Bronchial challenge test

There are some respiratory diseases such as exercise-induced asthma that are not apparent unless the patient is exposed to some sort of trigger, such as a chemical irritant, an allergen, cold or dry air, or rigorous exercise. To diagnose these conditions, a bronchial challenge test is performed. This is to discover which environmental conditions may be triggering the bronchospasm, or sudden contraction of the bronchioles, and subsequent breathing problems. This test also helps determine the extent of the reaction.

As part of this test, the patient's medical history is taken and possible triggers are discussed. Spirometry tests are taken. The patient is exposed to whatever triggers the breathing problem, under controlled conditions. Spirometry tests are again taken, and compared with the earlier results.

In such tests, pharmaceutical agents such as methacholine or histamine may be used.

Sometimes, to assess the reversibility of a particular condition, a bronchodilator is administered before performing another round of tests for comparison. This is commonly referred to as a reversibility test, or a post bronchodilator test (Post BD), and is an important part in diagnosing asthma versus COPD.

A methacholine challenge test is a medical test used to assist in the diagnosis of asthma. The patient breathes in nebulized methacholine. This provokes narrowing of the airways (bronchoconstriction). This is detected when the patient performs spirometry. People with asthma react to lower doses of inhaled methacholine.

However it is possible to have false negatives, and false positives on this test. Asthma can also be temporary, due to an exposure to noxious stimuli (smoke inhalation, etc.). Regardless of the results of a methacholine test, anyone who appears to have asthma clinically, and who responds to asthma treatment, should have asthma treatment. Asthma treatment should not be withheld in such a patient who passed a methacholine challenge.

The test is physically demanding, and the results can be affected by muscular weakness or exhaustion. Methacholine can, sometimes, stimulate the upper airway sufficiently to cause violent coughing. This can make spirometry difficult or impossible.