Asthma is primarily a diagnosis of exclusion, meaning there is no one diagnostic test for this lung disease. However, many physicians use the methacholine challenge (MCT) to as an objective component of diagnosing asthma. According to the Cleveland Clinic, an MCT is more useful for ruling out asthma, as a positive methacholine challenge does not necessarily mean asthma.
About the Methacholine Challenge
The methacholine challenge involves breathing in and out of a tube at the direction of a respiratory therapist to measure airway reactivity. The test takes about an hour (less or more depending on the exact protocol). Increasing doses of the methacholine irritant are administered until the patient's airways register a significant reaction. Once the test is concluded, the patient will be instructed to either use their albuterol inhaler or a nebulizer treatment will be given to reverse any constriction in the airways.
Some clinics use a small sauna-like chamber with see-through doors. Patients sit inside and the methacholine is piped into the chamber as the patient is instructed on how to breathe by a respiratory therapist.
An MCT is not painful and patients should be fine to return to regular activity after the test is completed.
Preparing for a Methacholine Challenge
The key to an accurate MCT is to discontinue asthma medication far enough in advance of the test that it won't interfere with the results. The American Association for Respiratory Care (AARC) provides comprehensive information on how long medication should be stopped before the test.
In general, stop taking long acting bronchodialators (i.e. Serevent) and Singulair as much as 48 hours before the test. Fast-acting albuterol must be avoided for up to eight hours prior and medications like Tiotropium (brand name Spiriva) should be discontinued for up to a week.
Not all doctors follow the AARC Clinical Practice Guideline, which is unfortunate as it affects the test results. Patients should consider a second test or opinion when physicians and clinics do not follow established methacholine challenge guidelines, particularly if they garner a negative test despite the clear presence of asthma symptoms.
Positive Methacholine Challenge
A drop of 20% or more in forced expiratory volume in one second, commonly abbreviated as FEV1, is considered a positive methacholine challenge. Anything less than 20% and the test will come back as negative for asthma.
There are several medical conditions that can result in a positive MCT. This is why doctors need to factor in the patient's symptoms, medical history and any other diagnostic testing before conferring an asthma diagnosis.
Chronic obstructive pulmonary disease, or COPD, can result in a positive result as can congestive heart failure. Smoking, chronic coughing, rhinitis, or an active respiratory infection can also yield a positive MCT. In general, these conditions can be ruled out with further testing and have their own set of unique symptoms that are not seen in asthma.
Because insurance can change and doctors can move on to other practices or clinics, savvy patients are sure to get copies of any positive methacholine challenges for their files.
Negative Methacholine Challenge
Medicine typically views a negative methacholine challenge as a final ruling against an asthma diagnosis, but the possibility of a false negative does exist. Factors that can cause a false negative according to the American Journal of Respiratory and Critical Care Medicine include:
- improperly stopping or not stopping medication prior to the test
- high doses of anti-inflammatory medications that suppress the inflammation response to the methacholine during the test
- occupational asthma that only responds to a specific irritant or allergen
- poor technique or faulty equipment on the part of the clinic
- patients with allergic asthma may not respond well unless they are exposed to their trigger allergen
The MCT is a tool that can help both doctors and patients, but it is only a piece of the puzzle. The most important component in a methacholine challenge is an experienced doctor who can bring the MCT results together with patient symptoms to find the right diagnosis.
References:
American Association of Respiratory Care RC Journal. "AARC Clinical Practice Guideline: Methacholine Challenge Testing 2001 Revision & Update." (Retrieved October 8, 2010.)
American Journal of Respiratory and Critical Care Medicine. "Guidelines for Methacholine and Exercise Challenge Testing-1999." (Retrieved October 8, 2010.)
Cleveland Clinic One Minute Consult. "When Should a Methacholine Challenge Be Ordered for a Patient with Suspected Asthma?" (Retrieved October 8, 2010.)
Michelle Rasey - Michelle R. Rasey has extensive expertise in International Business and Health. She holds two BA degrees and an alternative medical ...
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