1. Vital capacity (VC), which is the maximum volume of air that can be inhaled or exhaled.
2. Peak expiratory flow rate (PEFR), commonly called the peak flow rate, which is the maximum flow rate that can be generated during a forced exhalation.
3. Forced expiratory volume (FEV1), which is the maximum volume of air expired in one second.
If the airways are obstructed, these measurements will fall. Depending on the results, the doctor will take the following steps:
- If measurements fall, the doctor typically asks the patient to inhale a bronchodilator. This drug is used in asthma to open the air passages. The measurements are taken again. If the measurements are more normal, the drug likely has cleared the airways and a diagnosis of asthma is strongly suspected.
- If measurement results fail to show airway obstruction, but asthma is still suspected, the doctor may perform a challenge test. This involves administering a specific drug (histamine or methacholine) that usually increases airway resistance only when asthma is present. The challenge test may be quite useful in ruling out occupational asthma. It is not always accurate, particularly in patients whose only symptom is persistent coughing.
- Administering cold air is another method for inducing airway resistance. This test is very accurate for ruling out asthma, but it is not sensitive enough to accurately identify adults who actually have asthma.
Allergy Tests
The patient may be given skin or blood allergy tests, particularly if a specific allergen is suspected and available for testing. Allergy skin tests may be the best predictive tests for allergic asthma, although they are not recommended for people with year-round asthma.
 | Click the icon to see an image of allergy testing. |
Other Tests
Tests that either rule out other diseases or obtain more information about the causes of asthma include:
- A complete blood count.
- Chest and sinus x-rays.
- Computed tomography (CT) scans. CT scans may be helpful in certain cases, such as for determining wall thickness in airways in patients who are difficult to treat, which could signify a higher risk for lung damage.
- Examination of the patient's sputum for eosinophils (white blood cells that in high levels are associated with severe allergic asthma). One 2002 study suggested that treatment goals based on achieving a normal eosinophil count might effectively manage asthma.
- Investigative measurements of certain chemicals in sputum or exhaled air that indicate airway inflammation. Such chemical markers include nitric oxide and hydrogen peroxide. For example, high levels of nitric oxide in exhaled air is proving to be a simple and noninvasive way of diagnosing asthma.
- If aspirin-induced asthma (AIA) is suspected, a non-invasive test called acoustic rhinometry may be useful. A solution of lysine acetylsalicylic acid (L-ASA) is instilled into the patient's nostril. Patients who experience symptoms such as sneezing, itching, congestion, and secretion are likely to have AIA.
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