How is asthma diagnosed in an adult patient?

Diagnosing Asthma in Adults

To diagnose asthma in an adult, you must demonstrate episodic respiratory symptoms (wheeze, cough, dyspnea, or chest tightness) PLUS objective evidence of variable expiratory airflow limitation on spirometry, specifically an FEV1 increase of ≥12% AND ≥200 mL after bronchodilator administration. 1

Core Diagnostic Algorithm

Step 1: Establish Clinical Probability

Identify key symptom patterns that increase asthma likelihood 1:

Cardinal symptoms:

• Recurrent wheeze
• Recurrent dyspnea
• Recurrent chest tightness
• Cough (particularly worse at night)

Critical pattern recognition - symptoms that:

• Occur or worsen with exercise, viral infections, allergen exposure (pets, dust mites, mold, pollen), irritants (smoke, chemicals), weather changes, strong emotions, stress, or menses
• Occur or worsen at night, awakening the patient
• Show episodic variability over time

Step 2: Perform Spirometry with Bronchodilator Reversibility Testing

This is the essential objective measure - history and physical examination alone cannot reliably diagnose or exclude asthma 1. Peak flow meters are inadequate for diagnosis due to high variability; they are monitoring tools only 1.

Diagnostic criteria for reversibility:

• FEV1 increase of >12% AND >200 mL from baseline after short-acting β2-agonist inhalation 1
• Some evidence suggests >10% of predicted FEV1 may better separate asthma from COPD 1

Step 3: If Spirometry is Normal or Near-Normal

When asthma is still suspected despite normal spirometry, proceed sequentially 1:

1. Bronchoprovocation testing (methacholine, histamine, cold air, or exercise challenge)

   • Must be performed by trained personnel for safety
   • A negative test is more useful to rule OUT asthma than a positive test is to rule it in, since airway hyperresponsiveness occurs in other conditions 1

2. FENO measurement (if available, age ≥5 years)

   • Use only as an adjunct when diagnosis remains uncertain after history, examination, and spirometry 2
   • Critical caveat: Allergic rhinitis and atopy increase FENO independent of asthma, so interpret carefully in context 2
   • Do NOT use FENO alone to diagnose asthma 2

3. Peak flow variability monitoring

   • Diurnal variation >20% over 1-2 weeks supports diagnosis 3

Physical Examination Findings

Important limitation: Normal examination does NOT exclude asthma, as signs may be absent between episodes 1. When present, look for 1:

• Upper respiratory tract: Increased nasal secretion, mucosal swelling, nasal polyps
• Chest: Wheezing during normal breathing or prolonged forced exhalation, hyperexpansion, accessory muscle use, hunched shoulders, chest deformity
• Skin: Atopic dermatitis, eczema

Critical Differential Diagnoses in Adults

You must actively exclude 1:

• COPD (chronic bronchitis/emphysema) - consider diffusing capacity testing
• Congestive heart failure
• Pulmonary embolism
• Mechanical airway obstruction (benign/malignant tumors)
• ACE inhibitor-induced cough
• Vocal cord dysfunction (look for flattened inspiratory flow loop on spirometry)
• GERD, obstructive sleep apnea (may coexist with asthma)

Common Diagnostic Pitfalls

Cough-variant asthma: Cough may be the only manifestation. Diagnosis confirmed by positive response to asthma medications 1.

Vocal cord dysfunction (VCD): Can mimic or coexist with asthma. Suspect in difficult-to-treat cases unresponsive to asthma medications or elite athletes with exercise-related breathlessness. Variable inspiratory flow loop flattening on spirometry is strongly suggestive 1.

Patients already on inhaled corticosteroids: Diagnosis becomes extremely difficult as treatment may normalize spirometry and mask reversibility 4, 5. If possible, obtain objective testing before initiating treatment.

Normal spirometry between episodes: Asthma is variable - patients can have completely normal lung function when asymptomatic 3. This is when bronchoprovocation or serial peak flow monitoring becomes essential.

Additional Testing (Not Routine, But Useful)

• Chest x-ray: To exclude alternative diagnoses when clinically indicated 1
• Blood eosinophils and serum IgE: For phenotyping, not diagnosis 4
• Allergy testing (skin prick tests): Should be systematically considered given high frequency of allergic asthma in adults 6

When to Refer to Specialist

Consider referral when 1:

• Signs and symptoms are atypical
• Diagnostic uncertainty persists despite testing
• Additional specialized testing is needed
• Severe or difficult-to-treat asthma is suspected

The fundamental principle: Asthma diagnosis requires BOTH characteristic symptoms AND objective demonstration of variable airflow obstruction. Never diagnose based on symptoms alone, as this leads to substantial overdiagnosis 7, 5.