Approach Considerations
A guideline summary on allergy testing is available from the American Academy of Allergy, Asthma and Immunology, American College of Allergy, Asthma and Immunology (Allergy Diagnostic Testing: An Updated Practice Parameter). [22]
Go to Irritant Contact Dermatitis, Pediatric Contact Dermatitis, and Protein Contact Dermatitis for complete information on these topics.
Laboratory Studies
Potassium hydroxide preparation and/or fungal culture to exclude tinea are often indicated for dermatitis of the hands and feet. This will identify disorders such as tinea pedis.
Patch Testing
Patch testing [23, 24, 25] is required to identify the external chemicals to which the person is allergic. The greatest quality-of-life benefits from patch testing occur in patients with recurrent or chronic allergic contact dermatitis (ACD). Patch testing is most cost-effective and reduces the cost of therapy in patients with severe allergic contact dermatitis.
Patch testing should be performed by healthcare providers trained in the proper technique. Most dermatologists can perform patch testing using the TRUE test, which can identify relevant allergies in as many as one half of affected patients. More extensive patch testing is indicated to identify allergies to chemicals not found in the TRUE test. Such testing typically is available only in a limited number of dermatology offices and clinics.
The patch testing procedure is as follows:
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Small amounts of appropriate labeled dilutions of chemicals are applied to the skin and occluded for 2 days
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Patch tests may be left on for 3 days before removal
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For reasons of scheduling, a chemical must remain under a skin patch for a minimum of 1 day to produce a positive patch test reaction 2-7 days following initial application
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The patch test must be read not only at 48 hours, when the patch tests customarily are removed, but again between 72 hours and 1 week following initial application
Individuals with suspected allergic contact dermatitis without positive reactions on the TRUE test or with chronic dermatitis or relapsing dermatitis, despite avoiding chemicals to which they are allergic (identified on TRUE test), need additional patch testing. Many individuals have more than 1 contact allergy and may be allergic to 1 or more chemicals found on the TRUE test and on special allergen trays or series.
Testing reactions to more allergens increases accuracy of the diagnosis of allergic contact dermatitis. Selection of allergens for testing requires consideration of the patient's history and access to appropriate environmental contactants.
Certain chemicals (eg, neomycin) typically produce delayed positive patch test reactions at 4 days or later following initial application. A tendency exists for elderly patients to manifest positive patch test reactions later than younger patients. Do not perform patch testing on patients taking more than 15 mg/d of prednisone. Oral antihistamines may be used during the patch test period if required.
Angry back syndrome or excited skin syndrome may occur. If a patient has a large number of positive patch test reactions, retesting the patient sequentially to a small series of these allergens may be necessary to exclude nonspecific false-positive reactions. The syndrome most likely occurs in individuals who have active dermatitis at the time of patch testing or who have a strong positive patch test reaction, both of which may induce local skin hyperreactivity in the area where patches were applied.
Additional patch test series or sets include the following:
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Corticosteroids, particularly tixocortol pivalate and budesonide
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Ingredients in cosmetics not found in the TRUE test
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Chemicals used in dentistry that may produce mucosal and lip dermatitis in dental clients or that may produce chronic dermatitis of the hands in dentists and dental team members
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Chemicals used in hairdressing that may produce facial, ear, and neck dermatitis in clients or chronic hand dermatitis or eyelid dermatitis in hairdressers
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Fragrances found in cosmetics and a wide range of consumer products
Important allergens not found in the TRUE test that are frequent causes of allergic contact dermatitis are as follows:
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Acrylates used in dentistry, artificial nails, and printing
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Chemicals used in baking
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Pesticides (many cases of dermatitis attributed to pesticides result from other causes, particularly from plants such as poison ivy)
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Chemicals used in machining, eg, cutting oils and fluids
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Photographic chemicals used by photographers and photographic developers
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Plants
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Chemicals in plastics and glues
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Chemicals found in rubber products not included in the TRUE test
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Chemicals in shoes and clothing
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Ultraviolet (UV) protective ingredients in sunscreens
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Other chemicals producing photo allergic contact dermatitis
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Miscellaneous allergens
The chemicals listed above are tested under Finn chambers, allergEAZE chambers, or the IQ Chamber patch test. In photopatch testing, the chemicals are applied in duplicate sets. One set receives 10 J/cm2 of UV-A (or 1 J/cm2 less than the minimum erythema dose, whichever is lowest) 24 hours after application of the allergens. The other series is protected from UV exposure to differentiate allergic contact dermatitis and photo-accentuated allergic contact dermatitis from photo-allergic contact dermatitis. Both sets are read at 48 hours after application, as well as at an additional time point as in routine patch testing.
