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Diagnosis of Allergy

Diagnosing Allergy

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Updated January 28, 2009

Allergy Diagnosis

Symptoms of allergic diseases can certainly give strong hints that a person is indeed suffering from allergies. However, in most cases, various tests are required to confirm a diagnosis. Testing depends on the type of allergic disease in question.

Atopic Dermatitis

There is no laboratory test that can be used to diagnose atopic dermatitis. Instead, a physician must examine a patient and review the history of her symptoms.

There are three criteria that must be present in order to diagnose atopic dermatitis:

  • Allergies -- The person must have allergies (at least one positive skin test) and/or a family history of allergic diseases in close relatives. There may be rare cases in which a person has atopic dermatitis without evidence of allergies.

  • Itching -- The patient must have itching and scratching in order for the rash to occur. If the skin or areas of the rash do not itch or have not been scratched, then the person does not have atopic dermatitis.

  • Eczema -- Eczema refers to the appearance of the rash in patients with atopic dermatitis; it also occurs in those with other skin diseases. The rash appears red, with small blisters or bumps. These may ooze or flake with further scratching. Over the long-term, the skin appears thickened and leathery.

In most cases, people with atopic dermatitis should have skin testing performed to aeroallergens (pollens, molds, pet dander and dust mite) as well as to common food allergens. Avoiding allergic triggers is an important step in the treatment of atopic dermatitis.

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Food Allergies

The diagnosis of food allergies is made when typical symptoms occur after eating a specific food and a patient receives a positive result after undergoing an allergy test to the food in question. Testing for the allergic antibody is best accomplished with skin testing, although can be done with a blood test as well.

The blood test, called a RAST, is not quite as good of a test as skin testing, but it can be helpful in predicting if a person has outgrown a food allergy. This is especially true since, in many cases, the skin test can still be positive in children who have actually outgrown the food allergy.

If the diagnosis of food allergy is in question despite testing, an allergist may decide to perform an oral food challenge. This involves having a person eat increasing amounts of the suspect food, over many hours, under close medical supervision. Given the potential for life-threatening allergic reactions, this procedure should only be performed by a physician experienced in the diagnosis and treatment of allergic diseases.

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Nasal Allergies

In many cases, allergic rhinitis is diagnosed when a person has convincing symptoms and physical exam findings consistent with this disease. However, in order for allergic rhinitis to be properly diagnosed, allergy testing is needed. Positive allergy tests are required to diagnose allergic rhinitis; negative allergy testing suggests non-allergic rhinitis. While allergy testing can be accomplished with skin testing or blood tests (RAST), skin testing is still the preferred method of testing.

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Asthma

The diagnosis of asthma is suggested by the presence of asthma symptoms; however, spirometry is needed to make a firm diagnosis. Spirometry can easily be performed in people 5 years and older. It may show a specific pattern in a person with asthma, which can make the diagnosis more concrete. For example, someone with asthma may display a certain increase in lung function after using a bronchodilator, such as Albuterol.

If the diagnosis of asthma is still in question despite performing spirometry, asthma specialists can perform additional tests to determine if a person has asthma. These include bronchoprovocation (causing a drop of lung function with the inhalation of certain chemicals, such as methacholine) and measurements of markers of inflammation in exhaled air, sputum, blood and urine.

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DISCLAIMER: The information contained in this site is for educational purposes only, and should not be used as a substitute for personal care by a licensed physician. Please see your physician for diagnosis and treatment of any concerning symptoms or medical condition.

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