118. Pulm PEEPs Pearls: Methacholine Challenge

Furf and Monty are back with another Pulm PEEPs Pearls episode. The topic of today’s discussion is an often discussed, but often misunderstood, test; the methacholine challenge. They’ll review when to utilize this test, how it should be performed, and the appropriate interpretation. Contributors This episode was prepared with research by Pulm PEEPs Associate Editor George Doumat. Dustin Latimer, another Pulm PEEPs Associate Editor, assisted with audio and video editing. Key Learning Points What the Test Measures Methacholine challenge is a direct bronchial provocation test of airway hyperresponsiveness (AHR), a core physiologic feature of asthma. Anyone will bronchoconstrict at high enough concentrations — the test looks for an abnormal threshold. The key endpoint is the PC20: the methacholine concentration causing a 20% fall in FEV1. Abnormal in adults: PC20 ≤ 8–16 mg/mL Test Performance Meta-analyses: pooled sensitivity ~60%, specificity ~90%. Real-world cohorts: sensitivity 55–62%, specificity 56–100% (varies by population, protocol, and threshold used). Not a standalone yes/no test — best used as part of a broader diagnostic pathway. Where It Fits in the Asthma Workup The test belongs in a stepwise approach: Step 1: Spirometry + bronchodilator response Step 2: Add FeNO and/or peak flow variability (if available) Step 3: If the picture is still unclear → methacholine challenge It is most useful for symptomatic patients with normal spirometry and no bronchodilator reversibility. Given its cost, mild risk, and discomfort, it should not be a first-line test — most asthma diagnoses do not require it. Technique and Medication Prep Technique ERS guidelines favor tidal breathing over deep inspiratory maneuvers. Deep breaths can be bronchoprotective and blunt the response, reducing sensitivity — especially in mild or well-controlled asthma. Medication Washout (to Avoid False Negatives) Medication ClassWashout PeriodShort-acting beta-agonists (SABA)≥ 6 hoursLong-acting beta-agonists (LABA)~24 hoursUltra-long-acting beta-agonists~48 hoursShort-acting anticholinergics (e.g., ipratropium)~12 hoursLong-acting muscarinic antagonists (LAMA, e.g., tiotropium)7 days Inhaled corticosteroids, leukotriene blockers, and antihistamines do not significantly affect the test acutely — continue these. Withdrawing ICS also carries its own risk for asthma patients. Practical tip: Spell out exactly what to hold and when — for both the patient and the PFT lab — at the time the test is ordered. Interpreting Results Negative Test (PC20 > 16 mg/mL) Very high negative predictive value in symptomatic adults. Makes current asthma quite unlikely (assuming proper test conduct). This is the test’s greatest strength: it is an excellent rule-out test. Positive Test (PC20 ≤ 8–16 mg/mL) More nuanced — airway hyperresponsiveness is not unique to asthma. Can be positive in: chronic cough, allergic rhinitis, COPD, and even some healthy asymptomatic individuals. A positive result raises probability but must be interpreted alongside the clinical story, variable respiratory symptoms, peak flow variability, FeNO, and ICS response. Safety and Risks Overall, the test is quite safe; significant adverse effects are rare. Temporary breathing discomfort is expected (bronchoconstriction is being induced). Severe bronchospasm is possible: A trained clinician should be available; SABA inhaler/nebulizer must be immediately on hand; a physician should be reachable in the facility. Contraindications / caut