Diagnostic testing

Spirometry

Spirometry helps you to diagnose asthma and assess asthma control, by allowing you to:

• assess change in airflow limitation
• measure the degree of airflow limitation compared with predicted normal airflow (or with personal best in patients who have previously undergone spirometry).

Spirometry is the lung function test of choice for diagnosing asthma and for assessing asthma control in response to treatment.

• Single or office-based measurements of peak expiratory flow (PEF) with conventional peak flow meters have significant limitations for assessing airflow limitation.
• A spirometer allows you verify that the patient has performed the manoeuvre correctly and to generate a precise permanent record of results.
• Most adults, and children over 7 years old, can perform spirometry.

Accurate measurement of respiratory function is necessary to assess and manage asthma. Measurements taken both before and after administration of a short-acting beta₂ agonist (SABA) bronchodilator allow you to:

• diagnose airflow limitation
• measure the degree of airflow limitation
• monitor the effects of treatment
• demonstrate the presence and reversibility of airflow limitation to the patient
• provide objective feedback to the patient about the presence and severity of asthma. 

The National Asthma Council Australia recommends that all doctors managing asthma should have access to and use a spirometer.

How to perform spirometry

Clearly explain and physically demonstrate correct spirometry technique:

• Sit upright with feet firmly on the floor and do not lean forward. (Standing is permissible in adults if they find it easier, but not in children.)
• Breathe in until lungs feel absolutely full. (Coaching is essential to do this properly.)
• Form a good seal around the mouthpiece.
• Blast air out as hard and fast as possible and for as long as possible, until the lungs are completely empty.

Repeat the test until you obtain three reproducible and acceptable measures, i.e all the following apply:

• Forced vital capacity (FVC) of the two highest readings does not vary by more than 150 mL.
• The manoeuvre must be performed with a good start.
• Forced expiration must be maintained throughout the test.
• The patient did not cough during the first second of the test.

Record the best forced expiratory volume in one second (FEV₁) and FVC obtained. For most people, it is not practical to make more than eight attempts.

Repeat spirometry at least 10 minutes after giving four separate doses of salbutamol 100 mcg via a metered-dose inhaler (MDI) and large-volume spacer. (A large-volume spacer is more efficient.)

Interpreting spirometry

Airflow limitation is judged to be reversible if either of the following applies:

• Baseline FEV₁ >1.7 L and post-bronchodilator FEV₁ at least 12% higher than baseline
• Baseline FEV₁ ≤1.7 L and post-bronchodilator FEV₁ at least 200 mL higher than baseline.

A similar rule is used to determine reversibility based on pre-and post-bronchodilator FVC.

Results should be expressed as absolute values and also as a percentage of predicted values, based on the patient's age, height and sex. (See Respiratory function tables)

Approximately 70% of bronchodilator response is achieved in the first 3-4 minutes after administering SABA.

For more information on spirometry, see Spirometry. The measurement and interpretation of respiratory function in clinical practice. The interpretation table makes it simple.

Spirometry is the lung function test of choice for diagnosing asthma and assessing asthma control in response to treatment. [√] Pay close attention to spirometry technique to ensure you get the most reliable readings. The absence of acute reversibility of airflow limitation in response to a short-acting bronchodilator does not exclude the diagnosis of asthma. [√]

Peak expiratory flow measurement

When diagnosing asthma, PEF is not a substitute for spirometry. (It is useful in the diagnosis of occupational asthma where very frequent testing is required, and is it useful way to monitor asthma control for some people). Single PEF measurements are not adequate for use in routine asthma management by doctors.

A peak flow meter is used to detect and measure a person's variation in best PEF, in order to assess variability of airflow limitation. Measurement of PEF:

• is effort-dependent
• varies considerably between instruments.

Isolated readings taken in the surgery or pharmacy with a meter other than the person's own must be interpreted with caution because there is a wide normal range.

Despite its limitations, monitoring of PEF at home or work is useful when:

• symptoms are intermittent
• symptoms are related to occupational triggers (See Occupational asthma)
• the diagnosis is uncertain
• monitoring treatment response (See Ongoing care).

In the absence of an acute bronchodilator FEV₁ response, monitoring of PEF over several days to weeks may be useful in making a diagnosis.

Content Updated (Tuesday, 17 April 2007)