Abnormality on pulmonary function testing Abnormal methacholine challenge test 2025-07-31T12:35:34Z Methacholine, a derivative of acetylcholine, directly induces bronchoconstriction by stimulating airway smooth muscle receptors. In some cases, hyperresponsiveness may be the only objective sign of asthma, making this test a valuable diagnostic tool. The procedure involves stepwise administration of methacholine, followed by spirometry to measure the forced expiratory volume in 1 second. This process continues until either a 20% reduction in the forced expiratory volume in 1 second or a 35-40% reduction in specific airways conductance is observed. The dose required to cause a 20% reduction is known as the provocation dose (PD20). A PD20 of 200 micrograms, or a provocation concentration (PC20) of 8 mg/mL or less, indicates a positive result (100 microgram or 4 mg/mL for specific airway conductance). An abnormal result of the methacholine challenge test, which is a bronchoprovocation technique used to evaluate airway hyperresponsiveness, particularly in patients with suspected asthma.