From the Guidelines
The primary purpose of a Methacholine challenge test is to assess airway hyperresponsiveness, a key feature of asthma, by measuring the response of the airways to methacholine, a neurotransmitter substance that directly interacts with receptors on airway smooth muscle.
Key Points
- The test is used to diagnose asthma, particularly when traditional methods such as spirometry have not established or eliminated the diagnosis 1.
- Methacholine challenge testing is more useful in excluding a diagnosis of asthma than in establishing one, as its negative predictive power is greater than its positive predictive power 1.
- The test involves measuring the provocative dose (PD20), which is the delivered dose of methacholine resulting in a 20% fall in FEV1 1.
- The results are interpreted using a categorical method, where a positive test is often defined as a PC20 < 8 or < 16 mg/ml, and a negative test is defined as a PC20 > 8-25 mg/ml 1.
- The test has a high negative predictive power, with a negative result ruling out asthma with reasonable certainty in patients who have had asthma symptoms during the previous 2 weeks 1.
Important Considerations
- The test should be performed with caution, as it can cause transient symptoms such as wheezing, cough, mild dyspnea, and chest tightness, particularly in patients with airway hyperresponsiveness 1.
- The technician should be alert to patient symptoms and make a record of any that occur, and prudent measures should be in place to minimize the risk of severe bronchospasm 1.
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. The purpose of a Methacholine challenge test is to diagnose bronchial airway hyperreactivity in adults and pediatric patients who do not have clinically apparent asthma 2, 2.
- The test is indicated for patients five years of age and older.
- It is used to assess airway responsiveness by measuring the response to increasing concentrations of methacholine chloride solutions.
- The test is considered positive if there is a reduction in FEV1 of 20% or more from post-diluent FEV1 2.
From the Research
Purpose of a Methacholine Challenge Test
The purpose of a Methacholine challenge test (MCT) is to assess airway hyperresponsiveness (AHR) and its severity, which has significant clinical value in the diagnosis and treatment of bronchial asthma 3. The test is used to evaluate the effectiveness of asthma treatment, diagnose asthma, and assess the severity of airway hyperresponsiveness 4.
Key Aspects of the Methacholine Challenge Test
- The test involves inhaling increasing concentrations of methacholine, a substance that can trigger airway constriction in people with asthma 3.
- The test measures the amount of methacholine required to cause a 20% decrease in forced expiratory volume (FEV1), which is a measure of lung function 5.
- A positive test result indicates airway hyperresponsiveness, which is a characteristic feature of asthma 6.
- The test can be used to diagnose asthma, monitor the effectiveness of treatment, and assess the severity of airway hyperresponsiveness 3, 4.
Interpretation of Results
- A positive test result is indicated by a 20% decrease in FEV1 or a significant increase in respiratory resistance (Rrs) 3, 5.
- The severity of airway hyperresponsiveness can be graded based on the dose of methacholine required to cause a 20% decrease in FEV1 (PD20) or the concentration of methacholine required to cause a 20% decrease in FEV1 (PC20) 3.
- A negative test result does not entirely rule out asthma, as some patients with asthma may not respond to methacholine 7.