To diagnose asthma, the doctor will see if three criteria are met:
- You have recurring episodes where your airflow becomes blocked, resulting in at least one of the asthma signs
- The blocked airflow can be at least partially reversed through medication (see spirometry, below)
- Other potential causes of your symptoms are ruled out
Medical history
The doctor will want to evaluate your medical history to identify what symptoms you have had, when your symptoms occur, and how long they last. The doctor will ask about possible triggers, such as allergens, exercise, or occupational exposure to chemicals. The doctor will also want to know if you have a family history of asthma, allergy, sinusitis, or nasal polyps.
Physical exam
The doctor will conduct a physical exam that focuses on the upper respiratory tract, chest, and skin. The doctor will listen for wheezing and may look for nasal secretions, eczema, and similar allergy-related symptoms.
Lung function tests
Spirometry is the most reliable test for diagnosing asthma. A spirometer is an instrument that measures the maximum volume you can exhale after breathing in as much as you can. The total volume you exhale is called "forced vital capacity," or FVC. The spirometer also measures the volume of air you exhale in the first second. (This is referred to as "forced expiratory volume in one second," or FEV1.) In general, the more air you breath out during the first second of a full exhalation, the better.
The doctor will take the reading several times. Your FVC and FEV1 results will be compared to what is normally seen in people without any blockage in their airways. Your doctor will have a good idea just from this data how much "obstruction" is in your airways.
Then, you will be given a bronchodilator, a drug that relaxes the muscles of your airways. After the drug has had time to take effect, you blow into the spirometer again. If you can breath out significantly more air during the first second than you could before, it indicates that your breathing obstruction is "reversible." This is a strong indication of asthma.
Many doctors use a "peak flow meter" to diagnose asthma. This simple device may allow a doctor to quickly and easily determine whether asthma is your problem.
Other tests
No one single test, or set of tests, is appropriate for every patient. Your doctor may use other tests to help rule out the possibility of other causes of your symptoms. For example, your doctor may perform additional pulmonary function tests to rule out bronchitis or emphysema (especially if you are a smoker or an older patient).
A test called "bronchoprovocation" may be performed if the doctor strongly suspects asthma but your spirometry readings were normal. (This test must be conducted by a trained specialist in an appropriate facility.) Other tests include chest x-rays, allergy testing, examinations for nasal polyps or sinusitis, and evaluation for gastroesophageal reflux.
If it's not asthma...
According to the National Institutes of Health, "recurring episodes of cough and wheezing are almost always due to asthma in both children and adults." However, there are other possibilities. A certain amount of airway inflammation and narrowing are NORMAL responses to airborne irritants. Recurrent wheezing could come from ongoing exposure to chemical fumes, for example. It's only asthma if the narrowing is out of proportion to the threat to the lungs. And in young adults, "vocal cord dysfunction" can cause symptoms very similar to asthma, such as shortness of breath and wheezing.
Regardless of age, if asthma is ruled out, your doctor will explain other potential causes.
