A 28-year-old woman with suspected asthma presents with episodic dyspnea and cough. Clinical examination is unremarkable. Which investigation is the gold standard to confirm the diagnosis of asthma?

Explanation

## Diagnosis of Asthma: Investigation Hierarchy **Key Point:** Spirometry with bronchodilator reversibility testing is the gold standard for confirming asthma diagnosis in a symptomatic patient with normal baseline spirometry. ### Why Spirometry with Bronchodilator Reversibility? Asthma is characterized by **reversible airflow obstruction**. The diagnostic criterion is: - FEV~1~ improvement of ≥12% AND ≥200 mL after inhaled short-acting β~2~-agonist (SABA) - This demonstrates the hallmark reversibility that distinguishes asthma from COPD **High-Yield:** In patients with: - Normal baseline spirometry but high clinical suspicion → proceed to bronchodilator reversibility test - Baseline FEV~1~/FVC <0.70 → reversibility test confirms asthma (vs. fixed obstruction in COPD) ### Investigation Comparison Table | Investigation | Utility | Sensitivity | Specificity | When Used | | --- | --- | --- | --- | --- | | Spirometry + bronchodilator | Gold standard for diagnosis | 60–80% | 90%+ | Symptomatic patient, suspected asthma | | Methacholine challenge | Confirms hyperresponsiveness | High (>90%) | Moderate (60–70%) | Normal spirometry + high clinical suspicion | | Peak flow monitoring | Demonstrates variability | Moderate | Low | Monitoring, not diagnosis | | Chest X-ray | Excludes other pathology | — | — | Rule out alternative diagnoses | **Clinical Pearl:** Methacholine challenge is more sensitive but less specific; it detects airway hyperresponsiveness, not asthma per se. A positive test does not confirm asthma—it is a **rule-out test** (negative result excludes asthma with high confidence). **Mnemonic: SABA-REV** — **S**hort-acting **B**ronchodilator **A**gent shows **REV**ersibility in asthma. ### Diagnostic Algorithm ```mermaid flowchart TD A[Suspected asthma on clinical grounds]:::outcome --> B[Perform spirometry]:::action B --> C{FEV1/FVC normal?}:::decision C -->|Yes| D[Give SABA, repeat spirometry]:::action C -->|No| E[Baseline obstruction present]:::outcome D --> F{FEV1 improves ≥12% AND ≥200 mL?}:::decision F -->|Yes| G[Asthma diagnosed]:::outcome F -->|No| H[Consider methacholine challenge]:::action E --> I[Assess reversibility]:::action I --> J{Reversible?}:::decision J -->|Yes| G J -->|No| K[COPD or fixed obstruction]:::outcome ``` [cite:Harrison 21e Ch 297]