Aim:
To develop the criteria of small airways response to bronchodilators (by spirometry indices maximal expiratory flow (MEF50 and MEF25) as the markers of uncontrolled asthma course.

Material and methods:
The study involved 92 participants (64 boys and 28 girls) aged 6 to 17 years (60 were less than 12 years old) with diagnosed asthma. Asthma control was assessed with the use of Asthma Control Test and Asthma Control Questionnaire. Spirometry and bronchodilator responsiveness testing were performed for all participants.

Results:
Mostly, the studied children had a normal level of forced expiratory volume in the first second (FEV1), even at unsatisfactory symptoms control. The indicators of the medium and small airways patency were significantly worse in uncontrolled asthma children even in normal FEV1. Among children, the lack of asthma control can be caused by small airways obstruction in up to 80% cases. Among children who need the high dose inhaled corticosteroids treatment 93.3% have uncontrolled asthma with small airways obstruction. We found out that MEF50 and MEF25 could be the signs of the reversibility of bronchial obstruction and uncontrolled asthma with high sensitivity and specificity.

Conclusions:
Indices MEF50 and MEF25 allow detecting the small airways obstruction and their reversibility as a mark of uncontrolled asthma (MEF25 has a higher diagnostic value). In case of MEF50 and/or MEF25 increasing for 22% or 25% accordingly in bronchodilator test in children, the asthma should be considered uncontrolled.