A methacholine challenge or bronchial challenge test is a test for asthma and other bronchial diseases. The patient is given methacholine or histamines to inhale in a fine powdered form. The drugs encourage the bronchial passages to constrict. The patient is then put through a series of tests of their lung capacity. Eventually, the patient will have a reduction in lung capacity, but in asthmatic patients this will happen at lower doses of the drugs.
The test is not always reliable. Other conditions, such as those that cause coughing, will interfere with the lung capacity tests. Coughing can also be caused by the test itself. The patient may also just be reacting to environmental factors, such as smoke, which also temporarily constrict the airways.