A methacholine challenge is a breathing test that is the gold standard for diagnosing asthma. You will blow into a spirometer before and after each increasing dose of an inhaled medicine. This test is positive for asthma if the result after the inhaled medicine is 20% lower than it was before the inhaled medicine.
When a person has a positive methacholine challenge, it usually means that person has asthma. However, it can be positive in a person who has hay fever, also called allergic rhinitis, where allergies are causing inflammation of the nose. In addition it can be positive in a person who has chronic obstructive pulmonary disease (COPD), also called emphysema. Most of the time this testing indicates asthma, especially when it is strongly positive.
The methacholine challenge measures the reactivity of the airways of the lungs; that is how twitchy are the airways. The airways are more likely to narrow in response to triggers, such as irritants, infection, exercise, heartburn, cold air, or allergies. The tendency for twitchy airways is an inherited trait. However, the degree of twitchiness varies naturally over time. It can also be dependent upon recent exposures. For example, it may be more positive during the spring if the person is allergic to tree pollens and more positive in the fall if allergic to weeds.
It's generally helpful as a guide on how twitchy your airways are. Some folks who have only exercise-induced asthma do NOT react during the methacholine challenge but are still asthmatics. They may require exercise challenge testing.
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