The results of spirometry test can be interpreted to provide valuable information about the function of the lungs and respiratory system and guide diagnostic and treatment decisions.
There are various measurements that may be determined using a spirometer, and the results can be interpreted to show normal lung function, restrictive pattern, obstructive pattern or a combination pattern.
Spirometry tests are primarily required to measure the volume and flow rate of the air when it is inhaled or exhaled from the lungs.
The most common spirometry measurements are peak expiratory flow (PEF), forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC).
- PEF is the maximum speed of the air when exhaling.
- FEV1 is the maximum volume of air excreted in a period of 1 second.
- FVC is the total volume of air exhaled in one breath.
There are many other different measurements that can be determined in a spirometry test, depending on the information required. Possible measurements may include:
- Peak expiratory flow (PEF)
- Forced expiratory volume in 1 second (FEV1)
- Forced vital capacity (FVC)
- Residual volume (RV)
- Expiratory reserve volume (ERV)
- Total lung capacity (TLC)
- Functional residual capacity (FRC)
- Slow vital capacity (SVC)
- Maximum voluntary ventilation (MVV)
- Diffusion capacity to carbon monoxide (DLCO)
- Forced expiratory flow 25% to 75%
The expected normal results of a spirometry test depend on the age, body type, gender and ethnicity of the individual. The results are usually presented as a percentage of the expected value for the individual, and any result less than 80% of the predicted value is considered to be abnormal.
An abnormal result is usually indicative of poor lung function related to a disease of the chest or lungs. These results can be classified as obstructive, restrictive or a combination of obstructive and restrictive, depending on the nature of the related disease.
An abnormal obstructive pattern is typical of diseases that cause narrowing of the airways, meaning that the lung can hold a smaller volume of air and it takes longer to empty. Conditions that are characteristic of this pattern include:
- Chronic obstructive pulmonary disease (COPD)
The results of an obstructive pattern typically display a reduction in FEV1 and a lower ration of FEV1/FEV than normal. However, the FVC is often very near to normal, as the total capacity of the lungs is not reduced.
Assessing the effect of bronchodilators with spirometry can help to differentiate between obstructive disorders such as asthma and COPD. For example, asthma exhibits greater reversibility and results tend to improve with administration of bronchodilators, whereas COPD should show less improvement.
COPD can be graded be severity according to the FEV1 result following bronchodilator medication as follows:
- Mild COPD: FEV1 > 80% of normal value (e.g. normal spirometry following bronchodilator medication)
- Moderate COPD: FEV1 50-79% of normal value with bronchodilator
- Severe COPD: FEV1 30-49% of normal value with bronchodilator
- Very severe COPD: FEV1< 30% of normal value with bronchodilator
Other diseases follow a restrictive pattern and involve scarring of the lung tissue and lead to a reduction in the air contained in the lungs and the efficacy of oxygen transfer into the blood. These include obesity, pulmonary fibrosis, sarcoidosis and lung cancer.
Results of a restrictive pattern typically display a reduced FVC in comparison to the expected normal results. As the FEV1 is also reduced in this pattern, the ratio of FEV1/FVC is usually normal.
A combined pattern is usually indicative of two disorders that cause abnormality in the lung function. An example of this is cystic fibrosis, which involves obstruction of the airways by excess mucus and fibrotic damage to the lung tissue causing restriction.