This cohort study included 126 patients with asthma, receiving controller medications, and 93 nonasthmatic controls. The sensitivity of the methacholine challenge test was 77% and the specificity was 96% with a threshold PC(20) (the provocative concentration of methacholine that results in a 20% drop in FEV(1)) of 8 mg/mL.
The sensitivity was lower in white than in African American participants (69% vs 95%). It was higher in atopic compared with nonatopic (82% vs 52%).
Increasing the PC(20) threshold from 8 to 16 mg/mL did not improve the performance of the test. African American race, atopy, and lower FEV(1) were associated with a positive test result.
Clinicians should take into account the reduced sensitivity of the methacholine challenge test in white and nonatopic asthmatic patients when using this test for the diagnosis of asthma.
References:
Methacholine challenge test: Diagnostic characteristics in asthmatic patients receiving controller medications. Sumino K, Sugar EA, Irvin CG, Kaminsky DA, Shade D, Wei CY, Holbrook JT, Wise RA, Castro M; American Lung Association Asthma Clinical Research Centers. J Allergy Clin Immunol. 2012 Mar 30.
Image source: Spirometry, from Wikipedia, the free encyclopedia, GNU Free Documentation License.
Very interesting study. It is hard to interpret these results because the patients defined as having "asthma" really meant "physician diagnosed stable asthma". Grouping the patients using this subjective criteria make the results very difficult to interpret.
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