The safety of patch testing in pregnancy has not been studied; however, the minute amounts of allergens applied appear unlikely to be absorbed in sufficient amounts to harm the fetus. Nonetheless, as with all treatments in pregnant women, the benefits of testing should be weighed against any potential, albeit undocumented, risk.
Individuals with atopic dermatitis may have more positive patch-test reactions to ingredients in personal care products, to the topical steroids that they use, as well as to antibiotics. [26]
Repeat Open Application Test
For individuals who develop weak or 1+ positive reactions to a chemical, the repeat open application test (ROAT) is useful in determining whether the reaction is significant. ROAT is most useful when an individual has a 1+ reaction to a chemical found in a leave-on consumer product.
For example, an individual with a weak reaction to a preservative found in a moisturizer may apply the moisturizer twice a day for a week to the side of the neck or behind an ear; if clinical dermatitis does not develop, the 1+ reaction likely was not meaningful. Conversely, if dermatitis develops after a few days of repeated application of the suspected product, then the weak patch test reaction is highly relevant.
Dimethylgloxime Test
The dimethylgloxime test is a useful and practical way to identify metallic objects that contain enough nickel to provoke allergic dermatitis in individuals allergic to nickel. Dermatology staff may test suspected metal products in the office, or the individual may purchase a test kit and test objects at home or at work, particularly jewelry or metallic surfaces.
Other chemical tests are available for other suspected allergens (eg, formaldehyde, cobalt, chromate). Occasionally, chemical analyses may be necessary to determine whether a material contains a suspected allergen or to identify new unknown allergens.
Skin Biopsy
Skin biopsy may help exclude other disorders, particularly tinea, psoriasis, and cutaneous lymphoma. Skin biopsy of skin lesions of the palms and soles has several potential pitfalls, however.
The stratum corneum and epidermis are particularly thick on the palms and soles. This makes the histologic diagnosis of psoriasis more difficult and increases the possibility that the biopsy specimen will lack sufficient dermis for optimal diagnosis.
An overly deep skin biopsy of the thenar area can cut the motor nerve, which is the recurrent branch of the median nerve. A biopsy from the sole may leave a chronic painful scar on which the patient must walk.
Collections of Langerhans cells on skin biopsy can be an important clue to the diagnosis of allergic contact dermatitis. Although eosinophils are typically associated with allergic contact dermatitis, a 2016 study did not identify a difference in eosinophil count between patch test–positive and patch test–negative cases. [27]
Histologic Findings
The histology of allergic contact dermatitis is similar to that found in other forms of eczematous dermatitis. A pattern of subacute chronic dermatitis or acute dermatitis may be seen. The inflammatory infiltrate in the dermis predominately contains lymphocytes and other mononuclear cells.
Epidermal edema (ie, spongiosis and microvesicle formation) may be seen, but these changes may be absent in long-standing dermatitis in which thickening of the epidermis (acanthosis) with hyperkeratosis and parakeratosis may be seen in the epidermis and stratum corneum. Allergic contact dermatitis may provoke atypical T-cell infiltrates, simulating mycosis fungoides.
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Chronic stasis dermatitis with allergic contact dermatitis to quaternium-15, a preservative in moisturizer. Allergic contact dermatitis produces areas of erythema in areas of atrophie blanche and varicose veins.
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Erythema multiformelike reaction that developed acutely following hair dying.
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Allergic contact dermatitis to nickel in a necklace.
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Severe allergic contact dermatitis resulting from preservatives in sunscreen. Patch testing was negative to the active ingredients in the sunscreen.
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Onycholysis developing from allergic contact dermatitis to formaldehyde used to harden nails.
