How affective is the methacholine test for asthma?

See below: It is difficult to calculate the exact sensitivity of a methacholine challenge for the diagnosis of asthma, as a gold standard for diagnosing asthma does not exist. The sensitivity is felt to approach 100 percent, or if there is asthma, it will almost always be detected. There may be false positives, though, when the test is positive but you don't really have asthma.

See below: Methacholine challenge test is an effective way to determine if someone has asthma. The dose of methacholine that is administered that triggers bronchoconstriction and a 20% decrease in lung function helps to determine the likelihood that a person has asthma. If asthma is present, the test will almost always reveal the excessive bronchoconstriction. If not triggered, unlikely to has asthma.