A bronchial challenge test is used to diagnose asthma by inducing airway constriction with drugs like methacholine or histamine, and measuring the degree of constriction with spirometry. False positives are possible, and the test can be physically taxing, so other diagnostic methods are often used. Bronchodilators can be used to confirm the diagnosis and test the efficacy of future treatments.
A bronchial challenge test is a diagnostic method used to test for asthma in patients who experience occasional breathing difficulties. In a bronchial provocation test, the patient inhales a certain amount of a drug, such as methacholine or histamine, which causes the airways to narrow. Asthma is usually characterized by hypersensitivity of the airways, so an individual with asthma generally responds to a lower dose of the drug being tested. Spirometry, a clinical test that can measure the rate and volume of breathing, is used to evaluate the degree of airway constriction. The test can be quite challenging and even painful for some patients, so other diagnostic methods are often used.
The drugs used in a bronchial challenge test are intended to target various receptors that cause the airways to constrict. Histamine, for example, targets the histamine H1 receptor. When this receptor is exposed to histamine, it triggers a series of signals that lead to airway constriction. Similarly, methacholine initiates a signal pathway that begins with the M3 receptor, also leading to airway constriction. A low dose of histamine or methacholine can often induce airway constriction in patients via these pathways, making these tests useful diagnostic tools.
In some cases, asthma can be diagnosed by examining the symptoms or attempting treatment without using a bronchial challenge test. This is often preferable, because bronchial provocation testing can sometimes return false positives and can be physically taxing. The induced constriction of the airways can lead to a violent and painful cough which, in addition to being unpleasant for the patient, makes spirometry difficult. The test can also be quite dangerous at worst and grossly unpleasant at best for patients who already have problems with airway constriction or obstruction. False diagnoses of asthma are possible when asthma symptoms are caused by exposure to harmful environmental factors or by strenuous exercise.
To further test and clarify the diagnosis made with a bronchial challenge test, drugs known as bronchodilators may be administered. Such substances are used to reverse the effects of substances that lead to airway constriction. Their efficacy can be used to confirm that the constriction is caused by the suspect signaling pathways. Additionally, the administration of substances used to reverse the constriction can be used to test the ability of those substances to cure the patient in the future. Inhalers, for example, contain bronchodilators that are meant to be used by asthma patients during asthma attacks.
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