Screening for occupational asthma: a word of caution.

The diagnosis of occupational asthma may be difficult due to the complex mechanisms inducing the disorder. Identification of the offending agent after historical documentation may be difficult without bronchial challenge. The hallmark of asthma is bronchial hyperreactivity as detected by methacholine challenge, and this test could be considered as a screening test for asthma in the workplace. Four cases are presented that document changes in methacholine airway reactivity dependent on temporal association with exposure to the workplace or the specific offending agent. This indicates a need for a careful evaluation of symptoms relative to exposure in patients suspected for workplace asthma as well as serial determinations of methacholine response to detect potential variability in the airway reactivity.