Background: Specific and nonspecific bronchial responsiveness may decline or disappear after cessation of exposure in the workplace in patients with occupational asthma, leading to false-negative specific inhalation challenge (SIC) results.
Methods: Twenty-two patients with suspected diisocyanate-induced asthma were studied. SIC with diisocyanates (toluene diisocyanate [TDI] or hexamethylene diisocyanate [HDI]) was carried out in a 7-m3 dynamic chamber up to a maximum concentration of 19 parts per billion for 120 min. Methacholine inhalation challenges were performed before and 24 h after SIC with TDI or HDI. Patients who did not show an asthmatic reaction after SIC but had a greater than twofold reduction in provocative concentration of methacholine causing a 20% fall in FEV1 (PC20) after the first isocyanate challenge underwent a second isocyanate SIC 2 days later.
Results: The first SIC with isocyanates elicited an asthmatic reaction in 13 patients (59%). In five patients who did not show an asthmatic reaction after the first SIC, PC20 exhibited more than a twofold reduction. In three of the five patients, a second SIC with isocyanates elicited an immediate positive asthmatic reaction. Therefore, 3 of 16 patients (19%) were ultimately shown to have bronchial responsiveness to isocyanate; occupational asthma was demonstrated due to post-SIC monitoring of bronchial hyperresponsiveness to methacholine.
Conclusion: PC20 should be systematically assessed before and after SIC with isocyanates in the absence of significant changes in FEV1 during SIC to avoid false-negative results.