A 28-year-old woman presents with a 6-month history of intermittent dyspnea and cough, worse at night and with exercise. Baseline spirometry is entirely normal (FEV1 98% predicted, FEV1/FVC 0.82). A histamine bronchial provocation test is performed. The patient develops a 20% fall in FEV1 when exposed to a histamine concentration of 2.5 mg/mL, as marked by **A** in the diagram. Which of the following best describes the clinical significance of this finding?

Explanation

## Why option 1 is correct The structure marked **A** represents PC20 (histamine concentration causing 20% fall in FEV1) <4 mg/mL, which defines airway hyperresponsiveness (AHR) consistent with asthma. In this patient, PC20 = 2.5 mg/mL falls within the range of 1–4 mg/mL (mild AHR). According to ATS/ERS standards and GINA 2024, a positive histamine challenge test (PC20 <4 mg/mL) in a symptomatic patient with normal baseline spirometry and negative bronchodilator reversibility has high sensitivity (90–95%) and high negative predictive value for asthma. This is precisely the clinical scenario for which BPT is indicated—to unmask AHR when baseline spirometry is deceptively normal. The finding directly supports an asthma diagnosis. ## Why each distractor is wrong - **Option 2**: While methacholine is the gold standard in some regions, histamine is equally valid and widely used internationally, including in India. A PC20 of 2.5 mg/mL on histamine is diagnostic of AHR; repeat testing is unnecessary. The histamine cutoff (<4 mg/mL) is well-established in ATS/ERS and GINA guidelines. - **Option 3**: COPD is characterized by irreversible airflow obstruction on baseline spirometry. This patient has normal baseline FEV1 (98% predicted) and normal FEV1/FVC ratio (0.82), ruling out COPD. BPT positivity can occur in COPD but is not diagnostic of it in isolation. - **Option 4**: This is the opposite of the correct interpretation. PC20 >16 mg/mL indicates normal airway responsiveness and makes asthma unlikely. A PC20 of 2.5 mg/mL is abnormally low and indicates heightened airway responsiveness—a cardinal feature of asthma. **High-Yield:** PC20 <4 mg/mL on histamine challenge = airway hyperresponsiveness = asthma diagnosis supported, especially when baseline spirometry is normal and clinical suspicion is high. [cite: ATS/ERS Technical Standard on Bronchial Challenge Testing, Coates et al. Eur Respir J 2017; GINA 2024; Crapo et al. Am J Respir Crit Care Med 2000]