What Spirometry Is: Spirometry is a type of pulmonary function test used in diagnosing asthma. It measures how well the lungs are working. The findings from this test are not enough to prove a person has asthma, but they can help narrow down a diagnosis of asthma.
What Spirometry Measures: Spirometry measures the maximum amount of air that is exhaled forcibly after deep inhalation. Abnormal findings suggest an airflow obstruction. Spirometry can also reveal signs that the obstruction is reversible. These 2 factors together suggest an asthma diagnosis.
Reversibility can be predicted to some extent by the numbers, but some patients with asthma symptoms may not demonstrate reversibility until after bronchodilator (inhaler) use. If there is no significant difference in measurements taken before and after use of the medication, the defect is considered irreversible. If there is, the defect is reversible.
What the Numbers Mean: The results of a spirometry test are expressed in terms of 2 findings:
- FVC -- forced vital capacity (the amount of air that can be forcibly inhaled during the test)
- FEV1 -- forced expiratory volume (the amount of air forcibly exhaled) in first second of the FVC
When the FEV1/FVC ratio is lower than normal (<70%), there may be an airflow obstruction.
It's important to note, too, that many asthma patients have normal spirometry results between asthma attacks.
Types of Defects: Abnormal lung function is usually classified as either restrictive or obstructive. When the FEV1/FVC ratio is reduced to a level less than 70%, the abnormality is probably due to an obstruction of the airway. When the FVC is reduced but the ratio to FEV1 is normal, then the abnormality is more likely of the restrictive type, and probably not asthma.
How and Where Spirometry Is Done: Spirometry can usually be done in your doctor's office, especially if you are seeing an allergy or asthma specialist, such as a pulmonologist or immunologist. Special equipment that must meet certain guidelines issued by the American Thoracic Society is needed. The correct technique and proper management of the machinery is also essential. The healthcare professional giving the test should also have received special training.
Getting the Most Accurate Results: It's crucial that the patient breathe in and out as strongly as possible during the test in order to get the most accurate results. For this reason, spirometry is most useful in adults and children over age 5, because younger children may not be able to complete the test correctly.
What Happens Next: As mentioned earlier, an abnormal spirometry test result alone is not enough to prove a diagnosis of asthma. The doctor will also perform a physical examination and look for a pattern of symptoms, including wheezing, chronic cough, shortness of breath, and / or chest tightness, especially at night or after physical exertion. It's a good idea to have the spirometry results confirmed by a specialized pulmonary function laboratory.
Additional Testing and Evaluation: The doctor may also consider doing other evaluations or ordering additional tests, in some cases, which may include:
- inspiratory and expiratory flow volume loops
- diffusing capacity test
- lung volumes measurement
- bronchoprovocation with metacholine, histamine, or exercise
- chest x-ray
- assessment of variations of peak flow rate over the course of a day
- allergy testing
- evaluating for nasal polyps or sinus disease
- assessing for gastroesophageal reflux disease (GERD)
Importance in Diagnosing Asthma: Medical and symptom history are important tools in diagnosing asthma, but they're not enough to be sure of the diagnosis. Some sort of objective measurement is needed. Peak flow readings are sometimes used, but they are not nearly as reliable, because of the variability between different types of peak flow meters. Spirometry is a much more effective measure.
In Summary: Spirometry testing is an essential step in the process of diagnosing asthma. Not only does it help your doctor decide if you do have asthma, it also helps determine how severe your asthma may be.
Source: National Institutes of Health, National Heart, Lung, and Blood Institute. Guidelines for the diagnosis and management of asthma. Expert panel report 3. NIH Publication No. 97-4053, August 2007.
