Methacholine challenge testing is used clinically in adults, often when spirometry or reversibility to bronchodilator has not been able to establish a diagnosis of asthma.
Since the negative predictive power of methacholine testing is large, the test can be more helpful to exclude a diagnosis of asthma.
Contraindications may compromise the quality of the overall inhalation challenge and output measurements, as well as the safety of the patient. Substantial falls in FEVj may occur during a methacholine inhalation challenge, and those individuals with low-baseline lung function are at risk (Martin et al. 1997). In general, the predicted accepted baseline FEV1 of <65% should be used as a cutoff for conducting a methacholine inhalation challenge. In addition, low-baseline spirometry measurements are indicative of airway obstruction, thus making it difficult to interpret positive methacholine inhalation challenge results. This is due to a strong correlation between airway hyperresponsiveness and the degree of baseline airway obstruction that is a characteristic of chronic obstructive pulmonary disease (COPD). However, if baseline spirometry indicates airway obstruction that can be significantly reversed following use of a bronchodilator (>12% and >0.2 L increases in FEV1 or FVC), then the diagnoses of asthma can be confirmed, thereby eliminating the need to do a methacholine inhalation challenge. Additional contraindications include cardiovascular problems. Cardiovascular events can be exacerbated in patients with uncontrolled hypertension or recent heart attack or stroke, following the added cardiovascular stress brought on by bronchospasms during an inhalation challenge. In particular, bronchospasms can induce ventilation-perfusion mismatching, which may in turn lead to arterial hypoxemia and compensatory changes in blood pressure, cardiac output, and heart rate (Stewart et al. 1989; Rodriguez-Roisin et al. 1991; Roca and Rodriguez-Roisin 1992; Goldman et al. 1995).