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A patch test relies on the principle of a type IV hypersensitivity reaction.. The first step in becoming allergic is sensitization. When skin is exposed to an allergen, the antigen-presenting cells (APCs) – also known as Langerhans cell or Dermal Dendritic Cell – phagocytize the substance, break it down to smaller components and present them on their surface bound major histocompatibility ...
A patch test is used to identify the causative allergen in allergic conjunctivitis. [ 31 ] Although conjunctival scrapes for cytology can be useful in detecting chlamydial and fungal infections, allergies, and dysplasia , they are rarely done because of the cost and the general dearth of laboratory staff experienced in handling ocular specimens.
The injection should be made with a tuberculin syringe, with the needle bevel facing upward. When placed correctly, injection should produce a pale wheal of the skin, 6 to 10 mm in diameter. The result of the test is read after 48–96 hours, ideally after 72 hours/3rd day. This procedure is termed the 'Mantoux technique'.
For metal allergens, patch test reproducibility is low, and the extent to which they predict implant failures is debated. If the person being tested has a rash already, it may be difficult to do a patch test. Patch testing may also worsen the allergy. it is also difficult to distinguish co-sensitivity from cross-sensitivity using a patch test.
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Patch test For most people, irritation from formaldehyde is temporary and reversible, although formaldehyde can cause allergies and is part of the standard patch test series. In 2005–06, it was the seventh-most-prevalent allergen in patch tests (9.0%). [ 101 ]