BACKGROUND: The diagnosis of occupational asthma (OA) can be challenging and needs a stepwise approach. However, the predictive value of the methacholine challenge has never been addressed specifically in this context.
OBJECTIVE: We sought to evaluate the sensitivity, specificity, and positive and negative predictive values of the methacholine challenge in OA.
METHODS: A Canadian database was used to review 1012 cases of workers referred for a suspicion of OA between 1983 and 2011 and having had a specific inhalation challenge. We calculated the sensitivity, specificity, and positive and negative predictive values of methacholine challenges at baseline of the specific inhalation challenge, at the workplace, and outside work.
RESULTS: At baseline, the methacholine challenge showed an overall sensitivity of 80.2% and a specificity of 47.1%, with positive and negative predictive values of 36.5% and 86.3%, respectively. Among the 430 subjects who were still working, the baseline measures displayed a sensitivity of 95.4%, a specificity of 40.1%, and positive and negative predictive values of 41.1% and 95.2%, respectively. Among the 582 subjects tested outside work, the baseline measures demonstrated a sensitivity and specificity of 66.7% and 52%, respectively, and positive and negative predictive values of 31.9% and 82.2%, respectively. When considering all subjects tested by a methacholine challenge at least once while at work (479), the sensitivity, specificity, and positive and negative predictive values were 98.1%, 39.1%, and 44.0% and 97.7%, respectively.
CONCLUSION: A negative methacholine challenge in a patient still exposed to the causative agent at work makes the diagnosis of OA very unlikely.