Tests to Diagnose Asthma
Sometimes, your doctor can be fairly certain whether or not you have asthma 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 is 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.

Pulmonary Function Tests Spirometry
The most commonly used pulmonary function test is called "spirometry" (from the Greek "spiro" - breathing - and "metry" - measurement). Patients are asked to breathe in to the tops of their lungs, then to blow out as fast and as completely as possible into a machine (the "spirometer") which measures how much and how fast they can exhale. The exhalation speed is reported in two numbers - the forced expiratory volume in the first second of exhalation (or FEV1), and the fastest or peak instantaneous expiratory flow rate during exhalation (or "peak flow", abbreviated as PF or PEF). The total volume of air exhaled is called the "forced vital capacity", or FVC.

During an asthma attack, FEV1 and PF become reduced (indicating obstruction to airflow through the bronchial tubes caused by airway narrowing). FVC often falls, too. These numbers provide your health care provider with an objective measurement of how severely narrowed your airways are at the time. They are more accurate than your own impression of how severe your asthma is just then, and they are much more accurate than your doctor's impression! This is why asthmatics should measure their own peak flow rates at home, using a simple, portable peak flow meter, rather than simply relying on their own impressions of how they are doing.

Post-bronchodilator spirometry
Some other diseases besides asthma can also reduce FEV1, PF, and FVC, and for this reason doctors sometimes request additional tests. 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 in FEV1 and PF (over 20% or more) 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. So if the measurements are substantially better after the quick reliever medicine than they were before, it suggests that the person has asthma.

Lung volume determination, diffusing capacity measurement, and arterial blood gases
After forcibly exhaling from the top to the bottom of the lungs (as one does during spirometry), there is still some air remaining within the lungs, and this is normal. To determine how much air is left in the lungs after complete exhalation (this amount is called the "residual volume"), pulmonary function labs use two kinds of procedures. The preferred way is for the patient to perform some panting maneuvers while sitting inside a big plastic box (a "body box", or "body plethysmograph"), but it is also possible to infer residual volume by measuring the dilution of inhaled, non-absorbed gases like helium. While sitting inside a plastic box may seem weird, neither of these procedures is painful or dangerous at all! From these measurements, your doctor can also calculate the biggest size your lungs can achieve at full inhalation (called "total lung capacity", equal to residual volume plus FVC).

In one additional test called "diffusing capacity" measurement, the ability of your lung to absorb inhaled gas is measured. Finally, occasionally it may be necessary to measure how much oxygen and carbon dioxide are actually in your blood. This is usually done by sampling blood from an artery in your wrist. The information gained from all these auxiliary tests can usually provide fairly strong evidence for or against asthma.

Bronchial provocation
Sometimes all the tests listed above are normal, even in a person who actually has asthma. This is because airflow obstruction in asthma is very commonly episodic - that is, airflow obstruction occurs during asthma attacks, but at other times, lung function is normal. In this case, your doctor may ask you to perform "bronchial provocation" testing.

There are certain substances that when inhaled will narrow everyone's airways to some degree. These substances are called "bronchoconstrictors", and include histamine and methacholine. However, a characteristic of asthmatics' airways is that they tend to narrow more than those of normal people, and at lower doses of bronchoconstrictor than are required to cause narrowing of normal airways.

To perform a bronchial provocation test, the patient inhales increasing doses of bronchoconstrictor (usually methacholine), starting with tiny doses. The patient has spirometry after each dose to see how much bronchoconstriction was induced. From this test, your doctor can infer whether the "reactivity" of your airways is normal or high. Very high airway reactivity to methacholine or histamine is almost exclusively seen in patients with asthma.

Exercise or cold air bronchial provocation
Many patients with asthma experience airway narrowing after exercise or after breathing cold air. For this reason, it is sometimes useful to measure spirometry after a patient performs exercise or breathes cold air from a special laboratory apparatus. These sorts of bronchial challenge tests can also reveal quite specific evidence for asthma, if there is substantial exercise- or cold air-induced bronchoconstriction.

Allergy tests
Many, though certainly not all, patients with asthma have allergies to one or more substances in the environment (called allergens). As discussed in Avoiding Asthma Triggers, an asthmatic patient can improve his/her asthma control, or at least reduce the likelihood of inducing an asthma attack, by limiting contact with substances to which he or she is allergic.

To determine whether allergies are present, and to which environmental allergens, your doctor may perform a series of skin "prick" tests, in which a drop of allergen is placed on the forearm, and a tiny prick into the very top of the skin is made with a needle. 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 a particular kind of antibody (immunoglobulin E, or IgE) that is often elevated in people with multiple allergies.