Interpreting Pulmonary Function Test Graphs

Introduction to Pulmonary Function Test Interpretation

The European Respiratory Society recommends interpreting PFT graphs using a systematic, step-wise algorithm: first assess test quality and technical adequacy, then analyze the flow-volume loop morphology to identify patterns, compare numerical values against reference standards matched for age, height, sex, and ethnicity, classify the physiologic defect, grade severity, and finally integrate findings with clinical context 1, 2

Never rely on numerical results without first reviewing the flow-volume loop (FVL) graph for technical adequacy 2
Verify proper technique was used and results meet acceptability and reproducibility criteria before making clinical decisions 2
Poor effort or technique will produce unreliable graphs that cannot be interpreted accurately 1

Calculate percent predicted for FEV1 and FVC by comparing actual measurements to reference values matched for age, height, sex, and ethnicity 1, 2
Use the Lower Limit of Normal (LLN), defined as the 5th percentile, rather than arbitrary fixed percentages like 80% 3, 1, 4
All spirometric parameters (FVC, FEV1, FEV1/FVC) must come from the same reference source to ensure consistency 3, 4

First, evaluate FEV1/FVC ratio (not FEV1 alone) 1, 2
If FEV1/FVC < LLN → Obstructive pattern 1, 2
If FEV1/FVC ≥ LLN → Proceed to next step 1
If non-obstructive, assess FVC and TLC: If FVC reduced but TLC normal → Not true restriction (common pitfall) 2, 4
If TLC < 80% predicted → Restrictive pattern 1, 2
Never confirm restriction without measuring TLC, as spirometry alone has poor positive predictive value 1, 2, 4

Once the pattern is identified, grade severity based on FEV1 % predicted (European Respiratory Society classification): Mild: FEV1 ≥70% predicted, Moderate: FEV1 60-69% predicted, Moderately severe: FEV1 50-59% predicted, Severe: FEV1 35-49% predicted, Very severe: FEV1 <35% predicted 1, 2
Never use the FEV1/FVC ratio to grade severity—it only identifies the presence of obstruction 1

The American College of Chest Physicians, American Thoracic Society, and Canadian Thoracic Society highlight critical issues with race-specific reference equations: Race-specific equations may mask clinically important disease in Black individuals by artificially lowering predicted values 5, 6, 7
Multiple studies demonstrate that race-neutral equations (GLI-Other) better predict mortality, morbidity, and CT findings than race-specific equations 6, 7

Never diagnose obstruction based on reduced FEV1 alone—always check the FEV1/FVC ratio first 1, 2, 4
Never confirm restrictive disease without measuring TLC, as reduced FVC on spirometry has poor positive predictive value for true restriction 1, 2, 4
Never rely solely on computer-generated interpretations without personally reviewing the FVL graph 2
Never use FEV1/FVC ratio to grade severity of obstruction—use FEV1 % predicted instead 1

For COPD patients: FEV1 correlates with symptom severity and prognosis, Inspiratory capacity (IC) and DLCO are important mortality predictors beyond FEV1 alone 2, 1
For asthma patients: Bronchodilator response graph comparison is critical—look for normalization of the FVL shape post-bronchodilator, Serial testing showing variability (>12% FEV1 changes) supports the diagnosis 1