About Methacholine Challenge Testing
When to Consider MCT?
Always consider objective measurements when contemplating an asthma diagnosis. There are several consequences associated with improper treatment such as: under treatment of the disease, potential exposure to inappropriate medications, lost employment opportunities (i.e., military, public services), social consequences and economic factors.
Several studies and abstract presentations have shown that patients with physician diagnosed asthma may have been misdiagnosed in approximately 1/3 of cases, and one study showed as many as 41% of patients had been misdiagnosed. The American Thoracic Society (ATS) Guidelines for Methacholine and Exercise Challenge Testing suggests that even asthma specialists are not able to reliably predict the outcome of the Methacholine Challenge Test. To confirm a diagnosis of asthma, objective testing should be performed.
Figure 2. Methacholine challenge testing sequence (flow chart).
*The choice of the FEV1 value considered a contraindication may vary from 60 to 70% of predicted.
**The final dose may vary depending on the dosing schedule used. Final doses discussed in this statement are 16, 25, and 32 mg/ml.
Clinical Process for MCT
- If a patient presents with reoccurring symptoms of airflow obstruction a detailed medical history should be taken along with a physical examination of the upper respiratory tract, chest and skin.1
- Spirometry should be performed for every patient (greater than 5 years of age) to determine whether any airway obstruction present is at least partially reversible. 1
- In patients that have a median probability of asthma the results of the MCT cannot accurately be predicted, even by asthma specialists. If baseline spirometry shows limited airflow obstruction and an insignificant response to a bronchodilator (<12% and <0.2-L in FEV1 or FVC), these patients should be sent for further testing to rule out asthma as the diagnosis.2
- The MCT can effectively rule out asthma with greater than 90% certainty, with consideration of the following; ensure that airway responsiveness was not suppressed by intensive anti-inflammatory medications, patient has current symptoms and a small percentage of workers with occupational asthma may react only to a single antigen or chemical sensitizer. The best possible diagnostic value of the MCT occurs when the pretest probability is in the range of 30 to 70 percent.2
To access the source publication, click the following link to exit our website and be directed to the American Thoracic Society website.
Click Statements>scroll down to 'Pulmonary Function and Exercise Testing' and select 'Guidelines for Methacholine and Exercise Challenge Testing'.