Methacholine Challenge Test

Anatomy and Physiology

Asthma is a chronic inflammatory disorder of the airways characterized by airflow obstruction, hyperresponsiveness, and remodeling, leading to symptoms such as coughing, wheezing, chest tightness, and shortness of breath. The pathophysiology involves a complex interplay of airway inflammation, bronchoconstriction, and increased mucus production. Airway hyperresponsiveness is a central component of the underlying pathophysiology of asthma. Because asthma symptoms can fluctuate, not all patients exhibit abnormalities noted on initial pulmonary function tests. 

Methacholine is a nonselective muscarinic receptor agonist that acts directly on airway smooth muscle receptors to induce bronchoconstriction.[7] Methacholine also indirectly stimulates mucus cell secretion. Methacholine is more useful for testing than acetylcholine due to its longer duration of action.[3]

Indications

Indications for bronchoprovocation testing are as follows:

• Patients with typical asthma symptoms and normal pulmonary function tests with no response to bronchodilators.
• Patients with nonspecific symptoms potentially caused by asthma, such as an isolated cough.
• Patients who require testing for medicolegal reasons, such as those suspected of having occupational asthma.
• Assessing a patient's response to therapy.
• Identifying potential triggers of a patient's asthma.
• As a screening test for scuba divers, military personnel, or other patients in whom asthma may cause significant personal risk.
• Patients with atypical symptoms, such as difficulty sleeping or speaking, and anxiety or panic, not due to other identifiable causes.[8][9][10][11][12][13][14][15]

Contraindications

Contraindications to methacholine testing include:

• Allergy to the challenge agent
• Presence of significant airflow obstruction, typically an FEV₁ less than 60% predicted for adults and children or 1.5 L for adults
• Inability to consistently reproduce quality spirometry
• Recent myocardial infarction or stroke within the past 3 months
• Unstable cardiovascular disease
• Uncontrolled hypertension
• Known aortic aneurysm
• Recent upper respiratory infection
• Recent ophthalmologic surgery
• Patients at risk for intracranial pressure elevation [5][16]

In younger patients with smaller airway caliber, hypoxemia is a concern. However, pulse oximetry (SaO₂) and transcutaneous oxygen pressure allow for safe testing. Methacholine is a pregnancy category C drug, and no studies are available to determine whether methacholine is excreted in breast milk or is associated with fetal abnormalities.[17]

Equipment

Essential equipment for methacholine challenge testing includes:

• Spirometer
• Nose clips
• Timer
• A chair with armrests for the patient
• Nebulization apparatus to administer aerosolized methacholine at an output of 0.13 mL/min
• Preparations of varying concentrations of methacholine
• Vials and holders for methacholine
• A refrigerator to hold the different concentrations of methacholine
• Diluent or saline
• Emergency supplies, including oxygen; bronchodilators such as albuterol; defibrillator; epinephrine; atropine; lidocaine; corticosteroids; bag valve mask; stethoscope; sphygmomanometer; and pulse oximeter [5][16]

Personnel

Skilled technicians are crucial in guiding patients through the correct breathing maneuvers required for spirometry and accurate FEV₁ measurement. Poor technique can compromise test validity. Once methacholine administration begins, the patient should not be left unattended at any point. A clinician or other qualified healthcare provider capable of managing acute bronchospasm must be readily available to respond immediately in case of an emergency. Appropriately trained personnel must be present to ensure both patient safety and test accuracy.

Preparation

Healthcare professionals must instruct patients to avoid alcohol for at least 4 hours and refrain from smoking for at least 1 hour before testing. Different asthma medications can alter test results, and clinicians may need to discontinue some medications before testing. In general, asthma medications, except bronchodilators such as inhaled and oral corticosteroids and leukotriene modifiers, are not discontinued before testing if the purpose is to evaluate the patient's response to therapy or to determine whether the patient's current symptoms are related to asthma.[18] Given the high sensitivity of the test, a negative test result for patients on inhaled corticosteroids strongly suggests that their current symptoms are not caused by asthma. However, these results cannot exclude a diagnosis of underlying asthma. If the clinician needs testing to be done without the anti-inflammatory effects of these medications, testing should be performed after a 4-week discontinuation period. Caffeine and caffeine-related products, such as chocolate and antihistamines, do not affect test results. The following list includes the minimum time necessary to omit asthma medications before bronchoprovocation testing:

