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Contact Dermatitis, Allergic: Differential Diagnoses & Workup

Author: Daniel J Hogan, MD, Clinical Professor of Internal Medicine (Dermatology), NOVA Southeastern University; Investigator, Hill Top Research, Florida Research Center
Contributor Information and Disclosures

Updated: Jul 31, 2009

Differential Diagnoses

Other Problems to Be Considered

Mycosis fungoides

Workup

Laboratory Studies

• Potassium hydroxide preparation and/or fungal culture to exclude tinea are often indicated for dermatitis of the hands and feet.

Procedures

• Patch testing^11,12,13
  • Patch testing 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 must be performed by health care providers trained in the proper technique. Most dermatologists can perform patch testing using the TRUE test (consult the Physicians' Desk Reference), 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.
  • Patch testing procedure
    • Small amounts of appropriate labeled dilutions of chemicals are applied to the skin and occluded for 2 days.
    • Patch tests may be left on for 3 days before removal.
    • 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.
    • 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 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: 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:
    • Corticosteroids, particularly tixocortol pivalate and budesonide
    • Ingredients in cosmetics not found in the TRUE test
    • 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
    • Chemicals used in hairdressing that may produce facial, ear, and neck dermatitis in clients or chronic hand dermatitis or eyelid dermatitis in hairdressers
    • 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
      • Bacitracin
      • Acrylates used in dentistry, artificial nails, and printing
      • Chemicals used in baking
      • Pesticides (many cases of dermatitis attributed to pesticides result from other causes, particularly from plants such as poison ivy)
      • Chemicals used in machining, eg, cutting oils and fluids
      • Photographic chemicals used by photographers and photographic developers
      • Plants excluding poison ivy
      • Chemicals in plastics and glues
      • Chemicals found in rubber products not included in the TRUE test
      • Chemicals in shoes and clothing
      • UV protective ingredients in sunscreens
      • Other chemicals producing photo allergic contact dermatitis
      • Miscellaneous allergens
    • The chemicals listed above are tested under Finn chambers or IQ patch test. In photopatch testing, the chemicals are applied in duplicate sets. One set receives 10 J/cm² (or 1 J/cm² less than minimum erythema dose to UV-A, whichever is lowest) of UV-A 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.
• 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 the individual applies it twice a day for a week without developing clinical dermatitis, the 1+ reaction likely was not meaningful. Conversely, if the individual develops dermatitis following 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, chromate). Occasionally, chemical analyses may be necessary to determine whether a material contains a suspected allergen or to identify new unknown allergens.
• 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, which include the following:
  • 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.
• Guideline summaries
  • American Academy of Allergy, Asthma and Immunology, American College of Allergy, Asthma and Immunology - Allergy diagnostic testing: an updated practice parameter. Part 1¹⁴
  • American Academy of Allergy, Asthma and Immunology, American College of Allergy, Asthma and Immunology - Allergy diagnostic testing: an updated practice parameter. Part 2¹⁵

Histologic Findings

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 is provokes atypical T-cell infiltrates, simulating mycosis fungoides.

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