PATCH TEST UPDATE ! In view of some ongoing controversies I post for benefit of all an update about patch test. – Patch Test ∗ This tests the type IV hypersensitivity reaction and it is a confirmatory test for allergic contact dermatitis. ∗ Founder: Josef Jadassohn ∗ The test materials are applied to the back (or upper arms/ anterior thigs) under aluminum discs (Finn chambers, Scanpor tapes etc) or plastic chambers and covered with nonirritating and non allergic tape . Usually 20 mg of allergen in a petrolatum base is used; though different standard concentrations are used for different vehicles and allergens. The sites are inspected 1 hour after removing the patches on day 2 (48 hours). The one hour waiting is for resolution of pressure effects and better appreciation of changes. The sites are re-inspected at 96 hours (day 4) for delayed reaction though it may be read upto day 7. A single day 2 reading may be erroneous in labeling irritants as allergens as well as missing delayed reactors. If only one reading is possible then probably day 4 is best; though even this misses some important cases. Erythema, infiltration, vesiculation and even bulla formation is noted and graded . ∗ Some allergens produce a delayed reactions for example; neomycin, PPD dye, metals (like gold) and corticosteroids. ∗ Some allergens produce an early reaction peak like Carba, thiuram, Balsam of Peru. ∗ Irritant reactions are sometimes seen with following typical findings: erythema without edema; more burning sensation than itching; sharp margins; deep red, deep brown, purpuric/petechial (esp with cobalt chloride and sometimes with PPD), pustular eruptions (esp with nickel); crescendo-decrescendo effect (more severe on day 2 than on day 4), scaly/chapped and glistening “soap-effect”. ∗ Before the test; the test site skin should be normal, disease reasonably controlled and patient should be off steroids. If tested when disease is fulminant all tested sites become inflamed (False positive = angry back phenomenon; Figure [fig:Diagnostics] d). Conversely, if patient is on steroids/immunosuppressive he may give false negative result. ∗ Sunbathing in last 1 month also requires postponing the test. Patch test sites should also not be exposed to sun or UV light. ∗ Usually a battery of allergens (e.g. Indian standard series) is used rather than a single agent. ∗ The positive patch test indicates allergy. However, it may or may not be clinically relevant for the disease that was being evaluated. ∗ Persistent patch test reaction even after 1 month is usually due to gold. ∗ Thin layer rapid use epicutaneous test (TRUE) is a variant where patches are prepackaged and ready to use. ∗ In photoallergic contact dermatitis a duplicate set of patches are applied and one set is exposed to light (UV-A or B) after 24 hours and results are read after further 24 hours. (Photopatch test) Other Contact dermatitis trivia: * Iriitant dermatitis is more common than allergic dermatitis (80% and 20 % respectively) *Mc allergen: Nickel is the most frequent contact allergen, and sensitivity is more common in women than in men. ( ear prercing at an early age is a significant risk factor) *MC cause of allergic footwear dermatitis: Mercaptobenzothiazole (MBT) * MC cosmetic preservative to cause ACD: Quaternium-15 * MC sunscreen agent to cause photoallergic contact dermatitis: Oxybenzone (Benzophenone 3) * MC cause of Hair dye/ black henna dermatitis (esp in India as it is banned in many countries): Paraphenylene diamine (PPD) * Allergen in Poison ivy (Family: Anacardiaceae; Genus: Toxicodendron; Species: Rhus): Urushiol * MC irritant: detergent (some argue for water/wet work) * MC allergen/sensitizer in Indian females: Vegetables (Suman M, Reddy BS. Pattern of contact sensitivity in Indian patients with hand eczema. J Dermatol. 2003 Sep;30(9):649-54.)
Posted on: Tue, 21 Oct 2014 11:31:44 +0000
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