From the Guidelines
A methacholine challenge test is not always needed for asthma diagnosis, but it is valuable in certain clinical situations, particularly when patients have symptoms suggestive of asthma but normal baseline pulmonary function tests (PFTs). For patients with clear symptoms and positive PFTs showing reversible airflow obstruction, a methacholine challenge is unnecessary 1. However, when the diagnosis remains uncertain, a methacholine challenge can help reveal bronchial hyperresponsiveness, a hallmark of asthma. The test involves administering increasing concentrations of methacholine via nebulizer, with FEV1 measured after each dose, and a positive test is indicated by a 20% or greater drop in FEV1 1.
Key Considerations
- The decision to perform a methacholine challenge test depends on the clinical situation and the pretest probability of asthma, which can be influenced by symptoms, medical history, and exposure to potential asthma triggers 1.
- A methacholine challenge test is more useful in excluding a diagnosis of asthma than in establishing one, due to its higher negative predictive power compared to its positive predictive power 1.
- Other tests, such as peak flow monitoring, exhaled nitric oxide measurement, and exercise challenge tests, may complement the diagnosis of asthma, especially when the diagnosis remains uncertain after standard PFTs 1.
Recent Guidelines
The most recent guidelines, such as those from 2020, recommend the use of non-invasive measures of airway inflammation, like sputum cell counts and fractional exhaled nitric oxide (FENO), to support the diagnosis of asthma and determine its phenotype for targeted therapy 1. These guidelines also emphasize the importance of considering the clinical context and the patient's symptoms when deciding whether to perform a methacholine challenge test.
Clinical Application
In clinical practice, the methacholine challenge test should be performed by trained personnel with emergency medications available, and patients should withhold bronchodilators before testing (short-acting for 6-8 hours, long-acting for 24-48 hours) 1. The test results should be interpreted in the context of the patient's overall clinical presentation, including their symptoms, medical history, and the results of other diagnostic tests.
From the FDA Drug Label
Provocholine, used in a methacholine challenge test, is indicated for the diagnosis of bronchial airway hyperreactivity in adults and pediatric patients five years of age and older who do not have clinically apparent asthma. Before starting a methacholine challenge test, baseline spirometry must be performed For a patient to be able to undergo the test, he or she must present with baseline FEV1 (Forced Expiratory Volume in 1 second) greater than or equal to 60% of the predicted value (in adults and children) and greater than or equal to 1.5 L (in adults). The methacholine challenge test is considered positive if there is a reduction in FEV1 of 20% or more from post-diluent FEV1.
Methacholine challenge is needed for asthma testing in certain cases, specifically when the accuracy of the asthma diagnosis is in doubt, and the patient's baseline spirometry is normal after supervised withdrawal of asthma drugs. However, it is not recommended for patients with clinically apparent asthma or wheezing.
- Pulmonary function tests (PFTs), including baseline spirometry, are required before administering the methacholine challenge test.
- The test should only be performed under the responsibility of a healthcare practitioner trained in the technique and management of respiratory distress, with emergency equipment and medication available 2.
From the Research
Asthma Testing Methods
- Pulmonary function tests (PFTs) are commonly used to diagnose asthma, but a metacholine challenge test may be necessary in some cases to confirm the diagnosis 3.
- A study found that in patients with a negative bronchodilator response, 43% had a positive metacholine challenge test, indicating that metacholine challenge testing can be useful in diagnosing asthma in patients with a negative PFT result 3.
Importance of Metacholine Challenge Testing
- Metacholine challenge testing can help confirm asthma in patients with normal spirometry results, as it can detect airway hyperresponsiveness 4.
- The test can also help differentiate between asthma and other respiratory conditions, such as chronic obstructive pulmonary disease (COPD) 4.
Diagnostic Criteria
- A diagnosis of asthma is typically based on a combination of clinical symptoms, medical history, and diagnostic test results, including PFTs and metacholine challenge testing 3, 4.
- A thorough medical history is essential in diagnosing asthma, as diagnostic tests can only increase or decrease the probability of the diagnosis 4.