A 38-year-old man with known asthma presents with acute dyspnea and wheezing despite regular inhaled corticosteroid use. He has been using his salbutamol inhaler 4–5 times daily for the past 2 weeks. On examination, peak expiratory flow (PEF) is 65% of predicted. To assess the reversibility of airway obstruction and guide bronchodilator therapy, which investigation is most appropriate?