A 35-year-old woman with a history of allergic rhinitis and recurrent episodes of wheezing presents with intermittent dyspnea and chest tightness. Initial spirometry shows FEV₁/FVC ratio of 0.68 with FEV₁ at 78% predicted. The patient denies current respiratory symptoms. To differentiate asthma from COPD and assess the reversibility of airflow obstruction, which is the most appropriate next investigation?