Types of ventilatory defects and their diagnoses
| Abnormality | Diagnosis |
| Obstruction | FEV1/VC <5th percentile of predicted |
| A decrease in flow at low lung volume is not specific for small airway disease in individual patients | |
| A concomitant decrease in FEV1 and VC is most commonly caused by poor effort, but may rarely reflect airflow obstruction. Confirmation of airway obstruction requires measurement of lung volumes | |
| Measurement of absolute lung volumes may assist in the diagnosis of emphysema, bronchial asthma and chronic bronchitis. It may also be useful in assessing lung hyperinflation | |
| Measurements of airflow resistance may be helpful in patients who are unable to perform spirometric manoeuvres | |
| Restriction | TLC <5th percentile of predicted |
| A reduced VC does not prove a restrictive pulmonary defect. It may be suggestive of lung restriction when FEV1/VC is normal or increased | |
| A low TLC from a single-breath test should not be seen as evidence of restriction | |
| Mixed defect | FEV1/VC and TLC <5th percentile of predicted |
FEV1: forced expiratory volume in one second; VC: vital capacity; TLC: total lung capacity.