Best Diagnostic Measure for Asthma with Dry Cough and Expiratory Wheeze
Pulmonary Function Tests (PFT), specifically spirometry with bronchodilator reversibility testing, is the best diagnostic measure for a patient presenting with symptoms of asthma such as dry cough and expiratory wheeze. 1
Primary Diagnostic Approach
Spirometry is the essential first-line objective measure to establish the diagnosis of asthma because medical history and physical examination alone are not reliable means of excluding other diagnoses or assessing lung status. 1
Why Spirometry is Superior to HRCT
Spirometry directly demonstrates the pathophysiologic hallmark of asthma: reversible airflow obstruction, which can be assessed in patients 5 years of age and older. 1
HRCT is not routinely necessary for asthma diagnosis and should only be considered when other more targeted investigations are normal or when alternative diagnoses need to be excluded. 1
International guidelines uniformly recommend PFT as the gold-standard objective measure to confirm asthma diagnosis, with spirometry showing reversible airflow obstruction being the primary diagnostic tool. 1
Comprehensive Diagnostic Algorithm
Initial Testing Sequence
First-line tests should include spirometry, bronchodilator reversibility (BDR), and fractional exhaled nitric oxide (FeNO) as part of the diagnostic pathway. 1
Spirometry demonstrates obstruction (FEV1 or FEV1/FVC less than lower limit of normal and/or <80% predicted) and provides baseline lung function assessment. 1
Bronchodilator reversibility testing provides much better diagnostic accuracy than baseline lung function data alone, with a positive test defined as ≥12% and/or ≥200 mL improvement in FEV1. 1, 2
Peak flow meters should NOT be used for diagnosis because they are designed for monitoring, not as diagnostic tools, and have wide variability in measurements and reference values. 1
When Initial Spirometry is Normal
Normal spirometry does not exclude asthma, as baseline pulmonary function tests have poor diagnostic accuracy due to substantial overlap between healthy individuals and those with mild or intermittent asthma. 2
If spirometry is normal but clinical suspicion remains high:
Bronchoprovocation testing with methacholine may be useful when asthma is suspected and spirometry is normal or near normal. 1
A negative methacholine challenge test is more helpful to rule out asthma than a positive test is to confirm it, since airway hyperresponsiveness can be present in other conditions. 1
FeNO measurement ≥25 ppb supports the diagnosis and should be performed when spirometry is normal. 1
Critical Diagnostic Pitfalls to Avoid
Asthma is frequently overdiagnosed when objective measures are not used, with one-third of patients with physician-diagnosed asthma showing no evidence of asthma on spirometry and/or methacholine challenge testing. 2
Common Mistakes
Never rely on clinical history and physical examination alone to make the diagnosis, as symptoms of cough and wheeze are nonspecific and can be caused by multiple conditions. 1
Do not use trials of treatment where improvement of symptoms alone after empiric asthma preventer medication is used to confirm the diagnosis—this approach is specifically not recommended. 1
Avoid single PEF measurements for diagnosis, as they are not as accurate as FEV1 in diagnosing airflow obstruction. 1
Important Differential Diagnoses to Consider
When evaluating dry cough with expiratory wheeze, consider these alternative diagnoses that can mimic asthma:
Vocal cord dysfunction (VCD) can mimic asthma and may coexist with it; variable flattening of the inspiratory flow loop on spirometry is strongly suggestive. 1
COPD in adults, particularly chronic bronchitis or emphysema. 1
Cough variant asthma where cough is the principal or only manifestation; diagnosis is confirmed by positive response to asthma medications. 1
Gastroesophageal reflux disease (GERD) and obstructive sleep apnea may coexist with asthma and complicate diagnosis. 1
Role of HRCT in Asthma Diagnosis
HRCT scanning has not been properly evaluated for routine asthma diagnosis and should only be used in patients with persistent atypical cough in whom other more targeted investigations are normal. 1
HRCT is useful for excluding alternative diagnoses such as bronchiectasis, interstitial lung disease, or structural airway abnormalities, but it does not directly assess the functional impairment characteristic of asthma. 1
Chest x-ray may be needed to exclude other diagnoses but is not the primary diagnostic tool for asthma. 1
Confirmation Requires Multiple Abnormal Tests
The diagnosis of asthma in children should only be made when at least two objective test results are abnormal, and this principle applies to adults as well given the risk of overdiagnosis. 1
The most robust diagnostic approach combines: