Objectives

1. Learn the clinical indications for a methacholine challenge test (MCT).
2. Avoid factors that reduce MCT safety.
3. Summarize the procedures for performing a MCT.
4. Identify factors that influence bronchial responsiveness.
5. Understand pitfalls in the interpretation of MCT results.

Abbreviations

ATS = American Thoracic Society; BHR = bronchial hyperresponsiveness; MCT = methacholine challenge test; PC₂₀= provocative concentration for a 20% fall in FEV₁; PCP = primary care physician

Bronchial hyperresponsiveness (BHR) is a cardinal feature of asthma (when untreated and when poorly controlled). BHR may vary over time, often increasing during exacerbations and decreasing when factors causing airways inflammation are avoided and during treatment with antiinflammatory medications. A MCT is the most commonly performed test of airways responsiveness. Methacholine and histamine are categorized as direct (pharmacologic) tests, since they act directly on airway smooth muscle. Indirect bronchial provocation tests, which depend on the presence of mediators in the airways, are less commonly used in clinical practice and include the use of exercise, eucapnic voluntary hyperventilation, hypertonic aerosols, cold air, adenosine monophosphate, and mannitol.^1-3 The American Thoracic Society (ATS) has published clinical practice guidelines for the MCT, which are widely accepted and followed in this lesson.⁴

Indications and MCT Safety

An MCT should be considered when a patient has symptoms suggesting asthma but has normal spirometry test results (little, if any, airways obstruction). A history of asthma-like symptoms (such as episodic wheezing, cough, chest tightness, and/or dyspnea) increases the pretest probability of asthma. The highest combination of positive and negative predictive power for the MCT occurs when the pretest probability of asthma is about 50:50. The MCT may also be used in the evaluation of workplace asthma, to determine the risk of developing asthma and in research settings to assess the severity of asthma and the response to asthma therapy.^5-7 The MCT may be useful in children as young as age 6,⁸ as well as in elderly patients.⁹ The MCT is more sensitive and more specific for asthma than indices of peak flow variability¹⁰ and exercise testing.¹¹

A worldwide experience, of hundreds of thousands of tests, has shown the MCT to be very safe. However, severe bronchospasm may rarely occur due to a technician mistakenly administering the highest concentration (16 to 25 mg/mL) first to a patient with severe BHR. Any bronchospasm induced by the methacholine lasts < 1 h (even if untreated) and is fully reversed within minutes after albuterol treatment. It is not necessary to monitor BP, heart rate, or oxygen saturation during the test. About two thirds of patients receiving the MCT report no side effects during or after the procedure. However, the test does require repeated athletic-like breathing maneuvers, therefore, a few patients consider it a rather unpleasant experience. Technicians performing the test should be trained and observed while performing at least their first 20 MCTs. Their own provocative concentration for a 20% fall in FEV₁ (PC₂₀ ) should be measured before they test patients, and exposure of technicians or nurses to methacholine aerosol should be minimized.

In patients with asthma-like symptoms, if baseline spirometry shows airways obstruction (airflow limitation) and there is a significant bronchodilator response, the diagnosis of asthma is usually confirmed, and MCT is usually unnecessary. Airways obstruction is a relative contraindication for an MCT, since the overall risk of serious adverse events is small, even in patients with asthma with severe airways obstruction. However, it is difficult to interpret a "positive" result when baseline spirometry shows airways obstruction. The MCT may temporarily cause ventilation/perfusion mismatching if the induced bronchospasm is severe, which can result in arterial hypoxemia and compensatory changes in BP, cardiac output, and heart rate. One study found no significant increase in the number of supraventricular and ventricular arrhythmias during MCTs.¹² However, the additional cardiovascular stress of induced bronchospasm, theoretically, has the potential to precipitate cardiovascular events in patients with uncontrolled hypertension, recent heart attack, or stroke. Thus, these factors are contraindications for the MCT.⁴

The Test Procedure

When the test is ordered, the patient should be told to avoid medications and factors listed in the Tables 1 and 2. When the patient arrives for the test, a pretest questionnaire should be completed, which will be used to determine relative contraindications and the presence of factors that are likely to temporarily modify bronchial responsiveness. Methacholine solutions should be mixed, clearly labeled by a pharmacist, and stored in a refrigerator but brought to room temperature before use. Concentrations should be mixed to obtain "doubling doses" of methacholine solution in the range of 0.03 to 16 mg/mL. Either of two techniques for administering each dose may be used: the 2-min tidal breathing protocol or the 5-breath dosimeter protocol. In theory, the expensive dosimeter delivers more accurate doses of methacholine to the airways, but, in practice, the two methods give equivalent results.

