How is a methacholine challenge test performed to assess airway hyper‑responsiveness in suspected asthma with normal baseline spirometry?

Methacholine Challenge Test: Procedure and Performance

What Happens During the Test

A methacholine challenge test involves inhaling progressively increasing concentrations of methacholine aerosol while repeatedly measuring lung function (spirometry) after each dose until airways narrow by ≥20% or the maximum dose is reached. 1, 2

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Pre-Test Requirements

Baseline Spirometry

• Perform at least 3 acceptable FEV₁ maneuvers with the best two values agreeing within 0.10-0.20 L 2, 3
• Record the highest FEV₁ value as baseline 2, 3
• Baseline FEV₁ must be ≥60% predicted and ≥1.5 L in adults – values below this are absolute contraindications 1, 2, 3

Medication Withholding

• Short-acting β₂-agonists: withhold 8 hours 2
• Long-acting β₂-agonists: withhold 48 hours 2
• Theophylline: withhold 24-48 hours 2
• Anticholinergics: withhold 24 hours 2
• Testing should occur within 24 hours of workplace exposure for occupational asthma evaluation 1

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Step-by-Step Test Procedure

Step 1: Diluent (Saline) Inhalation

• Inhale nebulized saline as a control 1, 3
• Measure FEV₁ at 30 and 90 seconds post-inhalation 2, 3
• If FEV₁ falls ≥20% after diluent alone, this constitutes a positive test – stop and do not proceed with methacholine 2, 3
• If FEV₁ falls <20%, use the higher value between baseline and post-diluent as the reference for calculating subsequent declines 3, 4

Step 2: Incremental Methacholine Dosing

Two standardized protocols are endorsed by the American Thoracic Society 1, 2:

5-Breath Dosimeter Method:

• Patient inhales 5 breaths of methacholine at each concentration 2, 3
• Concentrations: 0.0625,0.25,1.0,4.0,16.0 mg/mL 2, 3

2-Minute Tidal Breathing Method:

• Patient breathes normally through nebulizer for 2 minutes at each concentration 2, 3
• Same concentration progression as dosimeter method 2, 3

Step 3: Spirometry After Each Dose

• Measure FEV₁ at 30 seconds and 90 seconds after completing each methacholine inhalation 2, 3
• Only acceptable FEV₁ measurements are required (complete FVC maneuvers are unnecessary during testing) 4
• Calculate percent fall: [(post-diluent FEV₁ - lowest post-methacholine FEV₁) / post-diluent FEV₁] × 100 1, 3
• Stop immediately if FEV₁ falls ≥20% from post-diluent baseline 2, 3
• Stop if maximum concentration (16 mg/mL) is reached 2, 3
• Interval between consecutive doses should not exceed 3 minutes 4

Step 4: Bronchodilator Administration and Recovery

• Administer 200-400 µg albuterol (2-4 puffs) immediately after reaching positive threshold 2, 3
• Wait 10 minutes and remeasure FEV₁ 3
• Patient cannot leave until FEV₁ recovers to within 90% of baseline 2, 3
• If recovery is inadequate after 10 minutes, repeat bronchodilator or add ipratropium 4
• Most patients recover within 5 minutes with bronchodilator; without bronchodilator, recovery takes 30-45 minutes 3

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Test Interpretation

Positive Test Definition

• ≥20% reduction in FEV₁ from post-diluent baseline 2, 3
• PC₂₀ (provocative concentration causing 20% fall) is calculated by linear interpolation between the last two concentrations 1, 3

Severity Grading (American Thoracic Society) 1, 2

PC₂₀ (mg/mL)	Interpretation
>16	Normal (negative test)
4-16	Borderline hyperresponsiveness
1-4	Mild hyperresponsiveness (positive)
<1	Moderate-severe hyperresponsiveness

• For children, the European Respiratory Society uses PC₂₀ ≤8 mg/mL as the positive threshold 2

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Safety Equipment and Monitoring

Required Emergency Equipment 1, 2

• Rapid-acting inhaled bronchodilators (albuterol, ipratropium) immediately available
• Supplemental oxygen
• Small-volume nebulizer
• Pulse oximeter and blood pressure monitor
• Stethoscope

Common Adverse Effects

• Transient cough, wheezing, dyspnea, chest tightness occur in 20-25% of patients 2
• Symptoms are mild, last only minutes, and resolve with bronchodilator 1
• Severe bronchoconstriction is rare but requires immediate bronchodilator treatment 1, 3

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Critical Pitfalls to Avoid

• Never perform the test if baseline FEV₁ <60% predicted – this is an absolute contraindication that increases risk without diagnostic benefit 1, 2, 3
• Inadequate medication withholding produces false-negative results 2
• Poor-quality spirometry maneuvers yield unreliable measurements 1, 2
• Testing rooms require adequate ventilation to minimize technician aerosol exposure 1
• A positive test does not automatically confirm asthma – bronchial hyperresponsiveness occurs in COPD, heart failure, allergic rhinitis, and other conditions 1
• The test has excellent negative predictive value but mediocre positive predictive value – it is more useful for excluding asthma than confirming it 1