Table 1

Differences between spirometry and FOT/IOS

Parameter	Spirometry	FOT/IOS
Main principle	Flow sensor/volume displacement helps measure flow rates and lung volumes	Forced oscillations of single frequency sound waves (FOT) or impulses of multiple frequency sound waves (IOS) are pushed into the lungs as pressure waves to measure respiratory resistance and reactance
Main parameters	Volumes: FEV₁, FVC	Z_rs, R_rs, X_rs, F_res, A_x
Main parameters	Flows: PEFR, FEF_25–75%	Z_rs, R_rs, X_rs, F_res, A_x
Patient co-operation required	+++	+
Type of breathing manoeuvre	Forced exhalation	Tidal breathing
Variability (intra-subject)	3–5%	5–15%
Sensitivity to airway location
Central	+	+++
Peripheral	++	+++
Cut off for bronchodilator response	12–15% for FEV₁	40% for R₅ or X₅
Cut off for bronchoconstrictor response	20% for FEV₁	50% for R₅
Insight into lung mechanics	+	+++
Standardised methodology	+++	++
Availability of robust reference values	+++	+

FEV₁: forced expiratory volume in 1 s; FVC: forced vital capacity; PEFR: peak expiratory flow rate; FEF_25–75%: forced expiratory flow at 25–75% of FVC; Z_rs: respiratory impedance; R_rs: respiratory resistance; X_rs: respiratory reactance; F_res: resonant frequency; A_x: reactance area; R₅: respiratory resistance at 5 Hz; X₅: respiratory reactance at 5 Hz.