It involves the administration of methacholine, a substance that causes airway narrowing, to determine the sensitivity of the airways.
This test is particularly useful when a patient’s symptoms suggest asthma, but routine pulmonary function tests are normal.
Understanding how this test works, its procedure, and its interpretation can help healthcare providers accurately diagnose and manage respiratory conditions.
What is a Methacholine Challenge Test?
A methacholine challenge test is a diagnostic test used to assess bronchial hyperreactivity or bronchial responsiveness. It’s commonly employed in situations where asthma is suspected but not confirmed by standard pulmonary function tests.
Methacholine is a substance that, when inhaled, can cause the airways to constrict or narrow in susceptible individuals.
People with asthma or other respiratory disorders might have an increased sensitivity to methacholine, causing their airways to constrict at lower doses compared to individuals without such conditions.
Procedure
The patient first undergoes a baseline spirometry test to measure airflow and lung volumes. If the baseline results are within a normal range, the patient is then given increasing concentrations of inhaled methacholine via a nebulizer.
After each dose, the patient’s lung function is measured again using spirometry.
The test continues until either there’s a significant reduction (usually 20% or more) in the patient’s Forced Expiratory Volume in one second (FEV1) or the highest dose of methacholine is reached.
A significant drop in FEV1 in response to low doses of methacholine indicates bronchial hyperreactivity, which is consistent with asthma.
If the patient does not show a significant reduction in FEV1 even after the highest dose of methacholine, the test is considered negative, suggesting that asthma or bronchial hyperreactivity is less likely.
Note: While the methacholine challenge test can be very helpful in diagnosing asthma, it’s just one piece of the puzzle. A comprehensive diagnosis should also consider a patient’s clinical history, symptoms, physical examination findings, and other diagnostic tests.
What is a Bronchoprovocation Study?
A bronchoprovocation study, sometimes called a bronchial challenge test, is a diagnostic procedure used to assess bronchial hyperreactivity or bronchial responsiveness.
This test evaluates how sensitive the airways are to specific stimuli that might cause them to constrict or narrow.
It’s particularly beneficial in individuals where asthma or other respiratory conditions are suspected but not evident through standard testing.
What to Expect During a Methacholine Challenge Test?
If you’re a patient scheduled for a methacholine challenge test, it’s natural to wonder what to expect. Here’s a step-by-step guide to provide some clarity:
- Preparation: Your doctor may ask you to avoid certain medications before the test. These can include bronchodilators, antihistamines, or other medications that can influence the test’s outcome. Typically, there’s no need to fast. However, a light meal is advisable to avoid nausea during deep inhalations. Wear loose and comfortable clothing, as you will be asked to perform spirometry, which involves deep breaths and forceful exhalations.
- Arrival: Once you arrive, a technician will review the procedure with you and answer any questions you might have. You’ll be provided with a mouthpiece connected to the spirometry machine.
- Baseline Spirometry: Before any methacholine is administered, your baseline lung function will be measured using spirometry. This helps to establish a reference point.
- Methacholine Administration: You’ll inhale an aerosolized saline solution (a control) followed by increasing concentrations of aerosolized methacholine. These are delivered via a nebulizer, which converts the solution into a fine mist that you can breathe in. You’ll likely be instructed on how to breathe (usually taking in deep breaths and holding for a few seconds) during the inhalation of each dose.
- Measurement After Each Dose: After inhaling each concentration, you’ll undergo spirometry again. The goal is to determine whether the methacholine is causing your airways to narrow and by how much.
- Completion: The test either concludes when there’s a significant drop in your lung function (usually a 20% or greater decrease in FEV1) or once the highest methacholine concentration is reached without a significant response.
- Reversal of Effects: If your airways do constrict during the test, you’ll be given a bronchodilator (like albuterol) to reverse the effects. After receiving the bronchodilator, another spirometry test might be done to ensure your lung function has returned to baseline or near-baseline levels.
- Post-Test: Once the test is over, the results are interpreted by a physician, often a pulmonologist. They’ll discuss the results with you, providing insights into your airway reactivity and what it means for your diagnosis and treatment. It’s advisable to have someone accompany you for the test or arrange for transportation, especially if you experience significant bronchoconstriction.
Note: It’s essential to follow pre-test instructions, especially regarding medications, to ensure the accuracy of the test. If at any point during the test you feel overly uncomfortable or anxious, communicate with the medical team. They’re there to assist and ensure your safety.
Is the Methacholine Challenge Test Safe?
Although the methacholine challenge test is generally safe, it can induce bronchospasm, leading to symptoms like wheezing or shortness of breath.
