Sometimes, your doctor can be fairly certain whether or not you have asthma or COPD based on your symptoms and physical examination. Often, though, more information is needed to distinguish between asthma and other respiratory problems. After all, asthma is not the only medical problem that can lead to breathing difficulties. If there are doubt about your diagnosis, your physician may ask you to perform some tests that provide additional information regarding how well your lungs work or whether you have allergies. These are called “pulmonary function tests” (PFTs) or allergy tests.
Spirometry
The most commonly used pulmonary function test is called “spirometry”. Patients are asked to breathe in to fill their lungs, then blow out as fast and as completely as possible into a machine (the “spirometer”) which measures how much and how fast they can exhale.
Post-bronchodilator spirometry
 Commonly, an asthmatic patient will be asked to repeat spirometry after inhaling a quick-reliever medicine, like albuterol. Patients with asthma typically show substantial improvements after inhaling such a medicine. Patients with other lung diseases (such as emphysema or chronic bronchitis from smoking) often do not exhibit such immediate benefit.
Some additional Pulmonary Function Tests your doctor may order are diffusing capacity, a methacholine challenge, or an exercise challenge test. Your doctor may also order an artificial blood gas sample.
Allergy tests
Many, though certainly not all, patients with asthma have allergies to one or more substances in the environment (called allergens). An asthmatic patient can improve his/her asthma control, or at least reduce the likelihood of inducting an asthma attack, by limiting contact with substances to which he or she is allergic.
To determine whether allergies are present, and to which environment allergens, your doctor may perform a series of skin “prick” tests, in which a drop of allergens is placed on the forearm, and a tiny prick into the very top of the skin is made with a needle or plastic device. Surprisingly, this doesn’t really hurt! When allergy to the substance is present, an itchy red reaction occurs.
Another kind of test for allergies is performed on a regular blood sample, and measures the quantity of particular kind of antibody (immunoglobulin E, or IgE) that is often elevated in people with multiple allergies.