Cough-Variant Asthma Diagnosis
A 49-year-old woman has 4 months of paroxysmal dry coughing spells that interrupt conversation. A glucocorticoid nasal spray for presumed allergic rhinitis gave no benefit. She has dust allergy. Examination shows slight nasal congestion and a dry cough, and the chest is clear. Chest radiograph is normal, and spirometry shows FEV1/FVC 0.78 with 6% change after bronchodilator. Which diagnostic test is most appropriate next?
- A.Allergy skin prick testing
- B.High-resolution CT of the chest
- C.Methacholine bronchoprovocation testingCorrect
- D.Measurement of serum total IgE
Explanation
Chronic dry cough in an atopic patient with a normal chest radiograph and non-diagnostic spirometry should be evaluated for cough-variant asthma, and methacholine bronchoprovocation is the test of choice. In cough-variant asthma the cough is often the only symptom, with no wheeze or dyspnoea, and baseline spirometry is frequently normal because airflow limitation is intermittent. Methacholine challenge detects bronchial hyperresponsiveness by measuring the provocative concentration causing a 20% fall in FEV1, and its greatest value is its very high negative predictive value: a normal challenge effectively excludes asthma as the cause of chronic cough. It is therefore reserved for exactly this situation, where asthma is suspected but spirometry is normal or borderline and bronchodilator reversibility is not diagnostic. The three commonest causes of chronic cough in a non-smoker with a normal radiograph and no ACE inhibitor use are upper airway cough syndrome, asthma including its cough variant, and gastro-oesophageal reflux, and failure of intranasal steroid therapy makes upper airway cough syndrome less likely here. Allergy skin testing and serum IgE identify atopy, which she already demonstrates clinically, but neither establishes that asthma is causing the cough. High-resolution CT is indicated for suspected interstitial lung disease or bronchiectasis, which are not suggested by a clear chest and a normal radiograph. Once a positive challenge confirms the diagnosis, an inhaled glucocorticoid for 8 weeks is both treatment and further diagnostic confirmation.
Why each option
- A.
- Skin prick testing documents allergic sensitisation but does not prove that asthma is the mechanism of the cough.
- B.
- High-resolution CT is for suspected interstitial disease or bronchiectasis, neither of which fits a clear chest with a normal radiograph.
- C.
- Correct. When asthma is suspected but spirometry is normal or borderline, methacholine challenge is the gold standard, and a negative test essentially excludes asthma.
- D.
- Serum total IgE is a marker of atopy used mainly to select patients for anti-IgE biologics, not to diagnose cough-variant asthma.
Reference: GINA Global Strategy for Asthma Management and Prevention, 2025; CHEST Guideline on Chronic Cough, 2018
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