A 28-year-old woman presents with a 3-month history of intermittent dyspnoea and dry cough, worse at night and with exercise. Clinical examination is unremarkable. Baseline spirometry shows FEV1 87% predicted with normal FEV1/FVC ratio. A methacholine bronchoprovocation test is performed following standard ATS/ERS protocol. The result marked **A** in the diagram (PC20 ≤ 4 mg/mL) is obtained. Which of the following best describes the clinical significance of this finding?
A. Suggests borderline airway hyperresponsiveness; the result is non-specific and could reflect recent viral infection or allergic rhinitis alone
B. Demonstrates that the patient requires immediate intubation due to severe bronchial obstruction
C. Confirms asthma diagnosis; the patient's airways are hyperresponsive to methacholine at concentrations consistent with mild airway hyperresponsiveness
D. Indicates a negative test; the patient's airways are normal and asthma is excluded
Explanation
Why "Confirms asthma diagnosis; the patient's airways are hyperresponsive to methacholine at concentrations consistent with mild airway hyperresponsiveness" is right
The methacholine challenge test is the gold-standard direct airway hyperresponsiveness (AHR) test for asthma, especially when baseline spirometry is normal or equivocal. A PC20 ≤ 4 mg/mL (the threshold marked A) indicates that the patient's airways bronchoconstricted at a low methacholine concentration, demonstrating intrinsic hyperresponsiveness. According to ATS/ERS 2017 standards, PC20 1–4 mg/mL is classified as mild AHR and is consistent with asthma. In this patient with normal baseline spirometry but typical asthma symptoms (nocturnal cough, exercise-induced dyspnoea), a positive methacholine challenge with PC20 ≤ 4 mg/mL confirms the diagnosis of asthma and justifies initiation of anti-inflammatory therapy.
Why each distractor is wrong
"Indicates a negative test; the patient's airways are normal and asthma is excluded": This reverses the interpretation. A negative methacholine challenge is defined as PC20 >16 mg/mL (marked B in the diagram), which has high negative predictive value and excludes asthma. The result marked A is positive and confirms AHR.
"Suggests borderline airway hyperresponsiveness; the result is non-specific and could reflect recent viral infection or allergic rhinitis alone": Borderline AHR is defined as PC20 4–16 mg/mL (marked B range), not ≤4 mg/mL. While PC20 4–16 mg/mL can be seen in allergic rhinitis or post-viral states, a PC20 ≤ 4 mg/mL in the context of typical asthma symptoms is highly specific for asthma diagnosis.
"Demonstrates that the patient requires immediate intubation due to severe bronchial obstruction": The test is designed to provoke a 20% fall in FEV1 and is immediately reversed with inhaled salbutamol. Severe AHR is PC20 <1 mg/mL; this patient has mild AHR (PC20 1–4 mg/mL). The test is safe and does not cause life-threatening obstruction when conducted per protocol.
