Test Interpretation and Reporting
Choosing the comparison
Modern interpretation uses GLI-2012 reference equations and the lower limit of normal, the fifth percentile or roughly z = -1.645, rather than a fixed FEV1/FVC of 0.70, which overdiagnoses obstruction in older adults and underdiagnoses it in younger ones. Report z-scores alongside percent predicted, because percent predicted varies with age, sex, height, and ethnicity.
Patterns
Obstruction is an FEV1/FVC below the lower limit of normal, with severity graded by the FEV1 z-score (mild at z above -2.5, moderate from -2.5 to -4, severe below -4). A reduced FVC with a preserved or high ratio suggests restriction but does not confirm it; total lung capacity below the lower limit of normal confirms restriction. A mixed pattern shows both a low ratio and a low TLC. DLCO helps separate processes: a low DLCO with normal lung volumes suggests parenchymal or vascular disease, while a normal DLCO with obstruction suggests asthma or chronic bronchitis. Bronchodilator responsiveness requires both a 12 percent and a 200 mL improvement in FEV1 or FVC. Flow-volume loop shapes identify variable intrathoracic and extrathoracic obstruction, fixed obstruction, and small-airway collapse.
Traps
Common traps are calling restriction from spirometry alone, using the fixed ratio, accepting a bronchodilator response that meets only the percentage criterion, and interpreting results without confirming that quality control criteria were satisfied.
Worked example
A 70-year-old has an FEV1/FVC of 0.66 with an FEV1 z-score of -1.9. The ratio is below the lower limit of normal, so obstruction is present, but the FEV1 z-score is above -2.5, so severity is mild.