volume curve shows convex pattern with FEV1/FVC typically >85-90%. 1
• Never use single-breath VA from DLCO testing to diagnose restriction, as it systematically underestimates TLC, especially in obstruction (by up to 3 liters in severe cases). 1

Mixed Pattern

• Defined by both FEV1/FVC AND TLC below the 5th percentile. 1
• Cannot be diagnosed without measuring TLC—reduced VC with low FEV1/FVC may be obstruction with hyperinflation, not mixed disease. 1

Nonspecific Pattern

• Reduced FEV1 or FVC with normal FEV1/FVC ratio and normal TLC. 2, 3
• Occurs in approximately 15% of patients referred for PFTs and usually has benign course. 3

Step 4: Grade Severity

• Base severity grading on FEV1 % predicted for obstructive, restrictive, and mixed defects: 2

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

• For DLCO, values <60% predicted indicate higher mortality (25%) and pulmonary morbidity (40%), particularly important for preoperative lung resection risk assessment. 2

• Do not use FEV1/FVC ratio to determine severity—this is a common error. 2

Step 5: Answer the Clinical Question

• Integrate PFT patterns with clinical history, symptoms (cough, phlegm, wheezing, dyspnea), smoking status, chest radiograph, hemoglobin, and suspected diagnoses. 1
• In COPD, FEV1 correlates with symptom severity and prognosis. 2
• In interstitial disease, VC may be only slightly impaired despite marked DLCO reduction—measure both systematically. 2
• Assess for bronchodilator response if performed (typically ≥12% and ≥200 mL improvement in FEV1 or FVC). 1
• Compare to prior PFTs if available to identify significant changes over time. 2

Critical Pitfalls to Avoid

• Never rely solely on computer interpretations without reviewing test quality and flow-volume curves. 1, 2
• Do not diagnose restriction without measuring TLC by body plethysmography. 1, 2
• Adjust DLCO for hemoglobin and carboxyhemoglobin, especially when monitoring for drug toxicity. 2
• Recognize that upper airway obstruction may be life-threatening despite being classified as only "mild" by FEV1 % predicted. 2
• When FEV1 and FVC are both reduced with normal FEV1/FVC, first suspect submaximal effort or patchy small airway collapse rather than true restriction. 1
• In severe obstruction, single-breath measurements (VA) can underestimate TLC by up to 3 liters, causing misclassification. 1