Monday, February 8, 2016
Predictive value of nonspecific bronchial responsiveness in occupational asthma
The diagnosis of occupational asthma (OA) poses challenges to the clinician and requires a stepwise approach. The American College of Chest Physicians has recently published a consensus on this approach, also providing guidelines that indicate “the absence of airway hyper-responsiveness on challenge testing has a fairly high negative predictive value (NPV) for current symptomatic asthma, and can generally be used to rule out active disease.” Pralong et al. have verified this statement, evaluating the sensitivity, specificity, and positive and negative predictive values of the methacholine challenge in the diagnosis of occupational asthma (J Allergy Clin Immunol 2016; 137(2): 412-416).
The authors used a Canadian database to review 1012 cases of workers who, between the years of 1983 and 2011, were referred for suspicion of having occupational asthma and who underwent a specific inhalation challenge (SIC). SIC is considered the gold standard for diagnosing OA. It entails a first day of testing during which a patient is exposed to a control substance followed by a methacholine challenge. The patient is then exposed to the suspected causative occupational agent and undergoes another methcholine challenge. Among the 1012 patients reviewed, the median exposure duration was seven years, the median symptom duration was one year, and the median delay between exposure cessation and testing was two months. SIC confirmed OA in 27.5% of the cases.
Results presented here are the first to confirm two current recommendations. First, a negative methacholine challenge during the time in which the patient is still working at the exposure site makes the diagnosis of OA highly unlikely, as the negative predictive value (NPV) of the test in this population while at the workplace was 95.2%. Second, the NPV rose to 97.7% when considering all patients who had undergone a methacholine challenge at least once while at work at the exposure site, and it fell to 82.2% among patients who were tested off-site. The data demonstrate the utility of the methacholine challenge and indicates that, when possible, an OA diagnostic workup is best done when the patient is still working at the location of exposure.