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How is asthma diagnosed?
The diagnosis of asthma begins with a detailed history and physical examination. A typical history is an individual with a family history of allergic conditions or a personal history of allergic rhinitis who experiences coughing, wheezing, and difficulty breathing, especially with exercise or during the night. There may also be a propensity toward bronchitis or respiratory infections. In addition to a typical history, improvement with a trial of appropriate medications is very suggestive of asthma.
In addition to the history and exam, the following are diagnostic procedures that can be used to help with the diagnosis of asthma:
- Lung function testing with spirometry: This test measures lung function as the patient breathes into a tube. If lung function improves significantly following the administration of a bronchodilator, such as albuterol, this essentially confirms the diagnosis of asthma. It is important to note, however, that normal lung function testing does not rule out the possibility of asthma.
- Measurement of exhaled nitric oxide (FeNO): This can be performed by a quick and relatively simple breathing maneuver, similar to spirometry. Elevated levels of exhaled nitric oxide are suggestive of "allergic" inflammation seen in conditions such as asthma.
- Skin testing for common aeroallergens: The presence of sensitivities to environmental allergies increases the likelihood of asthma. Of note, skin testing is generally more useful than blood work (in vitro testing) for environmental allergies. Testing for food allergies is not indicated in the diagnosis of asthma.
- Other potential but less commonly used tests include provocation testing such as a methacholine challenge, which tests for airway hyperresponsiveness. Hyperresponsiveness is the tendency of the breathing tubes to constrict or narrow in response to irritants. A negative methacholine challenge makes asthma much less likely. Specialists sometimes also measure sputum eosinophils, another marker for "allergic" inflammation seen in asthma. Chest imaging may show hyperinflation, but is often normal in asthma. Tests to rule out other conditions, such as cardiac testing, may also be indicated in certain cases.