How do I read and interpret pulmonary function tests?

How to Read and Interpret Pulmonary Function Tests

Begin by assessing test quality before looking at any numbers, then compare results to reference values using the 5th percentile as the lower limit of normal, identify the physiological pattern (obstruction, restriction, or mixed), and finally answer the specific clinical question. 1, 2

Step 1: Quality Assessment (Most Critical First Step)

Never rely on computer-generated interpretations or numerical results without first reviewing the raw data and technical quality. 1, 2

For Spirometry:

• Examine the flow-volume curve visually for rounded peaks (indicating submaximal effort), abrupt terminations, or coughing artifacts 1
• Verify that at least 3 acceptable maneuvers were performed with reproducibility within 150 mL for both FEV1 and FVC 1
• Check that expiration lasted at least 6 seconds in adults 1
• Document any quality issues in your interpretation because suboptimal tests can still provide useful information but you must state the direction and magnitude of potential errors 1, 2

For DLCO:

Use the grading system: Grade A requires inspired volume >90% of VC, breathhold time 8-12 seconds, and sample collection <4 seconds 1

For Lung Volumes:

Verify adherence to 2005 ATS/ERS acceptability and repeatability criteria 1

Step 2: Compare to Appropriate Reference Values

Measure height with a stadiometer at the time of testing—never use self-reported height. 1

Key Reference Value Principles:

• Use the 5th percentile of the reference population as the lower limit of normal for all parameters—never use fixed cutoffs like "80% predicted" in adults 1, 2
• **Never use FEV1/FVC <0.70 as a fixed threshold** because it causes false-positive COPD diagnoses in men >40 years and women >50 years, especially in elderly never-smokers 1, 2
• Match reference equations to the patient's age, sex, measured height, and ethnicity 1
• All spirometric parameters (FVC, FEV1, FEV1/FVC) must come from the same reference source to maintain internal consistency 1

Race/Ethnicity Adjustments (when specific equations unavailable):

• Black patients: multiply by 0.88 for FEV1, FVC, and TLC 1
• Asian Americans: multiply by 0.94 1
• Never apply these adjustment factors to FEV1/FVC or FEV1/VC ratios 1

Step 3: Identify the Physiological Pattern

Obstruction:

FEV1/FVC (or FEV1/VC) below the 5th percentile defines obstruction, even when absolute FEV1 is normal 2

• Severity is graded by FEV1 % predicted (not by the ratio): 2

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

• Measure TLC to assess hyperinflation; elevated TLC, RV, or RV/TLC supports emphysema or asthma 2

• If FEV1/FVC is borderline, examine FEF25-75% or maximal expiratory flows for early small airway disease, particularly in children with cystic fibrosis 2

Restriction:

TLC below the 5th percentile with a normal FEV1/VC ratio confirms true restriction—reduced VC alone is insufficient because only half of low-VC cases have low TLC 2

• Suspect restriction when VC is reduced, FEV1/VC ratio is increased (>85-90%), and the flow-volume loop shows a convex shape 2
• Critical pitfall: Single-breath alveolar volume (VA) from DLCO testing underestimates TLC by up to 3 L in severe obstruction, causing false restriction diagnoses 2
• In pneumothorax or non-communicating bullae, body plethysmography gives accurate TLC while gas-dilution falsely reports low TLC 2

Mixed Pattern:

Both FEV1/VC ratio and TLC fall below the 5th percentile 2

• If FEV1/VC is low and VC is reduced but TLC was not measured, state that VC reduction likely reflects hyperinflation and that superimposed restriction cannot be excluded without TLC 2

Normal FEV1/FVC with Reduced FEV1 and FVC:

This pattern suggests suboptimal effort, patchy peripheral airway obstruction, or inability to sustain expiration 2

• Repeat spirometry after bronchodilator; a significant increase (≥12% and ≥200 mL) supports reversible airflow obstruction 2

Step 4: Assess DLCO

DLCO <60% predicted indicates higher mortality (25%) and pulmonary morbidity (40%) in patients undergoing lung resection 2

Critical Adjustments:

• Always adjust DLCO for hemoglobin and carboxyhemoglobin, especially when monitoring for drug toxicity 2
• Elevated carboxyhemoglobin (in smokers) artificially lowers DLCO, causing false-negative assessments 2
• Altitude reduces measured DLCO; apply altitude-specific correction factors 2

Clinical Context:

• VC may be only slightly impaired in diffuse interstitial diseases despite marked loss of DLCO 2
• Both FEV1 and DLCO should be measured systematically in preoperative evaluation of lung cancer patients 2

Step 5: Answer the Clinical Question

PFTs are only one tool—interpret results in the full clinical context 2

• In COPD, FEV1 correlates with symptom severity and prognosis 2
• Upper airway obstruction may be life-threatening despite being classified as only "mildly reduced" by FEV1 % predicted 2
• In borderline cases near thresholds, additional testing (repeat PFTs, lung volumes, or diffusion studies) may be necessary 2

Common Pitfalls to Avoid

• Relying solely on computer interpretations without reviewing test quality is the most frequent error 1, 2
• Using FEV1/VC ratio (instead of FEV1 % predicted) to determine severity of obstruction 2
• Failing to measure lung volumes when spirometry suggests restriction 2
• Not adjusting DLCO for hemoglobin and carboxyhemoglobin 2
• Interpreting multiple parameters simultaneously inflates false-positive rates: examining 3 parameters yields 10% abnormality in healthy subjects, rising to 24% with 14 parameters 2
• Changing interpretation strategies within a laboratory creates the illusion of disease progression when it's actually due to analytical approach changes 2
• Patients near thresholds have the greatest risk of misclassification 1