A 28-year-old woman presents with a 3-month history of intermittent nocturnal cough and dyspnea on exertion. She denies wheeze. Baseline spirometry shows FEV1 88% predicted with FEV1/FVC ratio 0.78. A methacholine challenge test is performed. The result marked **C** in the diagram — PC20 <8 mg/mL — is obtained. Which of the following best explains the diagnostic significance of this finding in the context of her clinical presentation?

Explanation

## Why PC20 <8 mg/mL indicates airway hyperresponsiveness and strongly supports an asthma diagnosis is right The methacholine challenge test (MCT) is the gold standard for diagnosing asthma in patients with suggestive symptoms but normal baseline spirometry where bronchodilator reversibility cannot be demonstrated (Harrison 21e Ch 287). Methacholine is a synthetic muscarinic agonist that binds airway smooth muscle M3 receptors, causing bronchoconstriction. Asthmatics exhibit airway hyperresponsiveness (AHR) — they constrict airways at much lower methacholine doses than normal individuals. PC20 is the provocative concentration causing a 20% FEV1 fall. A PC20 <8 mg/mL (encompassing <1, 1–4, and 4–8 mg/mL ranges) indicates mild to moderate-severe AHR and strongly supports asthma diagnosis in the appropriate clinical context (ATS Methacholine Challenge Guidelines 2017). This patient's normal baseline spirometry, compatible symptoms (nocturnal cough, dyspnea on exertion), and positive MCT result together establish asthma as the diagnosis. ## Why each distractor is wrong - **PC20 <8 mg/mL is diagnostic of COPD**: COPD is diagnosed by fixed airflow obstruction (FEV1/FVC <0.70 persisting after bronchodilators) on baseline spirometry, not by MCT. This patient's FEV1/FVC is 0.78 (normal), excluding COPD. While COPD patients may show AHR, MCT is not used to diagnose COPD. - **PC20 <8 mg/mL indicates normal airway responsiveness and effectively excludes asthma**: This reverses the interpretation. PC20 >16 mg/mL indicates normal responsiveness and excludes asthma (NPV ~95%). PC20 <8 mg/mL is abnormal and indicates AHR, supporting asthma diagnosis. - **PC20 <8 mg/mL is pathognomonic for allergic rhinitis**: Allergic rhinitis is a nasal/sinus condition diagnosed clinically and by allergy testing. While allergic rhinitis may coexist with asthma and can cause AHR, MCT result alone does not diagnose allergic rhinitis. MCT has moderate positive predictive value for asthma; other conditions (allergic rhinitis, post-viral, smokers) can also cause AHR, but in this patient's clinical context (nocturnal cough, dyspnea on exertion, normal spirometry), asthma is the most likely diagnosis. **High-Yield:** PC20 <8 mg/mL = airway hyperresponsiveness supporting asthma in normal baseline spirometry; PC20 >16 mg/mL = normal, excludes asthma (high NPV); always hold ICS 1 week before MCT to avoid false negatives. [cite: Harrison 21e Ch 287; ATS Methacholine Challenge Guidelines 2017]