Background

 Impulse oscillometry (IOS) allows an effort-independent evaluation of small airway function in asthma. Unfortunately, well-determined minimal clinically important differences (MCID) for IOS-measures are lacking. Here, we provide MCIDs for frequently used IOS-measures, namely frequency dependence of resistance (FDR) and area of reactance (AX) in patients with asthma.

Methods 

We performed IOS at baseline and 1?year later in adult patients with mild to severe asthma (n=235). In a two-step approach, we first applied a distribution-based method to statistically determine the MCID. Next, we validated the proposed MCID according to patient-reported outcome measures (PROMs) of Asthma Quality of Life Questionnaire (AQLQ), Asthma Control Questionnaire (ACQ) and Asthma Control Test (ACT). We used multivariable analyses to investigate the proposed MCIDs as predictors for improvements in PROMs in comparison to the established MCID of FEV1.

Results 

The proposed MCID was a decline of?0.06?kPa·L?1·s?1 and?0.65?kPa·L?1 for FDR and AX, respectively. Patients who had changes beyond the MCID for both FDR and AX showed greater improvements in all PROMs than those who had not. The mean improvements in PROMs were beyond the established MCID for ACQ and AQLQ and approximated the MCID for ACT score. Multivariable analyses demonstrated the MCID for both FDR and AX as independent predictors for the MCID of all PROMs. The MCID for FDR was a stronger predictor of all PROMs than the MCID for FEV1.

Conclusion

 This study provides MCIDs for IOS-derived measures in adult patients with asthma and emphasizes that small airway function is a distinguished endpoint beyond the conventional measure of FEV1.