Standardized, detailed, step-by-step methods are given in the ATS guidelines. In summary, the patient is seated during the entire test, which may take up to 45 min. Baseline spirometry is performed with enough good quality FVC maneuvers to ensure accurate and repeatable results. If airways obstruction with FEV₁ <70% predicted is detected by the baseline test, a bronchodilator is usually given, and the MCT is abandoned. If good quality spirometry results were obtained during the baseline test, a diluent "dose" is unnecessary, because the rare fall in FEV₁ following inhalation of a diluent solution has unknown clinical meaning. The first (lowest) concentration (dose) of methacholine is administered with the FEV₁ measured at 30 and 90 s following the end of the dose. Additional FEV₁ maneuvers may be needed to obtain two FEV₁ measurements that match closely. At each dose, the highest FEV₁ from an acceptable maneuver is recorded. If the FEV₁ falls >20% from baseline, no further methacholine doses are given, any physical signs or symptoms are noted, and inhaled albuterol is administered. The FEV₁ should return to >90% of the baseline value before the patient is allowed to leave the testing area.

Quality Assurance

Change in FEV₁ is the primary outcome measure for an MCT. Special care should be taken to obtain high-quality baseline FEV₁ measurements, since unacceptable maneuvers may result in false-positive or false-negative results (misclassification). The attention of the technician to the details of administering the methacholine and obtaining optimal quality FEV₁ results is the most important factor in reporting accurate results. Maneuvers with hesitating starts (a back-extrapolated volume >150 mL) and poor blast efforts (time to peak flow >150 ms) should be identified as unacceptable. Full FVC efforts lasting at least 6 s with a flat volume-time plateau should be performed at baseline. Since FEV₁ is the only outcome being measured, the expiratory maneuver can be shortened to 1 to 2 s for the remainder of the test. Technicians should take considerable care to enthusiastically coach patients for deep inhalations, since incomplete inhalation (just prior to the forced exhalation) is the most common cause of a false reduction in FEV₁.

High quality test sessions increase confidence in the interpretation. Spirometry quality grades should be reported after each dose, as follows:

1. = Two acceptable FEV₁ measurements that match within 0.10 L.
2. = Two acceptable FEV₁ measurements that match within 0.20 L.
3. = Two acceptable FEV₁ measurements that do not match within 0.20 L.
4. = Only one acceptable FEV₁ maneuver.
5. = No acceptable FEV₁ maneuvers.

If vocal cord dysfunction is suspected, it may be helpful to record complete flow-volume loops (both forced exhalation and forced inhalation maneuvers) at baseline and after the highest dose of methacholine.¹³ The use of alternative lung function outcome variables (such as tests of airways resistance or so-called tests of small airways function) for inhalation challenge testing of adults is discouraged.

How To Interpret the Test Results

Always take into consideration the following factors when interpreting MCT results:

• The pretest probability of asthma, including current asthma symptoms.
• The presence or degree of baseline airways obstruction.
• The quality grades of the spirometry maneuvers.
• The pretest questionnaire results.
• The symptoms reported by the patient at the end-of-test.
• The degree of recovery following administration of a bronchodilator.
• The overall sensitivity and specificity of MCTs.
• The MCT repeatability (noise of measurement).

Using the degree of airways responsiveness to answer questions about individual patients assumes that the test was properly performed, no effect modifiers were present, and the patient's prior probability of having current asthma was estimated. If the prior probability of asthma is 30 to 70% and the PC₂₀ is >16 mg/mL, it may be stated with a high degree of confidence that the patient does not currently have asthma. If the same patient has a PC₂₀ of < 1 mg/mL, the result confirms the pretest clinical suspicion of asthma. However, when the PC₂₀ is between 1 and 16 mg/mL, the results are uncertain.

In people with a PC₂₀ between 1 and 8 mg/mL, who have no asthma symptoms (an unusual circumstance since MCT would not be clinically indicated in such a setting), several possibilities exist: (1) the patient has mild intermittent asthma but is a "poor perceiver" of asthma symptoms; (2) the patient experiences episodes of chest tightness that are perceived but not recognized as abnormal; (3) the patient never exercises or experiences environmental triggers of bronchospasm; (4) the mild BHR is due to a cause other than asthma (postviral upper respiratory infection, cigarette smoking, etc); or (5) there is subclinical (asymptomatic) asthma that will become clinical asthma in the future.

