and distilled water challenges means that each of these agents is widely used. Increased reactivity has been described in non-asthmatics, particularly in first degree relatives of asthmatics, cigarette smokers, people with allergic rhinitis, and some apparently normal individuals. In general, asthmatics are the most reactive (i.e., require the smallest concentration of agonists to effect a reduction in lung function). The risk of bronchial reactivity increases with increasing skin test reactivity (Burrows and Lebowitz, 1992; Lofdahl and Svedmyr, 1991). In asthmatics, the degree of reactivity correlates with other measures of disease severity. People with allergic rhinitis demonstrate an intermediate level of reactivity. A high proportion of cigarette smokers with airflow obstruction demonstrate increased airways reactivity, but the population distribution of airways reactivity among smokers without airflow obstruction is unknown at present.
Data from Pattemore and colleagues (1990) show a 24 percent prevalence of frequent wheezing in children with normal airway responsiveness and a lack of current asthma symptoms in 41 percent of people with hyperresponsiveness. This has led some to criticize the utility of measurement of methacholine or histamine responsiveness in clinical practice. Cockroft and Horgreave (1990) disagree, and point out that when spirometry is normal, methacholine and histamine hyperresponsiveness is a sensitive measure of abnormal airway function that correlates closely with the presence and degree of variable airway obstruction.
Cockroft and Horgreave (1990) specifically identify three areas of clinical utility for histamine and methacholine inhalation tests:
Exclusion or confirmation of a diagnosis of asthma, especially if the presentation is atypical.
Diagnosis and follow-up of occupational asthma.
Assessment of the severity of asthma and monitoring of asthma treatment.
Allergen-specific bronchoprovocation testing is a research tool used to diagnose specific immunologic diseases, identify new etiologic agents, and study the pathogenesis of asthma and hypersensitivity pneumonitis (Chan-Yeung and Lam, 1986; Pepys and Hutchcroft, 1975). Guidelines for allergen challenge have been proposed by the American Academy of Allergy and Immunology (Chai et al., 1975).
The limits of allergen inhalation challenge testing outside of a research setting are several. It is time-consuming and staff intensive, and may not be reimbursable. Patients must be monitored for up to 24 hours to detect and treat late reactions. Potentially toxic reactions must be avoided. When an individual has been removed from exposure to an allergen, several days'