The symptoms and severity of asthma vary from person to person.

When the condition is well controlled, the asthmatic subject experiences only mild symptoms, if any.

If the condition is severe or not adequately treated, asthma symptoms may become persistent.

Shortness of breath (dyspnea), with or without tightness of the chest, is caused by a narrowing of the airways (bronchi and bronchioles).

In children who are too young to describe what they are experiencing, parents will sometimes notice rapid breathing and indrawing (chest depression).

Wheezing is another sign of narrowing of the airways.

Coughing may be the only manifestation of asthma or one of the main symptoms, particularly in children. Coughing generally occurs at night or early in the morning.

Other common asthma symptoms include bronchial secretion (sputum or phlegm) and a sensation of tightness in the chest.

Some of the tools to diagnose asthma include:

• questions asked by your doctor
• physical examination including examination of the chest
• peak expiratory flow (PEF) measured on a peak flow meter (Peak expiratory flow is the maximum speed at which air is expelled from the lungs)

• forced expiratory volume (FEV) measured using a spirometer. This is a measurement of the volume of air that the lungs can expel in a specific amount of time (for example, in one second - FEV₁).
• Airway hyperresponsiveness test (histamine or methacholine) to measure sensitivity of airways. This test helps to determine the severity of asthma or confirm an uncertain diagnosis, i.e. in the case of persistent cough or unexplained shortness of breath.

The physical examination of someone with asthma may often be normal. Pulmonary function tests are always necessary to prove the presence of what characterizes asthma: variable airway obstruction or hyperresponsiveness (i.e. twitchy airway)

In diagnosing children, a peak flow meter or spirometer can usually be used from the age of five or six.

In some cases, the doctor may also ask the following tests:

• allergy skin tests: not to diagnose asthma but to indicate the presence of any allergies.

Anyone over age 5 with asthma should undergo at least one allergy test during their lifetime since 60% to 80% of all asthmatics suffer from allergies. Children under 5 sometimes are given allergy tests when there is a family history of allergy.

• chest X-rays to check for other lung conditions
• blood tests, especially in cases of allergic asthma
• specific bronchial provocation tests (in special cases such as occupational asthma).