In the setting of low to-moderate clinical suspicion of asthma plus no demonstrable obstruction on spirometry, Dr. Happel recommends moving on to bronchial hyperreactivity testing via a methacholine challenge.
“The beauty of a methacholine challenge is that is has a high negative predictive value to help exclude the diagnosis of asthma, particularly in people who are having symptoms suggestive of asthma,” Dr. Happel explained. “If it takes more than 8 mg/mL of methacholine to elicit a 20% or greater decrease in FEV1, it’s probably something else. It’s not a particularly specific test, though; you can get false positives from a methacholine challenge.”