Strategies for efficient diagnosing
Relying on MC results to minimize testing
A negative methacholine challenge (MC) test result can almost rule out a positive specific challenge test result in patients suspected with occupational asthma, reducing the need to perform the more laborious specific challenge test. That's the conclusion of German researchers who studied different methods of diagnosing occupational asthma.1
They performed inhalative MC in 229 patients presumed to suffer occupational asthma due to exposure to airborne latex allergens, flour, isocyanates, or irritants in hairdressers' salons. The patients also were subjected to specific challenges with the occupational agents they were exposed to. The researchers personally administered questionnaires and conducted interviews.
Bronchial hyperresponsiveness in MC was defined with a threshold cumulative MC dose of 0.3 mg. In 40% to 72% of the patients, workplace-related asthma complaints occurred with bronchial hyperreactivity in the MC ranging from 48% to 61%. But only 12% to 25% of the patients demonstrated a significant bronchoconstrictive reaction in the specific challenge test.
The researchers note that MC results are only moderately associated with workplace-related asthma case histories, while positive outcomes of challenges with occupational agents are well-correlated with positive MC results plus occupational asthma case histories.
"We conclude that in most cases, occupational asthma . . . is combined with bronchial hyperresponsiveness and workplace-related asthmatic symptoms," the researchers say. "However, subjects of each exposure group demonstrated bronchial hyperresponsiveness and complained of workplace-related asthmatic symptoms, but occupational asthma could not be proved in the specific challenge test."
Reference
1. Baur X, Huber H, Degens PO. Relation between occupational asthma case history, bronchial methacholine challenge, and specific challenge tests in patients with suspected occupational asthma. Am J Ind Med 1998; 33:114-122.
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