Summary of criteria for diagnosing obstructive lung disease

Expiratory airway obstruction	FEV1%(F)VC (corrected for age and sex)
No obstruction *	above 5th percentile, and no bronchodilator response
Mild obstruction *	below 5th percentile and FEV1%pred >60% , or above 5th percentile and bronchodilator response
Moderate obstruction *	below 5th percentile and FEV1%pred 40-59%
Severe obstruction *	below 5th percentile and FEV1%pred <40%

Bronchodilator response	Criteria for grading of response
No response	Increase in FEV1 < 200 mL and < 9% initial FEV1
Mild response	Increase in FEV1 > 200 mL and/or > 9% initial FEV1
Marked response	Increase in FEV1 > 12% initial FEV1 and > 200 mL
Possibly response	According to FEV1 no bronchodilator responsiveness, but increase in (F)VC > 340 mL and/or > 12% initial value and fall in FEV1%(F)VC < 2%: the increase in (F)VC might reflect better subject cooperation

VC too low	Criterion
Moderate	below 5th percentile but z-score > -3
Severe	z-score < -3.0

* There is no uniformity in recommendations. The criteria shown comply with the BTS 1997 recommendation, which has since been superseded.