- Spirometry is the preferred method to demonstrate airway obstruction because:
- It more clearly identifies airway obstruction than peak expiratory flow (PEF), and the results are less dependent on effort.
- PEF variability can be increased in people with conditions commonly confused with asthma.
- PEF should only be used if spirometry is unavailable.
- In view of the potential requirement for treatment over many years, it is important even in relatively clear cut cases, to try to obtain objective support for the diagnosis of asthma.
- Perform spirometry on all adults to assess for the presence and severity of airway obstruction.
- Airway obstruction is confirmed when forced expiratory volume in 1 second (FEV1)/Forced Vital Capacity (FVC) ratio is less than 0.7.
- Whether or not spirometry should happen before starting treatment depends on the certainty of the initial diagnosis and the severity of the presenting symptoms.
- Normal spirometry obtained when a person is asymptomatic does not exclude a diagnosis of asthma. Repeated assessment and measurement may be necessary.
- Spirometry is recommended for children with an intermediate probability of asthma if they are able to perform the test (usually older than 5 years).
- Spirographs require calibration to allow accurate interpretation of the results (for example Rosenthal normal values based on the child’s sex and height). Health care professionals require training on how to calibrate and interpret the results from a spirogram. Prodigy recommend that advice should be sought regarding carrying out spirometry in children and interpreting the results, unless the healthcare professional has received the appropriate training.
- Measuring lung function in young children is difficult and not usually possible in children under 5 years of age.
- Normal results obtained when the child is asymptomatic do not exclude a diagnosis of asthma.