A 38-year-old woman with a history of allergic rhinitis and childhood asthma (remitted for 10 years) presents with a 3-month history of exertional dyspnea and dry cough. Spirometry is normal (FEV₁ 92% predicted, FEV₁/FVC 0.82). She denies current wheeze or night-time symptoms. What is the most appropriate next step in management?

Explanation

## Clinical Scenario Analysis The patient has: - **History suggestive of asthma:** allergic rhinitis, childhood asthma, exertional dyspnea, dry cough - **Normal spirometry:** FEV₁ 92% predicted, FEV₁/FVC 0.82 (both normal) - **Current symptoms:** exertional dyspnea and cough without overt airway obstruction on testing This is a classic presentation of **asthma with normal spirometry** — a common scenario in mild asthma or asthma in remission. ## Why Methacholine Challenge is the Next Step **Key Point:** Methacholine challenge test (MCT) is the gold standard investigation for diagnosing asthma in patients with **clinical suspicion but normal resting spirometry**. It demonstrates airway hyperresponsiveness (AHR), a hallmark of asthma [cite:Harrison 21e Ch 242]. **High-Yield:** MCT is safe and appropriate when FEV₁ ≥ 70% predicted. A positive result (PC₂₀ ≤ 8 mg/mL) confirms AHR and supports asthma diagnosis. Negative MCT makes asthma unlikely [cite:GINA 2023 Guidelines]. ## Diagnostic Algorithm for Suspected Asthma ```mermaid flowchart TD A[Clinical suspicion of asthma]:::outcome --> B{Spirometry}:::decision B -->|Abnormal<br/>FEV1/FVC < 0.70| C[Obstructive pattern]:::outcome C --> D[BDRT testing]:::action B -->|Normal<br/>FEV1/FVC ≥ 0.70| E{Symptoms present?}:::decision E -->|Yes| F[Methacholine challenge]:::action E -->|No| G[Asthma unlikely<br/>Reassure]:::outcome F --> H{PC20 ≤ 8 mg/mL?}:::decision H -->|Yes| I[Asthma confirmed<br/>Start ICS]:::action H -->|No| J[Asthma unlikely<br/>Consider alternatives]:::outcome ``` **Clinical Pearl:** This patient's history of allergic rhinitis, childhood asthma, and exertional symptoms with normal spirometry is textbook for **exercise-induced asthma** or **mild intermittent asthma**. MCT will likely be positive and confirm the diagnosis. ## Why Not the Other Options | Option | Why Not Appropriate | |--------|---------------------| | Reassure; no further investigation | Premature. Symptoms persist and history is highly suggestive. Diagnostic confirmation is needed before ruling out asthma. | | HRCT chest | Not indicated. No features of structural lung disease (normal spirometry, no hemoptysis, no focal findings). HRCT would expose the patient to unnecessary radiation. | | Empiric ICS therapy | While ICS is appropriate for asthma, diagnosis should be confirmed first. Empiric therapy without MCT may mask other diagnoses and is not guideline-recommended for suspected asthma with normal spirometry. |