Background The forced oscillation technique (FOT) is regarded as useful to assess airway patency and response to bronchodilators but its agreement with spirometry in outpatients is still debatable.

Aims and objectives To assess the relationship between FOT and spirometry measurements in outpatients reporting recurrent respiratory symptoms.

Methods Seventy-five outpatients, aged 5-18 yrs (40 males) did FOT measurements at 8 Hz of inspiratory (ins) and expiratory (ex) resistance (Rrs) and reactance (Xrs), and spirometry, both at baseline and after inhaled salbutamol 200 mcg. Baseline FOT values and bronchodilator changes were expressed as z-scores (z-s). Cut-off values for airway patency were FEV₁<80% and FEF_25-75<60%; bronchodilator response (D) was set as DFEV₁>12% from baseline. Specificity was sought >90% to reduce false positive results. 

Results Inspiratory FOT variables correlated better with spirometry than those expiratory (Rrs_ins with FEV₁: r=-0.51, with FEF_25-75: r=-0.62, p<0.001). At specificity >90%, z-s values of Rrs_ins >2.46 predicted FEV₁<80% with a sensitivity of 53.8% and FEF_25-75<60% with a sensitivity of 66.7%. Post-salbutamol changes in impedance correlated better with DFEV₁ than DFEF_25-75 (DRrs_ins and DXrs_ins with DFEV₁: r=0.48 and r=0.50, p<0.001). At specificity>90%, a fall in DRrs_ins >-2.43 z-score (>-38.0%) predicted DFEV₁>12% with a sensitivity of 36.8%, while increases in DXrs_ins >1.65 z-score (>64.0%) predicted DFEV₁>12% with a sensitivity of 57.9%.

Conclusions FOT and spirometry correlate fairly; wide FOT z-scores are needed to reduce false positive results on airway patency and response to bronchodilators.