In groups of patients who are known to have asthma before the test, the correlation between PC₂₀ and the clinical severity of asthma is statistically significant but not strong. Recent exposures causing airways inflammation or residual effects of inhaled corticosteroids,¹⁴ prednisone, or a leukotriene antagonist¹⁵ can easily change the degree of airways responsiveness so that the PC₂₀ does not reflect the "usual" untreated severity of the patient's asthma (Tables 1-2).

It is difficult to interpret the meaning of a low PC₂₀ in a patient with moderate to severe baseline airways obstruction. For instance, most patients with COPD have BHR, but most have no symptoms of asthma and little, if any, response to bronchodilator therapy.¹⁶ It is even more difficult to interpret the significance of a change in PC₂₀ when there has also been a change in baseline FEV₁ (which often occurs after successful therapy).¹⁷ The most common clinical indication for an MCT is to evaluate the likelihood of asthma in patients in whom the diagnosis is suggested, but not obvious, by current symptoms. The continuous nature of airways responsiveness and the overlap in PC₂₀ between the response of persons with healthy lungs and patients who have unequivocal asthma require a decision analysis (formal or intuitive).¹⁸

The pretest probability (prior probability) is the likelihood that the patient has asthma before MCT results are considered. The posttest probability is the likelihood of asthma considering both the pretest probability and MCT results (posterior probability). The difference between the pretest and posttest probabilities represents the contribution of MCT.¹⁹ The MCT results can be helpful, of no clinical value, or misleading.²⁰

The prior probability of asthma for a given individual is equal to the prevalence of asthma when a randomly selected population sample is being tested and the subject's medical history is not considered. The prevalence of asthma is relatively low in the general population, usually about 5%, thus the pretest probability in the example is also likely to be about 5%. With pretest likelihoods in the 5 to 15% range, the pretest likelihood is a very strong determinant of the posttest likelihood of asthma.²¹

When a patient presents with symptoms suggesting asthma, the pretest probability is much higher than in the general population, and this probability is more difficult to define precisely. However, when the prior probability is between 30% and 70%, the MCT results are often clinically useful. For example, with a PC₂₀ of 1 mg/mL, the posttest likelihood of asthma is roughly 95% with pretest likelihood estimates in the midrange. If the prior probability is about one third, and the PC₂₀ is 4 mg/mL, the posttest likelihood is about 75%. The best combination of positive and negative predictive power occurs when the pretest probability of asthma is about 50:50.

The common "categorical" method for the clinical interpretation of MCTs makes three assumptions: (1) MCT results are either positive or negative for BHR; (2) asthma is either present or absent; and (3) there is a "gold standard" for diagnosing asthma. However, this method ignores the continuous spectrum of airways responsiveness, the continuous nature of the degree of uncertainty in the diagnosis of asthma, and the lack of a gold standard for the diagnosis. Using the categorical method, a negative MCT result is commonly defined as nonresponse to the highest concentration (usually 16 to 32 mg/mL). Studies using receiver-operator characteristic curves have concluded that a PC₂₀ in the range of 8 to 16 mg/mL works best to separate patients with asthma from those without asthma.

The false-positive rate for asthma is of concern when interpreting MCT results; in such cases, the PC₂₀ is <8 mg/mL, but the patient does not actually have asthma. In testing general population samples, patients with allergic rhinitis and smokers with COPD, the MCT has relatively high false-positive rates and, therefore, poor positive predictive power. See the example vignettes.

A falsely negative MCT result occurs when there was no response to the highest dose given in a patient who truly has asthma. This occurs much less frequently than do false-positive results. The negative predictive power of an MCT is very high when the pretest probability of asthma is in the range of 30 to 70%, and the patient reported asthma symptoms during the previous 2 weeks. However, two factors should be considered before accepting a negative test result as ruling out asthma: (1) airways responsiveness may have been suppressed if the patient was intensively taking antiinflammatory medications prior to the MCT; and (2) if the patient was asymptomatic for several weeks prior to the test (perhaps because the season for high aeroallergen exposures was over). An elite athlete who experiences respiratory symptoms only after strenuous exercise may also have negative MCT results but positive indirect challenge test results.²²

When an excellent quality assurance program is utilized for MCTs, studies of the short-term, within subject repeatability (1 to 8 weeks) when patients are in a stable clinical state, show that the 95% confidence interval for repeat determinations of methacholine PC₂₀ is about one doubling dose.^4,23 For example, if the PC₂₀ was measured as 4 mg/mL during a baseline clinic visit, a repeat MCT 2 weeks later will give a PC₂₀ between 2 and 8 mg/mL in 95% of cases. This estimate of the "measurement noise" for the MCT should be taken into consideration when interpreting the PC₂₀ obtained once and must be considered when interpreting changes in BHR.

