reexposure may be required for a measurable response to occur. Allergen extract in aerosol form is deposited differently in the bronchial system compared with allergen in its natural state.

Allergen challenge has been used in connection with bronchoalveolar lavage, in which a bronchoscope is used to sample the fluid in the alveoli. Bronchial biopsies can also be performed. Demonstration of lymphocytosis in bronchoalveolar fluid can suggest the diagnosis of hypersensitivity pneumonitis (Reynolds, 1988). Beasley and others (1989a) performed specific inhalation challenge with allergen followed by bronchial biopsy and lavage, and, later, histamine challenge. Their studies demonstrated an inverse correlation between the number of epithelial cells in lavage fluid and histamine reactivity.

Despite its limitations, specific inhalation challenge testing will continue to have a unique place in the study of the health effects of indoor allergens.

Lung Volumes

Measurement of lung volumes is useful to help evaluate reductions in forced vital capacity. The usual method (gas dilution) involves breathing a known concentration of an inert gas in a closed-circuit system, followed by measurement of the functional residual capacity (volume in the lungs at the end of a normal breath). Total lung capacity is then determined by summing the functional residual capacity and the inspiratory capacity (determined by spirometry).

Diffusing Capacity Testing

Measurement of diffusing capacity is indicated when the clinician suspects that gas exchange is impaired by the disease process. Diffusion capacity measurements are not necessary in most cases of suspected asthma, but they may be indicated occasionally to exclude interstitial lung disease.

In diffusing capacity testing, the patient inhales a known mixture of an inert (nondiffusible) gas and a readily diffusible gas such as carbon monoxide (ATS, 1987). The exhaled gas is collected and analyzed, and the uptake of carbon monoxide is expressed in ml/min/mmHg. Similarly to spirometry, reference values have been determined by cross-sectional studies; interpretation consists of comparing actual values with reference values and with any previously measured values for the patient. Possible confounding factors are also considered. For example, diffusing capacity may be falsely elevated by conditions that raise the metabolic rate, lung blood volume, or red blood cell count; it may be lowered by the presence of carboxyhemoglobin in the blood or by anemia (Gold and Boushey, 1988). Reference standards

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