What finding would support a diagnosis of asthma in an 8-year-old girl?

The increase in FEV1 by 15% following the use of a short-acting beta2-agonist is a significant indicator supporting a diagnosis of asthma. This response demonstrates the reversible nature of airway obstruction typically seen in asthma. In asthmatic patients, bronchial passages respond positively to bronchodilator medications by relaxing the airway muscles and enhancing airflow.

A 15% increase in FEV1 indicates that the airways were constricted prior to medication administration, which is characteristic of asthma, where bronchoconstriction and airway inflammation contribute to symptoms. This reversible obstruction is one of the hallmark features of asthma, distinguishing it from other chronic respiratory conditions.

The other findings such as an FEV1 of 92% predicted, an FEV1:FVC ratio of 85%, and a PEFR of 90% may suggest normal lung function or airflow but do not demonstrate the hallmark reversible airway obstruction that is seen in asthma. Therefore, the significant increase in FEV1 post-bronchodilator usage is the most definitive finding supporting an asthma diagnosis in this scenario.