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The type of test required depends on the allergy suspected. If a contact allergy is suspected, your dermatologist may send you for a patch test to evaluate for specific contactants based on your history, occupation, hobbies and location of the rash. On the other hand, a prick test or blood tests may be undertaken if a food allergy or drug allergy is suspected.

How is allergic contact dermatitis diagnosed?

Sometimes it is easy to recognise contact allergy and no specific tests are required. Taking a thorough history which includes hobbies, work environment, and products in use will aid in finding a diagnosis. The rash most often clears up if the allergen is no longer in contact with the skin, but may reoccur on exposure.

The open application test can be used to confirm contact allergy to a cosmetic product. The product under suspicion is applied several times daily for a few days to a small area such as the inner aspect of the upper arm. Contact allergy is likely if dermatitis arises in the treated area.

Dermatologists will also sometimes send patients for patch testing. This is for patients with suspected contact allergy that is severe, recurrent or chronic. The tests can help identify the specific allergen causing the rash, and avoidance of the contactant can reduce the risk of the rash reoccurring.

How is food allergy diagnosed?

Food allergy is diagnosed by taking a careful history of the symptoms (food diary), and their relationship to food. Specific food allergy tests can also be carried out for certain common allergens. Unfortunately, neither history nor tests are entirely reliable.

The main tests for food allergy are:

  • Skin prick tests
  • Specific IgE blood tests (RAST)

How is a drug allergy diagnosed?

Just like in suspected food allergies, possible drug allergies are first evaluated by taking a careful detailed history to ascertain the timeline of the rash in relation to the ingestion of various drugs. A thorough skin and general physical examination is also necessary to determine the type of drug eruption.

Specific drug allergy tests can also be carried out although very few drug reactions have a confirmatory test. Your dermatologist will first determine if a drug allergy is likely, and if this is suitable for further allergy testing. Skin prick tests may then be undertaken to check for immediate reactions to penicillin and a few other drugs.

Other blood tests may include a full blood count to look for eosinophilia, along with liver function and kidney function tests.

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