5th January 2024

Investigations of asthma in people aged 17 and over

There is no single diagnostic test to confirm diagnosis of asthma. Rather, clinical judgment should be used to determine the probability of asthma based on history and investigation findings.

Full blood count

• FBC may be normal or show raised eosinophils and/or Neutrophils. It is indicated in the first presentation and in acute exacerbations depending on history and examination findings.

Chest X-ray

• Chest X-ray could be normal or show hyperinflation of the lungs. It is often indicated in the first presentation to exclude other pathologies and in acute exacerbations.

Peak expiratory flow 

• 20% variability or more after monitoring twice daily for 2-4 weeks is considered a positive result. A positive result supports the diagnosis of asthma.
• Peak flow variability should be offered in adults with normal spirometry or in obstructive spirometry with positive BDR with FeNO level of 39 ppb or less. Consider monitoring in people with obstructive spirometry and negative BDR and FeNO level between 25 and 39 ppb.

Fractional exhaled nitric oxide (FeNO) testing

• FeNO testing may support asthma diagnosis in people aged 17 and over by confirming eosinophilic airway inflammation. FeNO level of 40 parts per billion or higher is considered a positive result in steroid-naive adults.
• FeNO testing should be considered in people aged 5 to 16 if there is diagnostic uncertainty after initial assessment or in case of normal spirometry or obstructive spirometry with negative bronchodilator reversibility test (BDR). A FeNO level of 35 parts per billion is a positive result in this group.

Spirometry

• All symptomatic people above the age of 5 years should be offered spirometry. Any value of FEV1/FVC less than 80% is abnormal and suggests airflow limitation. However, asthma cannot be ruled out when spirometry is normal in a symptomatic person.

Bronchodilator reversibility

• It should be offered to people aged 17 and above and considered in people aged 5 to 16 years with FEV1/FVC ratio less than 70%.
• Positive result in adults is an improvement of 12% or more in FEV1 with at least 200 ml increase in volume following administration of Beta-2 agonist or corticosteroids.
• Positive result in children is an improvement in FEV1 of 12%.

Bronchial challenge test with histamine or methacholine

• This test should be offered if the diagnosis is not clear after normal spirometry and either FeNO level of 40 ppb or more and no variability in peak flow readings or if FeNO level is 39 ppb or less with variability in peak flow values.