Diagnosis

The doctor will seriously consider a diagnosis of asthma if the child has a history of periodic attacks of shortness of breath, coughing, and wheezing, perhaps accompanied by tightness in the chest. The parent should describe the pattern of symptoms and possible precipitating factors, including:

• Whether symptoms are more frequent during the spring or fall (allergy seasons)
• Whether exercise, a respiratory infection, or exposure to cold air has ever triggered an attack
• Any family history of asthma or allergic disorders, such as eczema, hives, or hay fever

Ruling Out Other Diseases

A number of disorders may cause some or all of the symptoms of asthma: Panic disorder can coincide with asthma or be confused with it. Other diseases that must be considered during diagnosis are pneumonia, bronchitis, severe allergic reactions, psychosomatic illnesses, and certain rare disorders (such as tapeworm and trichomoniasis).

Pulmonary Function Tests

If symptoms and a patient's history are indicative of asthma, the doctor will usually perform tests known as pulmonary function tests to confirm the diagnosis and determine the severity of the disease.

Using a spirometer, an instrument that measures the air taken into and exhaled from the lungs, the doctor will determine several values:

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), the maximum volume of air expired in 1 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 has most likely 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. It involves administering a specific drug (histamine or methacholine) that usually increases airway resistance only when asthma is present.

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 test for allergic asthma, although they are not recommended for people with year-round asthma.

One of the most common methods of allergy testing is the scratch test or skin prick test. The test involves placing a small amount of the suspected allergy-causing substance (allergen) on the skin (usually the forearm, upper arm, or the back), and then scratching or pricking the skin so that the allergen is introduced under the skin surface. The skin is observed closely for signs of a reaction, which usually includes swelling and redness of the site. With this test, several suspected allergens can be tested at the same time, and results are usually available within about 20 minutes.

Other Tests

Tests that either rule out other diseases or obtain more information about the causes of asthma include the following:

• 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).
• 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.

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Review Date: 3/18/2006
Reviewed By: Harvey Simon, MD, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital.