What tests are used to diagnose asthma, including both objective and subjective assessments?

Asthma Diagnosis: Required Tests and Clinical Assessment

Asthma diagnosis requires a clinical assessment combined with at least two abnormal objective tests—specifically spirometry, bronchodilator reversibility (BDR) testing, and fractional exhaled nitric oxide (FeNO) as first-line diagnostic tools. 1

Subjective Assessment (Clinical History)

Evaluate for wheeze as the primary symptom, which demonstrates sensitivity of 55-86% and specificity of 64-90% for asthma diagnosis. 1

• Cough and breathing difficulty alone are nonspecific and should not be used to diagnose asthma. 1
• Document symptom patterns including frequency, specific triggers (exercise, allergens, cold air, viral infections), nocturnal awakening, and any previous response to bronchodilators or corticosteroids. 2
• Note that relying solely on clinical history results in considerable misdiagnosis rates in both children and adults. 3

Common pitfall: Many European healthcare settings diagnose asthma based on clinical history and examination alone without objective testing, leading to high rates of both over- and under-diagnosis. 3

Objective Testing: First-Line Tests

1. Spirometry

• Measure FEV1 and FEV1/FVC ratio. 1
• Abnormal results are defined as FEV1 or FEV1/FVC less than the lower limit of normal (LLN) and/or <80% predicted. 1
• Critical caveat: Normal spirometry does not exclude asthma, as airflow obstruction may be variable and absent during testing. 1, 4
• Abnormal spirometry alone does not confirm asthma—it may be present in other obstructive conditions. 1

2. Bronchodilator Reversibility (BDR) Testing

• Perform BDR testing even when spirometry is normal if clinical suspicion remains high. 1
• Measure FEV1 10 minutes after administering 200 mcg inhaled albuterol. 5
• A positive BDR indicates reversible airflow obstruction characteristic of asthma. 1
• This test may need repetition at multiple time points due to low sensitivity. 6

3. Fractional Exhaled Nitric Oxide (FeNO)

• Perform FeNO testing before spirometry. 1
• Use a cut-off of ≥25 ppb as abnormal. 1
• Elevated FeNO suggests eosinophilic airway inflammation. 1
• Limitation: FeNO has limited diagnostic utility in non-atopic asthma. 7

Diagnostic Decision Algorithm

Confirm asthma diagnosis when at least two objective test results are abnormal, such as:

• Abnormal spirometry + positive BDR, OR
• Abnormal spirometry + elevated FeNO, OR
• Positive BDR + elevated FeNO 1

When initial tests are normal but clinical suspicion remains high: Implement watchful waiting and repeat testing when the child is symptomatic. 1

Second-Line Testing (When Diagnosis Remains Unclear)

Peak Expiratory Flow Rate (PEFR) Variability

• Obtain twice-daily PEFR measurements for 2 weeks. 1
• Variability >12% suggests asthma. 1
• Calculate maximum within-day peak expiratory flow amplitude mean percentage. 5

Bronchial Challenge Testing

• Reserve for patients with negative first-line testing but persistent high clinical suspicion. 1, 7
• Options include methacholine challenge (sensitivity 91%, specificity 90%), exercise challenge, eucapnic voluntary hyperventilation, or mannitol challenge. 4, 5
• Caveat: Bronchial challenge testing has low specificity and may be positive in conditions mimicking asthma. 7

Sputum Eosinophil Count

• Induced sputum differential eosinophil count demonstrates 72% sensitivity and 80% specificity. 5
• Useful for confirming eosinophilic inflammation when other tests are equivocal. 5

Critical Pitfalls to Avoid

Never diagnose asthma based solely on:

• Symptomatic response to albuterol without objective testing—this leads to misdiagnosis. 4
• Improvement after a trial of preventer medication. 1
• Clinical history alone, even if symptoms appear classic. 3

Do not diagnose asthma based on a single abnormal objective test—at least two abnormal results are required. 1

Consider alternative diagnoses such as vocal cord dysfunction or exercise-induced laryngeal dysfunction, which may coexist with asthma and present with similar symptoms. 4

Age-Specific Considerations

The European Respiratory Society guidelines specifically address children aged 5-16 years, as diagnostic approaches and test cut-offs differ between pediatric and adult populations. 3 Younger children may present with atypical symptoms, making diagnosis more challenging. 2