How Allergy Is Diagnosed

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Allergy testing involves placing a small drop of an allergen extract on the skin and pricking or scratching through the drop. The skin is observed for signs of a reaction, which usually includes swelling and redness of the site within about 15 minutes. adam.about.net

The symptoms and timing of symptoms can give your doctor strong hints as to whether you have an allergy. However, in most cases, various tests are required to confirm a diagnosis. Testing procedures depend on the type of allergic disease in question.

Self Checks

Diagnosis of allergies starts at home. Keeping a diary of your activities, what you ate, when you got symptoms, and a good account of the symptoms is useful. Not only can you share this with your doctor, it can help you pinpoint sources of allergens to avoid in the future.

Home allergy test kits often have not been approved by the FDA or they are testing for the wrong type of antibody—IgG rather than IgE. Major medical organizations in the U.S., Canada, and Europe recommend against using these kits. They should never replace seeing a board-certified allergy physician to get an accurate diagnosis of what may be causing your symptoms.

Labs and Tests

The diagnostic tests and criteria differ based on the type of allergy. The only two valid types of testing are skin testing (prick/puncture and intradermal) and specific IgE (sIgE) blood testing. Blood testing procedures include radioallergosorbent test (RAST), enzyme-linked immunosorbent assay (ELISA), and ImmunoCAP testing.

Skin Testing

Skin testing for allergies has been standard for over 100 years. In the prick/puncture procedure, a scratch is made on the skin and a drop of a specific allergen is placed and inspected for a reaction. A bump like a mosquito bite indicates a positive reaction.

Intradermal skin testing can be more sensitive, but also has more false positives. Instead of scratching, the allergen is injected just under the top layer of the skin. This type of testing can't be used for food allergies.

Patch testing is done for contact dermatitis. Patches with a small amount of the suspected allergen are placed on the skin and kept there for about two day.

Blood Testing

Your doctor may have blood drawn to be tested for IgE antibodies to specific allergens. The older test is RAST, which has largely been replaced by ELISA testing and ImmunoCAP testing.

Numerous tests other than RAST, ELISA, ImmunoCAP, and skin testing are performed by non-allergy practitioners or people who call themselves “allergists” but lack formal training and national board certification in the field of allergy and immunology. These controversial tests should be avoided.

Atopic Dermatitis

There is no blood test that can be used to diagnose atopic dermatitis. Instead, a physician must examine a patient and review the history of her symptoms. In most cases, people with atopic dermatitis should have skin testing performed to airborne allergens (pollens, molds, pet dander, and dust mite) as well as to common food allergens.

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.

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 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.

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, skin testing is still the preferred method of testing.

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 is done by blowing into a spirometer device. 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 in lung function with the inhalation of certain chemicals, such as methacholine) and measurements of markers of inflammation in exhaled air, sputum, blood, and urine.

Allergic Contact Dermatitis

In addition to a good history and physical exam, your physician may do skin testing see your reaction to different allergens.

Imaging

Imaging is not commonly used for diagnosing allergies. Studies such as a sinus CT scan might be used if chronic sinusitis is suspected.

Differential Diagnoses

Your doctor will consider whether rhinitis is has a non-allergic cause, which can be environmental or due to a virus like the common cold. Acute or chronic sinusitis can also produce some of the same symptoms.

Food allergy symptoms could be caused by conditions including celiac disease. But the symptoms may also have a non-allergic cause including chronic infection, enzyme maladies, and even psychosomatic reactions.

Atopic dermatitis can look like many other skin conditions, including psoriasis, seborrheic dermatitis, scabies, ringworm, and more. Allergic contact dermatitis shares symptoms with irritant contact dermatitis.

A Word From Verywell

It can be quite a relief to find what exactly is triggering your allergies or to rule out allergies as the cause. If allergies are diagnosed, you can begin to find ways to avoid the allergens and effective treatment for your symptoms.

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