What is the next diagnostic test for a 30-year-old woman with a severe dry cough, initially diagnosed with allergic rhinitis and dust allergy, who showed no improvement with intranasal steroids and had a forced expiratory volume (FEV1) to forced vital capacity (FVC) ratio less than 70% that increased by 20% after bronchodilator use?

Methacholine Challenge Test is the Next Diagnostic Test for This Patient

The methacholine challenge test (option B) is the most appropriate next diagnostic test for this 30-year-old woman with severe dry cough, abnormal spirometry showing FEV1/FVC ratio <70%, and 20% improvement after bronchodilator use.

Diagnostic Reasoning

This patient presents with:

• Severe dry cough occurring in attacks
• Initial diagnosis of allergic rhinitis with dust allergy
• No improvement with intranasal steroids
• Spirometry showing FEV1/FVC ratio <70% (obstructive pattern)
• 20% improvement in ratio after bronchodilator administration

Interpretation of Current Findings

1. Obstructive Pattern: The FEV1/FVC ratio <70% indicates airflow obstruction, consistent with asthma or COPD 1

2. Bronchodilator Response: A 20% improvement after bronchodilator administration suggests reversible airway obstruction, highly suggestive of asthma 1

3. Diagnostic Uncertainty: Despite these findings, confirmation of asthma diagnosis requires additional objective testing according to current guidelines 1

Why Methacholine Challenge Test is Indicated

The methacholine challenge test is specifically designed to:

• Assess bronchial hyperresponsiveness, a hallmark feature of asthma 1, 2
• Provide high negative predictive value (effectively ruling out asthma if negative) 2
• Confirm the diagnosis when clinical presentation and initial testing suggest asthma but are not definitive 1, 2

According to the American Thoracic Society guidelines, methacholine challenge testing is indicated when:

• Asthma is suspected but traditional diagnostic methods have not established or eliminated the diagnosis 1
• There is a need for objective confirmation before initiating long-term therapy 1

Why Other Options Are Less Appropriate

• Skin allergy test (A): While useful for identifying specific allergens in allergic asthma, it doesn't confirm the diagnosis of asthma itself. The European Respiratory Society guidelines recommend confirming asthma with objective tests of airway function first 1

• IgE antibody level (C): Has low sensitivity and moderate specificity for asthma diagnosis 3. It may identify atopy but doesn't confirm bronchial hyperreactivity

• Chest CT (D): Not indicated as a primary diagnostic tool for asthma. It's more useful for excluding alternative diagnoses when initial testing is inconclusive or response to therapy is poor 1

Methacholine Challenge Test Procedure

The test involves:

1. Baseline spirometry measurement
2. Inhalation of diluent (control)
3. Sequential inhalation of increasing concentrations of methacholine
4. Measurement of FEV1 after each concentration
5. Test is positive if FEV1 decreases by ≥20% from baseline 4, 5

Interpretation of Results

• Positive test: A decrease in FEV1 ≥20% at low methacholine concentrations strongly suggests asthma 5
• Negative test: Effectively rules out asthma due to high negative predictive value 2, 6

Important Considerations

• The test should be performed when the patient is clinically stable and free from respiratory infections 1
• Certain medications should be withheld before testing:
  • Short-acting bronchodilators (6 hours)
  • Long-acting β-agonists (12 hours)
  • Sustained-release theophyllines (24 hours) 1
• Safety measures should be in place, including availability of rescue bronchodilators 4, 5

Conclusion

Given this patient's clinical presentation, spirometry findings showing obstruction with bronchodilator reversibility, and lack of response to intranasal steroids, the methacholine challenge test is the most appropriate next step to confirm the diagnosis of asthma and guide appropriate treatment.