Clinical tests

The onset of the allergic reaction from the time of allergen contact with sensitized mast cells is rapid, taking place within minutes. Skin-prick testing, which is the classical test for allergy, can thus be performed as a rapid diagnostic procedure. Antigen is introduced into the skin, which is examined for the presence of a wheal some 15-20 min later; histamine is used as a positive control, saline as the negative. A positive skin test (wheal 3 mm in diameter larger than saline control) usually correlates with the laboratory test for allergen-specific IgE in blood, the radioallergosorbent test (RAST) and with provocation tests. However, the skin-prick test results must always be interpreted in the light of the patient's history, as IgE reactivity to allergen is not always associated with clinical symptoms. Conversely, a negative skin-prick test can exist where there is only local sensitization in the shock organ; such patients will only give positive results on provocation tests.

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