• Albuterol: 6 h
• Levalbuterol: 6 h
• Terbutaline: 6 h
• Formoterol and salmeterol: 36 h
• Ultra long-acting beta-agonists: 48 h
• Short-acting muscarinic antagonist such as ipratropium: 12 h
• Long-acting muscarinic antagonists such as aclidinium, glycopyrrolate, and tiotropium: 1 week
• Theophylline: 24 h
• Oral albuterol: 12 to 24 h
• Oral glucocorticoids: 2 to 3 weeks
• Cromolyn sodium: 4 h
• Leukotriene modifiers: 4 weeks
• Inhaled glucocorticoids: 4 weeks

Technique or Treatment

The following outlines the steps involved in performing a methacholine challenge test:

• Prepare methacholine solutions in sterile vials at appropriate concentrations. Store the vials in a refrigerator and remove them 30 minutes before testing to allow the sample to warm to room temperature. The tester administers methacholine sequentially in increasing concentrations, ranging from 0.016 to 16 mg/mL, using 2- to 4-fold dilutions. 

• Perform pretest spirometry to confirm that the patient can adequately perform spirometry and that the baseline airflow limitation is acceptable for the test.

• Add 3 mL of diluent to the nebulizer, apply the nose clip, and administer to the patient, ensuring at least 1 minute of quiet tidal volume breathing. Alternatively, 5 vital capacity breaths followed by a breath hold is another option. However, quiet tidal volume breathing provides a higher sensitivity.[19] 

• Perform post-diluent spirometry at 30 and 90 seconds, measuring FEV₁ and forced vital capacity (FVC). The tester must adequately coach the patient to ensure quality measurements. If FEV₁ is the only measurement, the tester can shorten the expiratory effort to 2 seconds instead of 6 seconds. However, if the clinician suspects vocal cord dysfunction, the tester should perform complete FVC breathing maneuvers with analysis of both the inspiratory and expiratory phases.[20] The diluent should not cause a significant change in FEV₁ from pre-diluent spirometry. If the FEV₁ change is less than 10%, the patient can receive the first dose of methacholine. A change in FEV1 of 10% to 20% warrants repeating the diluent trial. A decrease in FEV₁ of 20% or more requires cancellation of the test. The post-diluent FEV₁ serves as the baseline.

• Following methacholine administration, repeat spirometry at 30 and 90 seconds. If the FEV₁ falls by less than 20%, administer the next dose of methacholine and repeat spirometry and FEV₁ measurements. The tester increases the dose stepwise until the FEV1 drops by more than 20%. The methacholine dose corresponding to a greater than 20% drop in FEV₁ is the provocation dose (PD20)—also known as the provocation concentration (PC20) expressed in mg/mL. A 35% to 40% reduction in sG_aw, a measure of airway resistance, is an alternative endpoint for patients with difficulty performing spirometry.

• Once the patient experiences a decrease in FEV₁ of 20% or more, note their symptoms and administer a fast-acting bronchodilator. Repeat spirometry after 5 to 10 minutes. Ensure the patient's FEV₁ returns to baseline before discharge.[5] 

A test is considered positive if PD20 is less than or equal to 200 µg or if PC20 is less than or equal to 8 mg/mL. When using sG_aw, a concentration of 100 µg or 4 mg/mL or less indicates a positive test result. A test is negative if the PD20 is greater than 400 µg or the PC20 is greater than 16 mg/mL.

An additional proposed grading system based on the PC20 categorizes degrees of hyperresponsiveness as follows: 

• Normal: >16 mg/mL
• Borderline: 4.0-16 mg/mL
• Mild: 1.0-4.0 mg/mL
• Moderate to severe: <1.0 mg/mL [5][16]

However, the clinical utility of this system has yet to be determined.

