For a study, researchers sought to highlight new evidence of lung function approaches that could be used to identify asthma in preschool children. Spirometry (from age of 5), impulse oscillometry (>3 years), whole-body plethysmography (>3 years), fractional exhaled nitric oxide (FeNO) (>5 years), multiple breath washout (>3 years), structured light plethysmography (>1–2 years), and impedance pneumography (>1 year) were some of the techniques available to measure lung function and airway inflammation in preschool children. Spirometry (cut-off 80% predicted or below the lower limit of normal [LLN] defined as z-score 1.64) was beneficial for identifying preschool asthma (cut-off 80% predicted or below the lower limit of normal [LLN] defined as z-score 1.64) if applicable. Whole-body plethysmography (sRaw>1.6 kPa/s) and impulse oscillometry (Rrs and Xrs at 5 Hz z-score>2) may be effective for patients who were unable to conduct spirometry. Including a bronchodilator reversibility test (FEV1 rise>12%, sRaw drop>25–30%, Rrs at 5 Hz decrease>40%) or a bronchial challenge test, such as an exercise test (FEV1 decrease>10%), might improve the sensitivity of these tests. Elevated FeNO (>25–35 ppb) levels were a potential supplementary test for detecting preschool asthma.

Lung function tests might be performed with excellent reliability in children aged 2 to 4 years old when performed by competent individuals. Objective evaluation of lung function in preschool children was clinically crucial for avoiding over- and under-treatment of asthma.