In a 49-year-old woman with a 4‑month history of dry cough and paroxysmal coughing spells, normal chest radiograph, and spirometry showing a reduced Forced Expiratory Volume in 1 second/Forced Vital Capacity ratio (~70%) that improves >20% after bronchodilator, which diagnostic test should be performed next?

Diagnosis: Cough-Variant Asthma - No Further Testing Required

This patient has already confirmed asthma based on spirometry showing significant bronchodilator reversibility (20% improvement in FEV1/FVC ratio), and no additional diagnostic testing is needed. The spirometry results definitively establish the diagnosis, making options A through D unnecessary.

Why the Diagnosis is Already Confirmed

• The 20% improvement in FEV1/FVC ratio after bronchodilator exceeds the diagnostic threshold for reversible airway obstruction 1, 2
• The European Respiratory Society defines bronchodilator reversibility as ≥12% increase AND ≥200 mL absolute increase in FEV1 or FVC from baseline 1, 2, 3
• This patient's 20% improvement substantially exceeds the 12% threshold, confirming reversible airway disease consistent with asthma 1

Clinical Context Supporting Asthma Diagnosis

• Paroxysmal coughing spells lasting 4 months with normal chest radiograph are classic for cough-variant asthma 1
• The European Respiratory Society guidelines recommend that asthma diagnosis requires at least two objective abnormalities: this patient has (1) reduced FEV1/FVC ratio and (2) significant bronchodilator reversibility 1
• The baseline FEV1/FVC ratio of 70% represents mild airflow obstruction that reversed substantially with bronchodilator, distinguishing this from COPD 1, 4

Why Each Listed Test is Inappropriate

Methacholine Challenge (Option D) - Not Indicated

• Bronchoprovocation testing is only recommended when spirometry is NORMAL and asthma is still suspected 1, 5, 6
• The American Family Physician guidelines state methacholine challenge should be considered "if pulmonary function test results are normal, but the physician still suspects exercise- or allergen-induced asthma" 6
• This patient already has abnormal spirometry with documented reversibility, making challenge testing redundant 1

Allergy Skin Testing (Option A) - Not Diagnostic

• Allergy testing does not diagnose asthma; it only identifies potential triggers in patients with already-confirmed asthma 1
• The diagnosis of asthma is clinical and spirometric, not based on atopy 1

High-Resolution CT (Option B) - Not Indicated

• Chest CT is not part of the asthma diagnostic pathway for patients with typical presentation and confirmatory spirometry 1
• Normal chest radiograph already excludes structural lung disease 1

Serum IgE Levels (Option C) - Not Diagnostic

• IgE levels do not diagnose asthma and are not recommended in the diagnostic algorithm 1

Common Pitfall to Avoid

The critical error would be ordering additional tests when the diagnosis is already established by spirometry. The 20% bronchodilator response is highly specific for asthma (specificity 90-98%) 2. The European Respiratory Society explicitly recommends against empiric treatment trials and emphasizes objective confirmation, which this patient already has 1.

Appropriate Next Step

The correct management is to initiate asthma controller therapy (inhaled corticosteroids) at GINA step 2 or 3 based on symptom severity, not to perform additional diagnostic testing 1. The patient's 4-month history of symptoms interfering with conversation suggests at least moderate severity requiring anti-inflammatory therapy 1.