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How is skin testing done?
- The procedure involves the placement of a small drop of the allergen on the skin (usually the patient’s forearm), then a specialist nurse will prick the skin gently using a small lancet.
- In allergic individuals, this will elicit a localised wheal and flare response after 10 to 15 minutes. Some patients experience localised itching, but any reaction generally disappears within 30 minutes. Usually, 10 to 16 individual tests would be done in one visit.
- Two control tests are always run thus in order to properly interpret skin tests. The negative control (salt water) must not produce a response whereas the positive control should produce a wheal and flare.
- Negative control involves pricking a drop of salt-water on the skin to ensure that the skin is not overly reactive; for example, some patients will come up positive to every single skin test because they have sensitive skin.
- Positive control: a small drop of histamine solution is placed on the skin to determine that the skin has the capacity to react and produce a wheal or flare response. Antihistamine medication will prevent the skin from reacting to the positive control and other allergic tests which is why antihistamine medications must be stopped before the visit.