Published Online: April 2, 2012
The clinical diagnosis of asthma can be difficult at times, since there is no perfect laboratory test to definitely tell whether a patient has asthma. The methacholine challenge test (MCT) is a widely used clinical test to evaluate for airway hyperresponsiveness (AHR), a hallmark of asthma. Asthmatics usually demonstrate an excessive response to an inhaled dose of methacholine which causes little or no change in lung function in normal healthy individuals. This test has been long regarded to be a good test to exclude the diagnosis of asthma if it was normal. However, we do not know if the test is still valid in recent years given that many patients are on potent inhaled corticosteroids.
In a manuscript in The Journal of Allergy and Clinical Immunology (JACI), Sumino et al and the American Lung Association Asthma Clinical Research Centers examined the utility of the MCT on asthmatics who are on controller medications. The researchers evaluated the sensitivity (percentage of asthma patients who are correctly identified as having asthma) and specificity (percentage of healthy people who are correctly identified as not having asthma) of MCT in 126 patients with asthma diagnosed by a physician, and 93 normal healthy individuals.
The researchers reported that one in four patients with physician diagnosis of asthma who are on controller medications had a negative MCT (sensitivity 77%). Only four normal controls had positive MCT (specificity 96%). In Caucasians and people without allergy, MCT performance worsened with more false negative tests.
These finding suggest that utility of the methacholine challenge test to rule out a diagnosis of asthma depends on racial and atopic characteristics. Clinicians should take into account the reduced sensitivity of the MCT in Caucasian and non-atopic asthmatics when using this test for the diagnosis of asthma.
The Journal of Allergy and Clinical Immunology (JACI) is the official scientific journal of the AAAAI, and is the most-cited journal in the field of allergy and clinical immunology.