17th February 2025

Diagnosis of asthma

Clinical features of asthma

Asthma is a clinical diagnosis; there is no singular definition but criteria normally include:

• Presence of episodic symptoms (cough, breathlessness, wheeze, chest tightness)
• Variable airflow obstruction
• Evidence of airway inflammation

Table 1 Clinical features with a high and low predictive value for asthma

Investigations in chronic asthma

With a typical clinical picture of asthma, treatment can be initiated with inhaled corticosteroids (usually a 6-week trial). If there is a good objective response (shown using lung function tests or symptom scores), the diagnosis can be confirmed. Otherwise, investigations include the following:

• Spirometry with bronchodilator reversibility test:
  • FEV1/FVC <70%, i.e. an obstructive picture
  • Post-bronchodilator >12% improvement in FEV1 or >200 ml increase in volume
• Fractional exhaled nitrous oxide (FeNO): >40 parts per billion (a marker of airway inflammation)

Occupational asthma

Common triggers for occupational asthma include isocyanates (in spray paint, adhesives), resins and flour. To make the diagnosis, patients should record peak flow measurements at work and home. This diagnosis requires referral to a respiratory specialist.