Tests
1st tests to order
patch testing
Test
Gold standard for the diagnosis of allergic contact dermatitis.[16][22]
Screening trays are often used for patch testing and include the most frequent contact allergens. Various screening series are available commercially. These include the thin-layer rapid use epicutaneous test, the American Contact Dermatitis Society Baseline Series, the European Baseline Series, the Australian Baseline Series, and the North American Contact Dermatitis Group Series. Each series offers slightly different allergens based on regional prevalence.
Patch testing requires at least two readings: the first at patch test removal at 48 hours and the next at 72-96 hours after placement. A third reading can sometimes be performed 1 week after patch placement to detect delayed reactions that may occur to topical antibiotics, metals, and topical steroids.[16]
Photopatch testing can be used to detect a photoallergic contact dermatitis.
Negative testing may occur if the patient's dermatitis is not caused by an allergen, the testing technique was not adequate (e.g., patient is on systemic immunosuppression, receiving phototherapy, or sunburned), or the appropriate allergen was not tested.
False-positive tests results may be seen in patients with active dermatitis near the testing site.[23]
A standardized strip patch test is proposed to improve diagnosis of allergic contact dermatitis beyond the patch test.[25][26] Its application in routine clinical practice can be recommended, especially if the conventional patch test is presumed false negative.[26]
Result
positive result is graded on a 1+ to 3+ scale according to standards established by the International Contact Dermatitis Research Group; however, determination of relevance is essential as not all positive reactions are relevant to the patient’s allergic reaction
Tests to consider
repeated open application test (ROAT) or provocative use test (PUT)
Test
If patch testing is not available or the causative allergen not identified, ROAT/PUT can be performed by the patient.[16][22]
1) Draw a 2.5 cm circle on an unaffected/non sun-exposed area of skin such as the volar forearm.
2) Apply the suspected leave-on products (moisturizers, sunscreens, cosmetics) in the circle twice a day for 7 days.
3) If the product is not meant to be left on the skin (shampoos, soaps, etc.) then apply the product but wash it off twice a day for 7 days.
4) If a red, itchy rash occurs, then the reaction is positive and the patient is allergic to the product.
Do not perform ROAT or PUT to products not meant to be used on the skin at all such as detergent, industrial reagents, and known irritants.
Correlation between patch testing and ROAT can be variable.[30]
Result
inflammation or dermatitis at application site
skin biopsy
Test
Skin biopsy is required if the diagnosis is in doubt. A biopsy will not differentiate between irritant and contact dermatitis in most cases, but may be useful in determining whether an alternate inflammatory skin disorder is present (e.g., psoriasis, cutaneous T-cell lymphoma).[22]
Result
ICD: variable epidermal findings including ulceration, spongiosis, parakeratosis, necrosis with epidermal pallor, or acantholysis with neutrophils; ACD: spongiosis, vesiculation with or without dermal and intraepidermal eosinophils in the acute phase; in the subacute and chronic phases there is greater epidermal hyperplasia, parakeratosis, and a perivascular lymphohistiocytic infiltrate
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