- A 20% drop in FEV1 (PC20) to methacholine <8 mg/mL or a 15% fall in FEV1 (PD15) after inhaling a cumulative dose of mannitol of ≤635 mg indicating the presence of airway hyperresponsiveness. If sites customarily use histamine to perform tests of airway responsiveness, this may be used in place of methacholine.