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A bronchial challenge test is a diagnostic method used to test for asthma in patients experiencing occasional breathing difficulties. In a bronchial challenge test, the patient inhales some quantity of a drug, such as methacholine or histamine, that causes narrowing of the airways. Asthma is usually characterized by hypersensitivity of the airways, so an individual with asthma generally responds to a lower dose of the drug used for the test. Spirometry, a clinical test that can measure the speed and volume of breathing, is used to judge the degree of airway constriction. The test can be quite demanding and even painful for some patients, so other diagnostic methods are often used.
Drugs used in a bronchial challenge test are intended to target various receptors that cause airway constriction. Histamine, for instance, targets the H1 histamine receptor. When this receptor is exposed to histamine, it sets off a series of signals that lead to constriction of the airways. Similarly, methacholine initiates a signal pathway starting with the M3 receptor, also leading to airway constriction. A low dose of histamine or methacholine can often induce airway constriction in patients through these pathways, making these tests useful diagnostic tools.
In some cases, it is possible to diagnose asthma by examining symptoms or attempting treatment without resorting to the bronchial challenge test. This is often preferable, as the bronchial challenge test can sometimes return false positives and may be physically strenuous. Induced airway constriction can lead to violent and painful coughing which, in addition to being unpleasant for the patient, makes spirometry difficult. The test can even be somewhat dangerous at worst and severely 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 noxious environmental factors or by particularly strenuous exercise.
To test and greater 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 the substances that lead to airway constriction. Their effectiveness can be used to confirm that the constriction is caused by the suspected signaling pathways. Furthermore, the administration of substances used to reverse constriction can be used to test the ability of such substances to treat the patient in the future. Inhalers, for instance, contain bronchodilators to be used by asthma patients during asthma attacks.