How is asthma diagnosed?
In most cases the physician can make the diagnosis on the basis of typical symptoms and signs. The typical rapid changes in airway obstruction can be demonstrated by a fall in pulmonary function tests spontaneously, after exercise or inhalation of histamine or metacholine,and subsequent improvement with an inhaled bronchodilator medication. Many people with
asthma have allergies; positive allergy tests support a diagnosis of asthma and may help in identifying avoidable triggers.
Apart from listening to the patient's chest, the examiner should look for maximum chest expansion while taking in air. Hunched shoulders and contracting neck muscles are other signs of narrowed airways. Nasal polyps or increased amounts of nasal secretions are often noted in asthmatic patients. Skin changes, like atopic dermatitis or eczema, are a tipoff that the patient has allergic problems.
A doctor suspects asthma based largely on a person's report of characteristic symptoms. More commonly, the doctor performs spirometry or pulmonary function tests before and after giving the person an inhaled beta-adrenergic agonist. Spirometry is also used to assess the severity of the airway obstruction and to monitor treatment. Peak expiratory flow (the fastest rate at which air can be exhaled) can be measured using a small handheld peak flow meter.
Allergy skin testing may be used, although an allergic skin response does not always mean that the allergen being tested is causing the asthma. Also, the body's immune system produces antibody to fight off the allergen, and the amount of antibody can be measured by a blood test. This will show how sensitive the patient is to a particular allergen. If the diagnosis is still in doubt, the patient can inhale a suspect allergen while using a spirometer to detect airway narrowing. Spirometry can also be repeated after a bout of exercise if exercise-induced asthma is a possibility. A chest x ray will help rule out other disorders. Skin testing can help identify allergens that may trigger asthma symptoms. If a doctor suspects a particular allergen, a blood test that measures the level of antibody produced in response to the allergen can be performed to determine the degree of sensitivity.
To test for exercise-induced asthma, an examiner uses spirometry before and after exercise on a treadmill or stationary bicycle to measure forced expiratory volume in 1 second. If the forced expiratory volume in 1 second decreases more than 15%, the person's asthma can be induced by exercise.
A chest x-ray is not generally helpful in diagnosing asthma. Doctors use chest x-rays when considering another diagnosis. However, a chest x-ray is often obtained when a person with asthma needs to be hospitalized or is treated in the emergency department with severe asthma.