Check FEV1/FVC first
Low FEV1/FVC: obstructive pattern
Normal or high FEV1/FVC with low TLC: restrictive pattern
Low FEV1/FVC with low TLC: mixed pattern
Normal spirometry with symptoms: consider small airway disease, asthma, or early restriction
Assess FEV1 severity in obstruction
Mild obstruction: FEV1 ≥ 70% predicted
Moderate obstruction: FEV1 60% to 69% predicted
Moderately severe obstruction: FEV1 50% to 59% predicted
Severe obstruction: FEV1 35% to 49% predicted
Very severe obstruction: FEV1 < 35% predicted
Check bronchodilator response
Significant reversibility: increase in FEV1 or FVC by at least 12% and at least 200 mL
Low FVC with normal FEV1/FVC: confirm restriction with TLC
Low TLC: true restriction
Normal TLC with low FVC: pseudo-restriction or air trapping
Check RV and RV/TLC
High RV or RV/TLC: air trapping
Check DLCO
Low DLCO with obstruction: emphysema, pulmonary vascular disease, or combined disease
Normal DLCO with obstruction: asthma or chronic bronchitis
Low DLCO with restriction: interstitial lung disease or other parenchymal disease
Normal DLCO with restriction: extrapulmonary restriction
High DLCO: asthma, obesity, polycythemia, or alveolar hemorrhage
Use flow-volume loop shape
Scooped expiratory limb: obstruction
Flattened inspiratory limb: variable extrathoracic obstruction
Flattened expiratory limb: variable intrathoracic obstruction
Flattened both limbs: fixed upper airway obstruction
Compare results with predicted values
Interpret in clinical context
Confirm test quality before concluding
Consider hemoglobin correction for DLCO
Consider lung volumes when spirometry is inconclusive