Interpreting Lung Function Tests

Introduction to Lung Function Interpretation

The European Respiratory Society recommends a systematic approach to interpreting lung function tests, evaluating both FEV1 and FVC values against predicted norms and calculating the FEV1/FVC ratio to distinguish obstructive from restrictive patterns, integrating these findings with clinical context 1

The European Respiratory Society suggests calculating percent predicted for both FEV1 and FVC by comparing the patient's actual measurements to reference values matched for age, height, sex, and ethnicity 2
The lower limit of normal (LLN), defined as the 5th percentile of the normal distribution, should be used rather than arbitrary fixed percentages when possible 3, 4

An obstructive pattern is confirmed by an FEV1/FVC ratio below the LLN, indicating airflow obstruction 3, 4
The FEV1/FVC ratio should not be used to grade severity of obstruction, but rather to identify its presence 1

The European Respiratory Society classification grades severity using FEV1 percent predicted: mild (FEV1 ≥70% predicted), moderate (FEV1 60-69% predicted), moderately severe (FEV1 50-59% predicted), severe (FEV1 35-49% predicted), and very severe (FEV1 <35% predicted) 1, 3, 5, 6
The GOLD classification also grades severity using FEV1 percent predicted, but with different thresholds: mild (FEV1 ≥80% predicted), moderate (FEV1 50-79% predicted), severe (FEV1 30-49% predicted), and very severe (FEV1 <30% predicted) 3, 5, 6

Bronchodilator testing can suggest reversibility, with changes >12% and >200 mL in FEV1 indicating a positive response 7, 5
Inspiratory capacity (IC) and diffusing capacity (DLCO) are important predictors of mortality and should be considered in addition to FEV1 3, 1

Short-term changes (>5% in FEV1 or FVC) and week-to-week changes (>12% in FEV1 or >11% in FVC) can indicate significant changes in lung function 7
Year-to-year changes (>15% in FEV1) can indicate clinically meaningful progression 7

Never diagnose obstruction based on FEV1 alone, but rather evaluate the FEV1/FVC ratio first 1, 5
Never confirm restrictive disease without measuring TLC, as spirometry has poor positive predictive value for restriction 1