Asthma - Bronchial Challenge Testing
A challenge test may be used by the physician to demonstrate that a patient with a normal result on pulmonary function testing may indeed have asthma. This bronchial challenge or provocation testing would only be performed if the patient’s history and physical findings suggested that the patient is asthmatic but spirometry was normal. It is not a routine part of pulmonary function testing. The substances or agents commonly used for challenge testing include histamine, methacholine, and cold air.
Histamine is stored in allergy cells such as mast cells and is released during allergic or asthmatic attacks. It is thought to be one of the mediators of asthma. For this reason it is very suitable for provoking asthma in challenge testing. Methacholine is a chemical that stimulates one part of the nervous system called the parasympathetic nervous system to fire. If inhaled into the bronchial tubes in an asthmatic subject, methacholine will trigger impulses that produce airway constriction. Cold air irritates the bronchial tubes and may also be used for challenge testing. In an asthmatic subject with hyperreactive airways, inhaling cold air will produce significant tightening of the bronchial tubes.
In the patient thought to have occupational asthma the specific offending substance may be used to confirm the direct link between the substance and the patient’s asthmatic reaction. A similar challenge test has been used in patients to confirm allergy to sulfites and aspirin. With any challenge test there is a risk of a severe asthmatic reaction and for this reason these tests are reserved for difficult diagnostic situations and are only performed under careful observation and control.
Guidelines have been developed for performing and interpreting bronchial challenge testing. It is vital to standardize this type of testing to avoid “false positive” or “false negative” results. Generally, for a provocation test to be positive, there must be at least a 15 percent fall in airflow after inhaling the challenge material.
Tagged under:allergy Asthma asthmatic attacks cold air histamine mast cells methacholine occupational asthma parasympathetic nervous system spirometry