Clinical Significance

Methacholine challenge testing is a nonspecific bronchoprovocation tool primarily used to evaluate patients with an uncertain diagnosis of asthma. The test is most appropriate when baseline spirometry is normal but the clinical history raises suspicion for asthma.[21][22] The test is highly sensitive and has a strong negative predictive value, making it particularly valuable for ruling out the diagnosis of asthma. A positive result may support a diagnosis of asthma; however, the test has a limited positive predictive value, making it more reliable for excluding than confirming the diagnosis.[23]

Clinicians should be cautious, as false positives can occur in other conditions, such as chronic obstructive pulmonary disease, heart failure, bronchitis, allergic rhinitis, and cystic fibrosis. Up to 7% of asymptomatic individuals may have a positive test, a normal variant, or an indicator of imperceptible symptoms. In contrast, a negative methacholine challenge in a patient with symptoms suggestive of asthma should prompt evaluation for alternative diagnoses, such as inducible laryngeal obstruction (vocal cord dysfunction), occupational asthma, or central airway obstruction from tumors, polyps, or foreign bodies.[3][21] Because asthma symptoms wax and wane, and patients with occupational asthma may be asymptomatic if they have been off work for some time, clinicians may need to repeat testing when the patient is symptomatic. 

Enhancing Healthcare Team Outcomes

Methacholine challenge testing is a nonspecific bronchoprovocation method used to assess airway hyperresponsiveness, aiding in the diagnosis of asthma, particularly when baseline spirometry is normal but clinical suspicion remains. Methacholine, a cholinergic agonist, induces bronchoconstriction by stimulating airway smooth muscle receptors. The test involves administering increasing doses of methacholine with serial spirometry to measure FEV₁. A drop in FEV₁ of 20% or more indicates a positive result. This highly sensitive test has a strong negative predictive value, making it especially useful for ruling out asthma. However, due to the potential for false positives in conditions such as chronic obstructive pulmonary disease and allergic rhinitis, results should be interpreted in the context of the overall clinical picture. Emergency resuscitation equipment and trained personnel must be present during testing to manage potential bronchospasm.

Effective methacholine challenge testing relies on a coordinated interprofessional approach to ensure patient safety, diagnostic accuracy, and optimal clinical outcomes. Clinicians and advanced practitioners play a central role in determining the appropriateness of the test, educating patients about its purpose, and interpreting the results within the broader clinical picture. These healthcare professionals must manage acute bronchospasm or adverse effects that may occur during the procedure. Nurses and respiratory therapists play a critical role in patient preparation, ensuring that they clearly communicate instructions and that patients perform reproducible spirometric maneuvers, which are essential for test validity. These team members also continuously monitor the patient throughout the procedure for signs of respiratory compromise and must be available to respond promptly to emergencies. Pharmacists contribute by ensuring the availability of methacholine at the appropriate dilutions, reviewing medications that may interfere with testing, and ensuring the availability of rescue agents, such as short-acting beta-agonists. Transparent, real-time communication among all healthcare professionals is crucial for coordinating care, managing risks, and optimizing patient outcomes. A shared understanding of each team member's role fosters mutual accountability, enhances team performance, and supports patient-centered care throughout the testing process.

Nursing, Allied Health, and Interprofessional Team Interventions

Clinicians perform methacholine challenge testing with the support of a trained nurse who plays a key role in ensuring patient safety and comprehension. Before the procedure, the nurse confirms that informed consent is signed, the patient understands the purpose and process of the test, and that all necessary resuscitation equipment, including bronchodilators, is readily available in the testing area. Throughout the procedure, the nurse closely monitors the patient's vital signs and observes for any signs of respiratory distress. After the procedure, the nurse continues to monitor the patient and provides clear instructions on recovery and self-care. The nurse advises the patient on what to expect after the test and when to resume regular medications. Patients should also be educated on warning signs of worsening respiratory symptoms and instructed to contact their healthcare provider or seek immediate medical attention if they experience increased shortness of breath, wheezing, chest tightness, or reduced responsiveness to usual asthma medications. This guidance helps ensure continuity of care and promotes patient safety following bronchoprovocation testing.

Nursing, Allied Health, and Interprofessional Team Monitoring

In addition to monitoring the patient's vital signs and oxygenation, the nurse is responsible for thoroughly documenting the methacholine challenge test. Proper documentation includes noting the start time of the procedure, any medications administered, and the conclusion time of the test. The nurse must also provide detailed documentation of the patient's respiratory status throughout the procedure and recovery period, capturing any changes or symptoms observed to ensure accurate clinical records and continuity of care.