What are the diagnosis criteria for asthma?

Diagnosis of Asthma

Asthma diagnosis requires a compatible clinical history of respiratory symptoms (wheeze, dyspnea, chest tightness, cough) combined with objective confirmation of variable expiratory airflow limitation through pulmonary function testing—no single test alone is sufficient, and at least two abnormal objective tests should be documented before confirming the diagnosis. 1

Core Diagnostic Approach

Clinical History Requirements

The diagnosis begins with identifying characteristic respiratory symptoms that vary over time and in intensity 1:

• Wheeze (most specific symptom for asthma) 1
• Dyspnea, chest tightness, and cough (less specific) 1
• Symptoms typically worse at night or early morning 1
• Symptoms triggered by exercise, allergens, cold air, or viral infections 1

Objective Testing: The Two-Test Rule

The 2021 European Respiratory Society guidelines explicitly recommend against diagnosing asthma based on a single abnormal test—at least two objective test results must be abnormal. 1

First-Line Diagnostic Tests

1. Spirometry with Bronchodilator Reversibility (BDR)

This is the primary diagnostic test 1:

Positive BDR criteria:

• FEV₁ improvement ≥12% AND ≥200 mL after bronchodilator 1
• If spirometry shows obstruction (FEV₁/FVC < lower limit of normal or <80% predicted) AND BDR is positive, asthma is confirmed 1

Important caveat: Normal spirometry does not exclude asthma, as up to 97% of children with current asthma may have FEV₁/FVC >70% 1. BDR testing can still be performed even with normal baseline spirometry if clinical suspicion is high 1

2. Fractional Exhaled Nitric Oxide (FeNO)

This measures airway inflammation 1:

Diagnostic threshold:

• FeNO ≥25 ppb supports asthma diagnosis 1
• When combined with abnormal spirometry or positive BDR, confirms diagnosis 1
• FeNO <25 ppb makes asthma unlikely but does not exclude it 1

Critical limitation: FeNO is most useful in atopic/eosinophilic asthma and has limited utility in non-atopic phenotypes 2

Second-Line Tests (When Initial Tests Are Inconclusive)

3. Peak Expiratory Flow Rate (PEFR) Variability

Used when spirometry and FeNO are non-diagnostic 1:

• Measure twice-daily for 2 weeks 1
• Positive result: ≥12% variability 1
• This is an inferior alternative to bronchial challenge testing but acceptable when challenge testing is unavailable 1

4. Bronchial Challenge Testing

Reserved for patients with normal spirometry and ongoing clinical suspicion 1:

• Uses methacholine, histamine, cold air, or exercise 1, 3
• Detects airway hyperresponsiveness (AHR) 3
• A positive test confirms AHR (characteristic of asthma but not specific) 1
• A negative test is more valuable to rule out asthma 1

5. Trial of Inhaled Corticosteroid (ICS) Treatment

The 2024 GINA guidelines include improvement in lung function after 4 weeks of ICS as one method to confirm variable airflow limitation 1. However, the European Respiratory Society strongly recommends against using symptom improvement alone after empiric treatment to confirm diagnosis 1. If this approach is used:

• Repeat objective testing (spirometry, BDR) after 4-8 weeks 1
• Spirometry improvement confirms diagnosis 1
• Symptom improvement without objective improvement warrants specialist referral 1

Special Populations and Situations

Children Ages 5-16 Years

The diagnostic algorithm applies uniformly across this age range 1:

• Requires at least two abnormal objective tests 1
• Spirometry, BDR, and FeNO are first-line 1
• Watchful waiting with repeat testing during symptomatic periods is acceptable when initial tests are inconclusive 1

Children Under 5 Years

Diagnosis is more challenging due to difficulty obtaining objective measurements 1. The diagnosis relies more heavily on clinical pattern recognition, though this increases misdiagnosis risk 1.

Patients Already on ICS Treatment

For symptomatic patients on ICS with normal lung function and normal FeNO, consider alternative diagnoses 1. For asymptomatic patients on ICS, review at 6-12 month intervals and step down treatment; if symptoms recur, apply the full diagnostic algorithm 1.

Patients on ICS Who Cannot Demonstrate Variability

The 2024 GINA guidelines recommend repeating objective lung function measures and trialing a step-down of ICS-containing treatment to unmask variable airflow limitation 1.

Common Diagnostic Pitfalls

Avoid these errors:

• Do not diagnose based on symptoms alone without objective testing 1
• Do not rely on a single abnormal test 1
• Do not use symptom improvement after treatment as sole diagnostic criterion 1
• Do not assume normal spirometry excludes asthma 1
• Do not overlook alternative diagnoses in patients with atypical presentations 1

Differential Diagnoses to Consider

When objective tests are negative or atypical, evaluate for 1:

• Vocal cord dysfunction (can mimic or coexist with asthma; look for inspiratory flow-volume loop flattening) 1
• COPD in adults (use diffusing capacity and lung volume measurements) 1
• Gastroesophageal reflux disease (may coexist with asthma) 1
• Foreign body aspiration in children 1
• Cardiac causes (congestive heart failure in adults) 1

When to Refer to Specialist

Refer when 1:

• Initial objective tests fail to confirm diagnosis despite high clinical suspicion 1
• Symptoms persist despite treatment with normal objective tests 1
• Atypical features or diagnostic uncertainty 1
• Severe or difficult-to-control asthma requiring phenotyping for biologic therapies 1