ABSTRACT
A 65-year-old woman presents with dyspnea following exercise. She has never smoked. Physical examination is normal. Spirometry shows a normal FVC and FEVb with a normal FEVi/FVC ratio and no response to bronchodilator. Because of the concern that the patient may have asthma, a methacholine challenge test is obtained (Fig. 13A). In te rp re ta tio n As increasing concentrations of methacholine are administered by inhalation, the patient has a gradual decline in FEVb eventually reaching and surpassing the 20% threshold for a positive response at a dose less than 8 mg/mL. In this case, the provocative concentration causing a 20% fall in FEVi (the PC20 FEVi) was 2.2 mg/mL, indicative of significant airways hyperresponsiveness. In the proper clinical context, this result supports the diagno sis of asthma.
