Spirometry

In children older than 6 years, adolescents and adults with suspected asthma, perform spirometry to identify airflow limitation.

Spirometry should preferably be done before starting regular preventer treatment, as it is harder to confirm the diagnosis of asthma in patients receiving treatment. In patients with significant symptoms, empirical treatment with an inhaled corticosteroid can be started, but spirometry should be done as soon as possible.

Normal spirometry when the patient is asymptomatic does not exclude asthma. Ideally, repeat spirometry when the patient is symptomatic. If spirometry is normal when the patient is symptomatic (especially if short of breath), consider another diagnosis.

Note: Consider an alternative diagnosis if spirometry is normal when the patient is symptomatic.

Some patients only have reduced lung function seasonally (eg patients with allergic asthma related to grass pollen allergy). In these patients, performing spirometry in the pollen season can confirm the diagnosis. It is also important to consider whether such a patient may be susceptible to thunderstorm asthma.

Spirometry findings that support a diagnosis of asthma include:

  • reversible airflow limitation—an increase in forced expiratory volume in 1 second (FEV1) of at least 200 mL and 12% from baseline 10 to 15 minutes after giving a short-acting beta2 agonist (SABA) (200 to 400 micrograms inhaled salbutamol or equivalent). A larger increase in FEV1 (eg more than 400 mL) in response to a SABA is strongly supportive of asthma
  • expiratory airflow limitation—reduced FEV1 to forced vital capacity (FVC) ratio (FEV1/FVC ratio).

Spirometry testing requires staff training and reliable equipment to ensure good quality testing, particularly in young children. For further information on performing spirometry; see here.