For this reason, the test is always conducted under close medical supervision with equipment and medications on hand to reverse any adverse reactions promptly.
Methacholine Challenge Test Practice Questions
1. What is the methacholine challenge test?
Also known as the bronchoprovocation test, it is performed to evaluate how reactive or responsive a patient’s lungs are to things in the environment.
2. What can be determined by the methacholine challenge test?
If a patient has asthma
3. How does the methacholine challenge test work?
The patient inhales doses of methacholine, which is a drug that can cause narrowing of the airways (as seen in asthma). A breathing test is performed after each dose of methacholine to measure the degree of constriction in the airways. The patient starts with receiving a very small dose of methacholine and then the doses will be increased until either there is a 20 percent drop in breathing ability, or they receive the maximum dose with no change in lung function.
4. What bronchodilators should be withheld for 24 hours before bronchial challenge testing?
Long-acting inhaled B-agonists and anticholinergic agents.
5. When should theophylline be withheld?
For 48 hours before bronchial challenge testing.
6. A decrease in sGaw of 35 to 45% may be considered what?
A positive methacholine or histamine response.
7. What should you conclude after a decrease of 15% after 5 minutes of hyperventilation?
The patient is breathing cold air and has hyperreactive airways.
8. What are the two methods of delivering methacholine?
5-breath dosimeter or 2-minute tidal breathing
9. How do you know if a patient has asthma from a methacholine challenge test?
If their FEV1 decreases at least 20% or more at a low dose.
10. What are the nebulizer factors that need to be controlled?
Nebulizer output, particle size, and breath-hold time.
11. What is a negative mannitol challenge test?
When there is a cumulative dose of 635 mg and the FEV1 has not dropped more than 15 percent.
12. What do you call the direct bronchoprovocation test?
Methacholine
13. When is a mannitol test indicated?
When a patient has a cough after exertion and their spirometry and lung volumes are normal.
14. A patient should not do vigorous exercise for four hours before what test?
An exercise bronchoprovocation test.
15. How many hours after an exercise test should you wait before another test is performed?
4 hours
16. What is methacholine?
A drug that causes bronchoconstriction by increasing the parasympathetic tone in bronchial smooth muscle.
17. What does the methacholine challenge test help with?
An asthma diagnosis
18. What are the symptoms of asthma?
Wheezing, dyspnea, chest tightness, and persistent coughing.
19. What are the absolute contraindications of the methacholine challenge test?
Severe airflow obstruction (FEV1 less than 50% predicted), heart attack or stroke within last 3 months, uncontrolled HTN (systolic greater than 200, or diastolic greater than 100), and a known aortic aneurysm.
20. What are the relative contraindications of the methacholine challenge test?
Inability to perform spirometry and pregnant/nursing mothers.
Access our quiz with sample TMC practice questions and detailed explanations to help you master pulmonary function testing (PFT).
21. What are the hazards of the methacholine challenge test?
Shortness of breath, cough, chest tightness, wheezing, and headache.
22. What should the patient withhold before performing a methacholine challenge test?
Short-acting bronchodilators, medium-acting bronchodilators, long-acting bronchodilators, oral bronchodilators, and mediator modifiers.
23. What is the ventilatory reserve?
It is the difference between the highest level of ventilation in exercise and the ventilatory capacity.
24. In what type of patients is bronchoprovocation testing used?
It is used in patients with bronchospasms who have normal PFTs or nonspecific results of bronchodilator studies.
25. What is a direct bronchoprovocation test?
It has high sensitivity and low specificity.
26. What is an indirect bronchoprovocation test?
It has low sensitivity and high specificity.
27. What are the indications for bronchoprovocation tests?
Diagnosis of asthma, airway hyperresponsiveness severity assessment, and seeing the response of asthma treatment.
28. What are the absolute contraindications of bronchoprovocation testing?
Severe airflow limitation where the FEV1 is less than 50 percent, myocardial infarction or ischemic stroke during the past three months, high blood pressure that cannot be controlled, and having a known aneurysm.
29. What are the relative contraindications of bronchoprovocation testing?
Moderate airflow limitation where the FEV1 is less than 60 percent, recent upper respiratory tract infection within 2 weeks, pregnancy or breastfeeding, and spirometry measurements cannot be performed at an acceptable level.
30. What type of cells does a direct stimulus affect?
Effector cells
31. What type of cells does an indirect stimulus affect?
Intermediary cells
32. What makes up effector cells?
Airway smooth muscle cells, bronchial endothelial cells, and mucus-producing cells.
33. What types of cells make up intermediary cells?
Inflammatory cells and neuronal cells.