Conclusion

Using MCTs may help to increase or decrease the probability of asthma in patients with a history of asthma-like symptoms but relatively normal spirometry findings. BHR is a common feature of asthma in patients whose airways inflammation is poorly controlled. The test is very safe but requires a well-trained and motivated technician to obtain accurate results. Many factors must be considered when interpreting the reported PC₂₀. It is important to estimate the pretest probability of asthma and the presence of factors that may have temporarily influenced BHR on the day of the test. The use of a single threshold for determining a positive or negative test is naïve. The astute physician will be willing to accept uncertainty in some cases.

Acknowledgment

The author thanks Sandra Anderson for her careful review and helpful suggestions.

Example Vignettes

Case 1, a 45-year-old lawyer with a life-long history of allergic rhinitis and sinusitis is referred to you by her primary care physician (PCP) with a 2-year history of chronic cough, which is worse in the summer and with exposures to her damp office building. She has no history of asthma or heartburn and has smoked for 10 years but stopped 2 years ago. She takes no medications and never exercises. Her office spirometry test results were normal (FEV₁ /FVC% = 80% and FEV₁ = 86% predicted); her chest radiograph was normal; and her physical exam findings were normal, except for some nasal congestion and postnasal drainage. She reports coughing every day last week. You perform an MCT, and, after the final concentration (16 mg/mL), her FEV₁ fell only 8%. She remained asymptomatic, except for a mild productive cough.

Interpretation: Given the patient's history of allergies and chronic cough, her pretest probability of asthma was in the middle range (40 to 60%). Her normal spirometry test results and MCT PC₂₀ was >16 mg/mL "negative" in the presence of recent symptoms, but the absence of any airways medications makes her posttest probability of asthma very low (<5%). You should tell her PCP that she probably doesn't have asthma.

Case 2, a 25-year-old pilot, is referred to you by the local Air Force base to rule out asthma, because he has applied for training to fly a new stealth fighter. He denied any history of asthma, allergies, any respiratory symptoms, or ever smoking, but his spirometry test results were interpreted as "GOLD stage II COPD" with a pre- and postbronchodilator FEV₁ /FVC% = 68% and FEV₁ = 110% predicted. You perform a MCT, and, after inhaling 8 mg/mL of methacholine, his FEV₁ fell to 78% of the baseline. Upon auscultation of his chest during forced exhalation, your nurse heard some wheezing. He denied symptoms, but was given 2 puffs of albuterol, increasing his FEV₁ to 92% of the baseline.

Interpretation: Your automated spirometer calculated the patient's PC₂₀ as 7.8 mg/mL. When using the categorical (naïve) method of interpreting MCT results, this is near the lower limit of the normal range. However, you realize that the patient's pretest probability of asthma was very low (about 7%, the same as that of the general population) and that the specificity of the MCT in such cases is mediocre, so you report his posttest probability of asthma as the same as his pretest probability (very low). His baseline spirometry results (a low ratio but high FEV₁ ) were misinterpreted as abnormal but are a normal finding in athletic young people.

Case 3, a 50-year-old accountant, is referred to you by his PCP with a 5-year history of intermittent bouts of chest tightness with shortness of breath. About 6 months ago, his PCP started him on a regimen of fluticasone 100 m g bid (which he also took yesterday), without much symptom relief. The patient has been a pack-a-day smoker since age 18 and has a chronic productive morning cough but denies any allergies, workplace exposures, or prior asthma history. He is overweight and experiences heartburn for which he takes an over-the-counter acid blocker. His only exercise is golfing (about twice per month). His baseline spirometry results in your office are normal (FEV₁ /FVC% = 74% and FEV₁ = 84% predicted), and his chest radiograph was normal. You performed an MCT, and his FEV₁ fell by 35% after the 2 mg/mL dose. He started to cough and wheeze and was given 2 puffs of albuterol through a spacer, which improved his FEV₁ back to 95% of the baseline value.

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