A methacholine challenge test is a diagnostic test used in the evaluation of suspected asthma when reversibility is not demonstrated on initial spirometry. The methacholine challenge is also used for research purposes to study airway hyperreactivity. It is one type of a class of specialized tests called bronchoprovocation tests. Cold-air exercise tests are another example of a bronchoprovocation test.
A bronchoprovocation test might be ordered in the evaluation of suspected asthma. It is not considered a routine test. Consider a scenario where a patient describes subtle symptoms suggestive of asthma. Spirometry (before and after bronchodilator) and other pulmonary function testing are entirely normal. History, physical examination, blood tests, and X-rays fail to reveal an alternative diagnosis or medical explanation for the reported symptoms.
The bronchoprovocation test is then indicated to further evaluate for the possibility of asthma. It is an extremely powerful test for eliminating or “ruling out” asthma. In other words, if the test is negative, then asthma is not present. However, if the test is positive and if the symptoms correspond, then the patient likely has asthma. People with asthma demonstrate an increased sensitivity to the inhalation of methacholine and there-fore obtain a “positive” test result. The converse statement is not true: Although everyone with asthma has a positive result on methacholine testing, not everyone demonstrating a positive result on methacholine testing will have asthma. The actual methacholine challenge test is usually per-formed in a hospital pulmonary function lab.
The test requires obtaining a baseline spirometry measurement and then repeating spirometry after inhalation of higher and higher concentrations of methacholine. The baseline spirometry values should be normal, which is why the challenge test is indicated. If the spirometry measurements remain close to baseline values after inhalation of increasing doses of methacholine, the test is said to be “negative,” and asthma is effectively ruled out.
If, on the other hand, the spirometry values decline significantly after the methacholine inhalation, or if wheezing or other symptoms develop, the test is reported as “positive.” The testing immediately stops at that point, and an inhaled, short-acting bronchodilator is promptly administered to relieve symptoms and to reverse the abnormal lung function.