There are three primary methods of allergy skin tests. All are effective at diagnosing a patient’s allergies, but each has its own distinct advantages and disadvantages. These tests are:

• Prick test/scratch test. Tiny amounts of different allergens are placed on the skin, and a series of tiny scratches or needle pricks force the allergens into the skin. This epicutaneous test is usually the first type of skin test attempted, because it is highly unlikely to trigger anaphylaxis, a potentially dangerous reaction. Both prick tests and scratch tests are very good at identifying specific allergies. However, they are not as sensitive as intradermal tests and sometimes show no reaction to an allergen even when an allergy is present.
  
  
• Intradermal test. Tiny amounts of an allergen are injected just below the skin. Intradermal tests are more sensitive, but less specific, than prick or scratch tests. They sometimes are done when a skin prick test does not reveal an allergy, but the allergy nonetheless is still suspected.
  
  
• Patch test. An absorbent pad is soaked in an allergen and taped onto the skin of the patient for 24 to 72 hours. A patch test generally is used to identify allergic contact dermatitis, a rash that results from direct skin contact with an allergen. The chemicals used in these tests often come from a kit containing the 20 most common causes of contact dermatitis. These include substances used in jewelry, hair dyes, shampoos, medicinal creams, clothing, glues and cosmetics.

As part of the skin testing process, physicians perform control tests that help to determine how accurate skin testing will be on the patient. These control substances are administered in the same manner as the allergen (e.g., prick, scratch, injection, patch) and should produce a predictable result. Two control substances commonly used are:

• Histamine (used in a positive control test). Patients should automatically have a reaction to the histamine – if they do not, allergy tests are unlikely to reveal interpretable results. A positive control test is not used for patch testing.
  
  
• Saline (used in a negative control test). Saline should not provoke any reaction. If it does, the patient likely has very sensitive skin, and any results from the allergy testing may be less conclusive.