34. If the direct stimulus is methacholine, then the indirect stimulus is what?
Mannitol
35. If the direct stimuli are prostaglandins, then what is in the indirect stimulus?
Exercise
36. How long should inhaled short-acting beta-agonists be withheld before a bronchial challenge test?
8 hours
37. How long should inhaled long-acting beta-agonists be withheld before a bronchial challenge test?
48 hours or longer
38. How long should anticholinergic agents (e.g., Ipratropium) be withheld before a bronchial challenge test?
24 hours
39. How long should histamine be withheld before a bronchial challenge test?
72 to 96 hours
40. How long should caffeine-containing drinks be withheld before a bronchial challenge test?
6 hours
41. How long should leukotriene modifiers be withheld before a bronchial challenge test?
24 hours
42. What is the definition of PC20?
It is the provocative concentration of methacholine that causes a 20% decrease in the parameter of interest.
43. What does the PC20 describe?
It describes airway hyperresponsiveness. For example, if the PC20 value is high, airway hyperresponsiveness is normal and asthma is unlikely.
44. What does PC20 represent?
A 20% decline in FEV1.
45. What PC2O value is consistent with daily symptoms and requires more serious treatment?
A PC20 value of less than 2 mg/mL.
46. How is nebulizer quality control done before a methacholine challenge test?
The nebulizer is weighed on an accurate scale before and after the delivery of saline.
47. How is the delivered dose of methacholine standardized?
By using a fixed number of breaths (usually 5) or by breathing for a fixed length of time (2 minutes).
48. What is the baseline spirometry requirement during a methacholine challenge test?
It should be greater than 60 to 70 percent of the predicted or the previously reported best value.
49. What might cause a negative methacholine challenge result?
Asthma that has been suppressed by anti-inflammatory medications, or occupational asthma that is triggered by a specific agent.
50. What are two common side effects of the inhalation of antihistamines?
Flushing and headaches
51. When can a histamine challenge be repeated?
Within 2 hours after the patient has returned to baseline.
52. When is the peak action of the histamine challenge?
30 to 120 seconds
53. How is the histamine challenge carried out?
The patient inhales aerosols via tidal volume breathing for two minutes.
54. What device is the mannitol challenge carried out with?
Dry powder inhaler
55. At a molecular level, how does the mannitol challenge work?
It causes a hypertonic stimulus that results in the release of mediators from mast cells and basophils.
56. What FEV1 makes a subject ineligible for an exercise challenge?
If the FEV1 is less than 65%.
57. What does the exercise challenge analyze?
Airway heat and water loss during increased ventilation with exercise.
58. When is bronchospasm expected to occur during exercise testing?
It should occur immediately after the exercise rather than during the exercise. Most of the time, a short period of moderately heavy work is all that is required to trigger exercise-induced bronchospasm.
59. When is a repeat exercise test done?
After 4 hours because of the refractory period where the bronchoconstriction lessens.
60. What are the symptoms of methacholine?
Methacholine, when inhaled, can induce bronchoconstriction. Symptoms include coughing, wheezing, shortness of breath, and chest tightness.
61. Can a patient pass the methacholine challenge and still have asthma?
Yes, a patient can pass the methacholine challenge and still have asthma. While the test is sensitive, no diagnostic test is perfect. Clinical judgment based on a comprehensive review of the patient’s symptoms, history, and other tests is essential.
62. Does everyone react to methacholine?
No, not everyone reacts to methacholine. Individuals with normal bronchial reactivity might not show any significant response, even at higher doses. However, those with bronchial hyperreactivity, such as many asthmatics, will likely have a bronchoconstrictive response at lower doses.
63. What gas concentration is used in eucapnic voluntary hyperventilation?
5 percent mixture of CO2 with air
64. What PC20 value is asymptomatic for diagnosing asthma?
PC20 value of more than 20mg/ml
65. What is the maximum dose of methacholine that can be delivered?
16 mg/mL
Final Thoughts
In the field of respiratory medicine, the methacholine challenge test stands as an invaluable tool for gauging bronchial responsiveness.
When conducted under standardized conditions and interpreted correctly, the test offers significant insight into a patient’s pulmonary function.
However, as with any diagnostic tool, the results should be considered in conjunction with clinical history, physical examination, and other relevant tests to ensure an accurate and holistic assessment of the patient’s respiratory health.
Written by:
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
References
- Faarc, Mottram Carl Ba Rrt Rpft. Ruppel’s Manual of Pulmonary Function Testing. 11th ed., Mosby, 2017.
- Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
- Sayeedi I, Widrich J. Methacholine Challenge Test. [Updated 2023 May 22]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.