Skin prick test

What it is

The skin prick test is the most commonly used allergy test, as it is easy to perform and inexpensive, and it is a test for immediate allergic reactions. Drops of diluted allergen are placed on the skin, after which the skin is pierced using a needle. A large number of allergens can be tested in one session. It normally includes a histamine positive control and a sterile water negative control. A wheal will typically develop if a patient has an allergy, and the test can be interpreted after 15 minutes.

How it is diagnosed

The test is useful for food allergies and also pollen. Its listed indications are:

• Atopy
• Chronic urticaria
• Latex allergy
• Food allergy
• Bee allergy

It carries weight in two conditions in particular. Allergic rhinitis, an IgE mediated hypersensitivity to foreign allergens giving nasal obstruction with sneezing, clear rhinorrhea containing eosinophils and itching of eyes with tearing, is confirmed by skin prick test, though the diagnosis is mainly clinical. In asthma, the skin-prick test is one of the tests used at the initial diagnosis of a chronic attack, where its role is to identify the allergen.

Two other tests are used alongside it. The radioallergosorbent test, RAST, determines the amount of IgE that reacts specifically with suspected or known allergens, for example IgE to egg protein; results are given in grades from 0, negative, to 6, strongly positive, and it is useful for food allergies, inhaled allergens such as pollen, and wasp or bee venom. Blood tests may be used when skin prick tests are not suitable, for example if there is extensive eczema or if the patient is taking antihistamines. Skin patch testing is useful for contact dermatitis: around 30-40 allergens are placed on the back, irritants may also be tested for, and the patches are removed 48 hours later with the results being read by a dermatologist after a further 48 hours.